From 0b183bf2d608bedf118607b1471fbf1e68813a08 Mon Sep 17 00:00:00 2001
From: Kelvin Nilsen <kdnilsen@openjdk.org>
Date: Tue, 3 Mar 2026 09:39:06 +0000
Subject: [PATCH] 8312116: GenShen: make instantaneous allocation rate triggers
 more timely

Reviewed-by: wkemper
---
 .../shenandoahAdaptiveHeuristics.cpp          | 559 ++++++++++++++++--
 .../shenandoahAdaptiveHeuristics.hpp          |  76 +++
 .../shenandoahGenerationalHeuristics.cpp      |   6 +
 .../shenandoahGenerationalHeuristics.hpp      |   2 +
 .../heuristics/shenandoahHeuristics.cpp       |  30 +-
 .../heuristics/shenandoahHeuristics.hpp       |  33 +-
 .../heuristics/shenandoahYoungHeuristics.cpp  |   3 +-
 .../gc/shenandoah/shenandoahConcurrentGC.cpp  |   1 +
 .../gc/shenandoah/shenandoahControlThread.cpp |  19 +-
 .../gc/shenandoah/shenandoahDegeneratedGC.cpp |   1 +
 .../share/gc/shenandoah/shenandoahFreeSet.cpp |  62 +-
 .../share/gc/shenandoah/shenandoahFreeSet.hpp |  70 ++-
 .../share/gc/shenandoah/shenandoahFullGC.cpp  |   4 +-
 .../gc/shenandoah/shenandoahGeneration.cpp    |   7 +-
 .../gc/shenandoah/shenandoahGeneration.hpp    |   2 +-
 .../shenandoahGenerationalControlThread.cpp   |   3 +-
 .../shenandoah/shenandoahGenerationalHeap.cpp |  22 +-
 .../shenandoah/shenandoahGenerationalHeap.hpp |   3 +
 .../share/gc/shenandoah/shenandoahHeap.cpp    |  27 +-
 .../share/gc/shenandoah/shenandoahHeap.hpp    |   3 +
 .../gc/shenandoah/shenandoahOldGeneration.cpp |   2 +
 .../shenandoah/shenandoahRegulatorThread.cpp  |  16 +-
 .../shenandoah/shenandoahRegulatorThread.hpp  |   3 +
 .../gc/shenandoah/shenandoah_globals.hpp      |  53 ++
 24 files changed, 899 insertions(+), 108 deletions(-)

diff --git a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp
index 7a8bd55c795..ac8b3ebdf37 100644
--- a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp
+++ b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp
@@ -33,6 +33,7 @@
 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
 #include "gc/shenandoah/shenandoahHeapRegion.inline.hpp"
+#include "gc/shenandoah/shenandoahYoungGeneration.hpp"
 #include "logging/log.hpp"
 #include "logging/logTag.hpp"
 #include "runtime/globals.hpp"
@@ -59,14 +60,95 @@ const double ShenandoahAdaptiveHeuristics::HIGHEST_EXPECTED_AVAILABLE_AT_END = 0
 const double ShenandoahAdaptiveHeuristics::MINIMUM_CONFIDENCE = 0.319; // 25%
 const double ShenandoahAdaptiveHeuristics::MAXIMUM_CONFIDENCE = 3.291; // 99.9%
 
+
+// To enable detection of GC time trends, we keep separate track of the recent history of gc time.  During initialization,
+// for example, the amount of live memory may be increasing, which is likely to cause the GC times to increase.  This history
+// allows us to predict increasing GC times rather than always assuming average recent GC time is the best predictor.
+const size_t ShenandoahAdaptiveHeuristics::GC_TIME_SAMPLE_SIZE = 3;
+
+// We also keep separate track of recently sampled allocation rates for two purposes:
+//  1. The number of samples examined to determine acceleration of allocation is represented by
+//     ShenandoahRateAccelerationSampleSize
+//  2. The number of most recent samples averaged to determine a momentary allocation spike is represented by
+//     ShenandoahMomentaryAllocationRateSpikeSampleSize
+
+// Allocation rates are sampled by the regulator thread, which typically runs every ms.  There may be jitter in the scheduling
+// of the regulator thread.  To reduce signal noise and synchronization overhead, we do not sample allocation rate with every
+// iteration of the regulator.  We prefer sample time longer than 1 ms so that there can be a statistically significant number
+// of allocations occuring within each sample period.  The regulator thread samples allocation rate only if at least
+// ShenandoahAccelerationSamplePeriod ms have passed since it previously sampled the allocation rate.
+//
+// This trigger responds much more quickly than the traditional trigger, which monitors 100 ms spans.  When acceleration is
+// detected, the impact of acceleration on anticipated consumption of available memory is also much more impactful
+// than the assumed constant allocation rate consumption of available memory.
+
 ShenandoahAdaptiveHeuristics::ShenandoahAdaptiveHeuristics(ShenandoahSpaceInfo* space_info) :
   ShenandoahHeuristics(space_info),
   _margin_of_error_sd(ShenandoahAdaptiveInitialConfidence),
   _spike_threshold_sd(ShenandoahAdaptiveInitialSpikeThreshold),
   _last_trigger(OTHER),
-  _available(Moving_Average_Samples, ShenandoahAdaptiveDecayFactor) { }
+  _available(Moving_Average_Samples, ShenandoahAdaptiveDecayFactor),
+  _free_set(nullptr),
+  _previous_acceleration_sample_timestamp(0.0),
+  _gc_time_first_sample_index(0),
+  _gc_time_num_samples(0),
+  _gc_time_timestamps(NEW_C_HEAP_ARRAY(double, GC_TIME_SAMPLE_SIZE, mtGC)),
+  _gc_time_samples(NEW_C_HEAP_ARRAY(double, GC_TIME_SAMPLE_SIZE, mtGC)),
+  _gc_time_xy(NEW_C_HEAP_ARRAY(double, GC_TIME_SAMPLE_SIZE, mtGC)),
+  _gc_time_xx(NEW_C_HEAP_ARRAY(double, GC_TIME_SAMPLE_SIZE, mtGC)),
+  _gc_time_sum_of_timestamps(0),
+  _gc_time_sum_of_samples(0),
+  _gc_time_sum_of_xy(0),
+  _gc_time_sum_of_xx(0),
+  _gc_time_m(0.0),
+  _gc_time_b(0.0),
+  _gc_time_sd(0.0),
+  _spike_acceleration_buffer_size(MAX2(ShenandoahRateAccelerationSampleSize, 1+ShenandoahMomentaryAllocationRateSpikeSampleSize)),
+  _spike_acceleration_first_sample_index(0),
+  _spike_acceleration_num_samples(0),
+  _spike_acceleration_rate_samples(NEW_C_HEAP_ARRAY(double, _spike_acceleration_buffer_size, mtGC)),
+  _spike_acceleration_rate_timestamps(NEW_C_HEAP_ARRAY(double, _spike_acceleration_buffer_size, mtGC)) {
+  }
 
-ShenandoahAdaptiveHeuristics::~ShenandoahAdaptiveHeuristics() {}
+ShenandoahAdaptiveHeuristics::~ShenandoahAdaptiveHeuristics() {
+  FREE_C_HEAP_ARRAY(double, _spike_acceleration_rate_samples);
+  FREE_C_HEAP_ARRAY(double, _spike_acceleration_rate_timestamps);
+  FREE_C_HEAP_ARRAY(double, _gc_time_timestamps);
+  FREE_C_HEAP_ARRAY(double, _gc_time_samples);
+  FREE_C_HEAP_ARRAY(double, _gc_time_xy);
+  FREE_C_HEAP_ARRAY(double, _gc_time_xx);
+}
+
+void ShenandoahAdaptiveHeuristics::initialize() {
+  ShenandoahHeuristics::initialize();
+}
+
+void ShenandoahAdaptiveHeuristics::post_initialize() {
+  ShenandoahHeuristics::post_initialize();
+  _free_set = ShenandoahHeap::heap()->free_set();
+  assert(!ShenandoahHeap::heap()->mode()->is_generational(), "ShenandoahGenerationalHeuristics overrides this method");
+  compute_headroom_adjustment();
+}
+
+void ShenandoahAdaptiveHeuristics::compute_headroom_adjustment() {
+  // The trigger threshold represents mutator available - "head room".
+  // We plan for GC to finish before the amount of allocated memory exceeds trigger threshold.  This is the same  as saying we
+  // intend to finish GC before the amount of available memory is less than the allocation headroom.  Headroom is the planned
+  // safety buffer to allow a small amount of additional allocation to take place in case we were overly optimistic in delaying
+  // our trigger.
+  size_t capacity = ShenandoahHeap::heap()->soft_max_capacity();
+  size_t spike_headroom = capacity / 100 * ShenandoahAllocSpikeFactor;
+  size_t penalties      = capacity / 100 * _gc_time_penalties;
+  _headroom_adjustment = spike_headroom + penalties;
+}
+
+void ShenandoahAdaptiveHeuristics::start_idle_span() {
+  compute_headroom_adjustment();
+}
+
+void ShenandoahAdaptiveHeuristics::adjust_penalty(intx step) {
+  ShenandoahHeuristics::adjust_penalty(step);
+}
 
 void ShenandoahAdaptiveHeuristics::choose_collection_set_from_regiondata(ShenandoahCollectionSet* cset,
                                                                          RegionData* data, size_t size,
@@ -76,8 +158,8 @@ void ShenandoahAdaptiveHeuristics::choose_collection_set_from_regiondata(Shenand
   // The logic for cset selection in adaptive is as follows:
   //
   //   1. We cannot get cset larger than available free space. Otherwise we guarantee OOME
-  //      during evacuation, and thus guarantee full GC. In practice, we also want to let
-  //      application to allocate something. This is why we limit CSet to some fraction of
+  //      during evacuation, and thus guarantee full GC. In practice, we also want to let the
+  //      application allocate during concurrent GC. This is why we limit CSet to some fraction of
   //      available space. In non-overloaded heap, max_cset would contain all plausible candidates
   //      over garbage threshold.
   //
@@ -108,6 +190,7 @@ void ShenandoahAdaptiveHeuristics::choose_collection_set_from_regiondata(Shenand
   size_t cur_cset = 0;
   size_t cur_garbage = 0;
 
+  // Regions are sorted in order of decreasing garbage
   for (size_t idx = 0; idx < size; idx++) {
     ShenandoahHeapRegion* r = data[idx].get_region();
 
@@ -126,6 +209,88 @@ void ShenandoahAdaptiveHeuristics::choose_collection_set_from_regiondata(Shenand
   }
 }
 
+void ShenandoahAdaptiveHeuristics::add_degenerated_gc_time(double timestamp, double gc_time) {
+  // Conservatively add sample into linear model If this time is above the predicted concurrent gc time
+  if (predict_gc_time(timestamp) < gc_time) {
+    add_gc_time(timestamp, gc_time);
+  }
+}
+
+void ShenandoahAdaptiveHeuristics::add_gc_time(double timestamp, double gc_time) {
+  // Update best-fit linear predictor of GC time
+  uint index = (_gc_time_first_sample_index + _gc_time_num_samples) % GC_TIME_SAMPLE_SIZE;
+  if (_gc_time_num_samples == GC_TIME_SAMPLE_SIZE) {
+    _gc_time_sum_of_timestamps -= _gc_time_timestamps[index];
+    _gc_time_sum_of_samples -= _gc_time_samples[index];
+    _gc_time_sum_of_xy -= _gc_time_xy[index];
+    _gc_time_sum_of_xx -= _gc_time_xx[index];
+  }
+  _gc_time_timestamps[index] = timestamp;
+  _gc_time_samples[index] = gc_time;
+  _gc_time_xy[index] = timestamp * gc_time;
+  _gc_time_xx[index] = timestamp * timestamp;
+
+  _gc_time_sum_of_timestamps += _gc_time_timestamps[index];
+  _gc_time_sum_of_samples += _gc_time_samples[index];
+  _gc_time_sum_of_xy += _gc_time_xy[index];
+  _gc_time_sum_of_xx += _gc_time_xx[index];
+
+  if (_gc_time_num_samples < GC_TIME_SAMPLE_SIZE) {
+    _gc_time_num_samples++;
+  } else {
+    _gc_time_first_sample_index = (_gc_time_first_sample_index + 1) % GC_TIME_SAMPLE_SIZE;
+  }
+
+  if (_gc_time_num_samples == 1) {
+    // The predictor is constant (horizontal line)
+    _gc_time_m = 0;
+    _gc_time_b = gc_time;
+    _gc_time_sd = 0.0;
+  } else if (_gc_time_num_samples == 2) {
+    // Two points define a line
+    double delta_y = gc_time - _gc_time_samples[_gc_time_first_sample_index];
+    double delta_x = timestamp - _gc_time_timestamps[_gc_time_first_sample_index];
+    _gc_time_m = delta_y / delta_x;
+
+    // y = mx + b
+    // so b = y0 - mx0
+    _gc_time_b = gc_time - _gc_time_m * timestamp;
+    _gc_time_sd = 0.0;
+  } else {
+    _gc_time_m = ((_gc_time_num_samples * _gc_time_sum_of_xy - _gc_time_sum_of_timestamps * _gc_time_sum_of_samples) /
+                  (_gc_time_num_samples * _gc_time_sum_of_xx - _gc_time_sum_of_timestamps * _gc_time_sum_of_timestamps));
+    _gc_time_b = (_gc_time_sum_of_samples - _gc_time_m * _gc_time_sum_of_timestamps) / _gc_time_num_samples;
+    double sum_of_squared_deviations = 0.0;
+    for (size_t i = 0; i < _gc_time_num_samples; i++) {
+      uint index = (_gc_time_first_sample_index + i) % GC_TIME_SAMPLE_SIZE;
+      double x = _gc_time_timestamps[index];
+      double predicted_y = _gc_time_m * x + _gc_time_b;
+      double deviation = predicted_y - _gc_time_samples[index];
+      sum_of_squared_deviations += deviation * deviation;
+    }
+    _gc_time_sd = sqrt(sum_of_squared_deviations / _gc_time_num_samples);
+  }
+}
+
+double ShenandoahAdaptiveHeuristics::predict_gc_time(double timestamp_at_start) {
+  return _gc_time_m * timestamp_at_start + _gc_time_b + _gc_time_sd * _margin_of_error_sd;;
+}
+
+void ShenandoahAdaptiveHeuristics::add_rate_to_acceleration_history(double timestamp, double rate) {
+  uint new_sample_index =
+    (_spike_acceleration_first_sample_index + _spike_acceleration_num_samples) % _spike_acceleration_buffer_size;
+  _spike_acceleration_rate_timestamps[new_sample_index] = timestamp;
+  _spike_acceleration_rate_samples[new_sample_index] = rate;
+  if (_spike_acceleration_num_samples == _spike_acceleration_buffer_size) {
+    _spike_acceleration_first_sample_index++;
+    if (_spike_acceleration_first_sample_index == _spike_acceleration_buffer_size) {
+      _spike_acceleration_first_sample_index = 0;
+    }
+  } else {
+    _spike_acceleration_num_samples++;
+  }
+}
+
 void ShenandoahAdaptiveHeuristics::record_cycle_start() {
   ShenandoahHeuristics::record_cycle_start();
   _allocation_rate.allocation_counter_reset();
@@ -133,6 +298,10 @@ void ShenandoahAdaptiveHeuristics::record_cycle_start() {
 
 void ShenandoahAdaptiveHeuristics::record_success_concurrent() {
   ShenandoahHeuristics::record_success_concurrent();
+  double now = os::elapsedTime();
+
+  // Should we not add GC time if this was an abbreviated cycle?
+  add_gc_time(_cycle_start, elapsed_cycle_time());
 
   size_t available = _space_info->available();
 
@@ -185,6 +354,7 @@ void ShenandoahAdaptiveHeuristics::record_success_concurrent() {
 
 void ShenandoahAdaptiveHeuristics::record_degenerated() {
   ShenandoahHeuristics::record_degenerated();
+  add_degenerated_gc_time(_precursor_cycle_start, elapsed_degenerated_cycle_time());
   // Adjust both trigger's parameters in the case of a degenerated GC because
   // either of them should have triggered earlier to avoid this case.
   adjust_margin_of_error(DEGENERATE_PENALTY_SD);
@@ -236,6 +406,24 @@ bool ShenandoahAdaptiveHeuristics::should_start_gc() {
   size_t available = _space_info->soft_mutator_available();
   size_t allocated = _space_info->bytes_allocated_since_gc_start();
 
+  double avg_cycle_time = 0;
+  double avg_alloc_rate = 0;
+  double now = get_most_recent_wake_time();
+  size_t allocatable_words = this->allocatable(available);
+  double predicted_future_accelerated_gc_time = 0.0;
+  size_t allocated_bytes_since_last_sample = 0;
+  double instantaneous_rate_words_per_second = 0.0;
+  size_t consumption_accelerated = 0;
+  double acceleration = 0.0;
+  double current_rate_by_acceleration = 0.0;
+  size_t min_threshold = min_free_threshold();
+  double predicted_future_gc_time = 0;
+  double future_planned_gc_time = 0;
+  bool future_planned_gc_time_is_average = false;
+  double avg_time_to_deplete_available = 0.0;
+  bool is_spiking = false;
+  double spike_time_to_deplete_available = 0.0;
+
   log_debug(gc, ergo)("should_start_gc calculation: available: " PROPERFMT ", soft_max_capacity: "  PROPERFMT ", "
                 "allocated_since_gc_start: "  PROPERFMT,
                 PROPERFMTARGS(available), PROPERFMTARGS(capacity), PROPERFMTARGS(allocated));
@@ -250,7 +438,6 @@ bool ShenandoahAdaptiveHeuristics::should_start_gc() {
 
   _last_trigger = OTHER;
 
-  size_t min_threshold = min_free_threshold();
   if (available < min_threshold) {
     log_trigger("Free (Soft) (" PROPERFMT ") is below minimum threshold (" PROPERFMT ")",
                  PROPERFMTARGS(available), PROPERFMTARGS(min_threshold));
@@ -271,55 +458,227 @@ bool ShenandoahAdaptiveHeuristics::should_start_gc() {
       return true;
     }
   }
-  // Check if allocation headroom is still okay. This also factors in:
-  //   1. Some space to absorb allocation spikes (ShenandoahAllocSpikeFactor)
-  //   2. Accumulated penalties from Degenerated and Full GC
-  size_t allocation_headroom = available;
 
-  size_t spike_headroom = capacity / 100 * ShenandoahAllocSpikeFactor;
-  size_t penalties      = capacity / 100 * _gc_time_penalties;
+  // The test (3 * allocated > available) below is intended to prevent triggers from firing so quickly that there
+  // has not been sufficient time to create garbage that can be reclaimed during the triggered GC cycle.  If we trigger before
+  // garbage has been created, the concurrent GC will find no garbage.  This has been observed to result in degens which
+  // experience OOM during evac or that experience "bad progress", both of which escalate to Full GC.  Note that garbage that
+  // was allocated following the start of the current GC cycle cannot be reclaimed in this GC cycle.  Here is the derivation
+  // of the expression:
+  //
+  // Let R (runway) represent the total amount of memory that can be allocated following the start of GC(N).  The runway
+  // represents memory available at the start of the current GC plus garbage reclaimed by the current GC. In a balanced,
+  // fully utilized configuration, we will be starting each new GC cycle immediately following completion of the preceding
+  // GC cycle.  In this configuration, we would expect half of R to be consumed during concurrent cycle GC(N) and half
+  // to be consumed during concurrent GC(N+1).
+  //
+  // Assume we want to delay GC trigger until:    A/V > 0.33
+  //     This is equivalent to enforcing that:      A > 0.33V
+  //                                 which is:     3A > V
+  //              Since A+V equals R, we have: A + 3A > A + V  = R
+  //                     which is to say that:      A > R/4
+  //
+  // Postponing the trigger until at least 1/4 of the runway has been consumed helps to improve the efficiency of the
+  // triggered GC.  Under heavy steady state workload, this delay condition generally has no effect: if the allocation
+  // runway is divided "equally" between the current GC and the next GC, then at any potential trigger point (which cannot
+  // happen any sooner than completion of the first GC), it is already the case that roughly A > R/2.
+  if (3 * allocated <= available) {
+    // Even though we will not issue an adaptive trigger unless a minimum threshold of memory has been allocated,
+    // we still allow more generic triggers, such as guaranteed GC intervals, to act.
+    return ShenandoahHeuristics::should_start_gc();
+  }
 
-  allocation_headroom -= MIN2(allocation_headroom, spike_headroom);
-  allocation_headroom -= MIN2(allocation_headroom, penalties);
+  avg_cycle_time = _gc_cycle_time_history->davg() + (_margin_of_error_sd * _gc_cycle_time_history->dsd());
+  avg_alloc_rate = _allocation_rate.upper_bound(_margin_of_error_sd);
+  if ((now - _previous_acceleration_sample_timestamp) >= (ShenandoahAccelerationSamplePeriod / 1000.0)) {
+    predicted_future_accelerated_gc_time =
+      predict_gc_time(now + MAX2(get_planned_sleep_interval(), ShenandoahAccelerationSamplePeriod / 1000.0));
+    double future_accelerated_planned_gc_time;
+    bool future_accelerated_planned_gc_time_is_average;
+    if (predicted_future_accelerated_gc_time > avg_cycle_time) {
+      future_accelerated_planned_gc_time = predicted_future_accelerated_gc_time;
+      future_accelerated_planned_gc_time_is_average = false;
+    } else {
+      future_accelerated_planned_gc_time = avg_cycle_time;
+      future_accelerated_planned_gc_time_is_average = true;
+    }
+    allocated_bytes_since_last_sample = _free_set->get_bytes_allocated_since_previous_sample();
+    instantaneous_rate_words_per_second =
+      (allocated_bytes_since_last_sample / HeapWordSize) / (now - _previous_acceleration_sample_timestamp);
 
-  double avg_cycle_time = _gc_cycle_time_history->davg() + (_margin_of_error_sd * _gc_cycle_time_history->dsd());
-  double avg_alloc_rate = _allocation_rate.upper_bound(_margin_of_error_sd);
+    _previous_acceleration_sample_timestamp = now;
+    add_rate_to_acceleration_history(now, instantaneous_rate_words_per_second);
+    current_rate_by_acceleration = instantaneous_rate_words_per_second;
+    consumption_accelerated =
+      accelerated_consumption(acceleration, current_rate_by_acceleration, avg_alloc_rate / HeapWordSize,
+                              (ShenandoahAccelerationSamplePeriod / 1000.0) + future_accelerated_planned_gc_time);
 
-  log_debug(gc)("average GC time: %.2f ms, allocation rate: %.0f %s/s",
-          avg_cycle_time * 1000, byte_size_in_proper_unit(avg_alloc_rate), proper_unit_for_byte_size(avg_alloc_rate));
-  if (avg_cycle_time * avg_alloc_rate > allocation_headroom) {
-    log_trigger("Average GC time (%.2f ms) is above the time for average allocation rate (%.0f %sB/s)"
-                 " to deplete free headroom (%zu%s) (margin of error = %.2f)",
-                 avg_cycle_time * 1000,
-                 byte_size_in_proper_unit(avg_alloc_rate), proper_unit_for_byte_size(avg_alloc_rate),
-                 byte_size_in_proper_unit(allocation_headroom), proper_unit_for_byte_size(allocation_headroom),
-                 _margin_of_error_sd);
-    log_info(gc, ergo)("Free headroom: %zu%s (free) - %zu%s (spike) - %zu%s (penalties) = %zu%s",
-                       byte_size_in_proper_unit(available),           proper_unit_for_byte_size(available),
-                       byte_size_in_proper_unit(spike_headroom),      proper_unit_for_byte_size(spike_headroom),
-                       byte_size_in_proper_unit(penalties),           proper_unit_for_byte_size(penalties),
-                       byte_size_in_proper_unit(allocation_headroom), proper_unit_for_byte_size(allocation_headroom));
+    // Note that even a single thread that wakes up and begins to allocate excessively can manifest as accelerating allocation
+    // rate. This thread will initially allocate a TLAB of minimum size.  Then it will allocate a TLAB twice as big a bit later,
+    // and then twice as big again after another short delay.  When a phase change causes many threads to increase their
+    // allocation behavior, this effect is multiplied, and compounded by jitter in the times that individual threads experience
+    // the phase change.
+    //
+    // The following trace represents an actual workload, with allocation rates sampled at 10 Hz, the default behavior before
+    // introduction of accelerated allocation rate detection.  Though the allocation rate is seen to be increasing at times
+    // 101.907 and 102.007 and 102.108, the newly sampled allocation rate is not enough to trigger GC because the headroom is
+    // still quite large.  In fact, GC is not triggered until time 102.409s, and this GC degenerates.
+    //
+    //    Sample Time (s)      Allocation Rate (MB/s)       Headroom (GB)
+    //       101.807                       0.0                  26.93
+    //                                                                  <--- accelerated spike can trigger here, around time 101.9s
+    //       101.907                     477.6                  26.85
+    //       102.007                   3,206.0                  26.35
+    //       102.108                  23,797.8                  24.19
+    //       102.208                  24,164.5                  21.83
+    //       102.309                  23,965.0                  19.47
+    //       102.409                  24,624.35                 17.05   <--- without accelerated rate detection, we trigger here
+    //
+    // Though the above measurements are from actual workload, the following details regarding sampled allocation rates at 3ms
+    // period were not measured directly for this run-time sample.  These are hypothetical, though they represent a plausible
+    // result that correlates with the actual measurements.
+    //
+    // For most of the 100 ms time span that precedes the sample at 101.907, the allocation rate still remains at zero.  The phase
+    // change that causes increasing allocations occurs near the end ot this time segment.  When sampled with a 3 ms period,
+    // acceration of allocation can be triggered at approximately time 101.88s.
+    //
+    // In the default configuration, accelerated allocation rate is detected by examining a sequence of 8 allocation rate samples.
+    //
+    // Even a single allocation rate sample above the norm can be interpreted as acceleration of allocation rate.  For example, the
+    // the best-fit line for the following samples has an acceleration rate of 3,553.3 MB/s/s.  This is not enough to trigger GC,
+    // especially given the abundance of Headroom at this moment in time.
+    //
+    //    TimeStamp (s)     Alloc rate (MB/s)
+    //    101.857                 0
+    //    101.860                 0
+    //    101.863                 0
+    //    101.866                 0
+    //    101.869                53.3
+    //
+    // At the next sample time, we will compute a slightly higher acceration, 9,150 MB/s/s.  This is also insufficient to trigger
+    // GC.
+    //
+    //    TimeStamp (s)     Alloc rate (MB/s)
+    //    101.860                 0
+    //    101.863                 0
+    //    101.866                 0
+    //    101.869                53.3
+    //    101.872               110.6
+    //
+    // Eventually, we will observe a full history of accelerating rate samples, computing acceleration of 18,500 MB/s/s.  This will
+    // trigger GC over 500 ms earlier than was previously possible.
+    //
+    //    TimeStamp (s)     Alloc rate (MB/s)
+    //    101.866                 0
+    //    101.869                53.3
+    //    101.872               110.6
+    //    101.875               165.9
+    //    101.878               221.2
+    //
+    // The accelerated rate heuristic is based on the following idea:
+    //
+    //    Assume allocation rate is accelerating at a constant rate.  If we postpone the spike trigger until the subsequent
+    //    sample point, will there be enough memory to satisfy allocations that occur during the anticipated concurrent GC
+    //    cycle?  If not, we should trigger right now.
+    //
+    // Outline of this heuristic triggering technique:
+    //
+    //  1. We remember the N (e.g. N=3) most recent samples of spike allocation rate r0, r1, r2 samples at t0, t1, and t2
+    //  2. if r1 < r0 or r2 < r1, approximate Acceleration = 0.0, Rate = Average(r0, r1, r2)
+    //  3. Otherwise, use least squares method to compute best-fit line of rate vs time
+    //  4. The slope of this line represents Acceleration. The y-intercept of this line represents "initial rate"
+    //  5. Use r2 to rrpresent CurrentRate
+    //  6. Use Consumption = CurrentRate * GCTime + 1/2 * Acceleration * GCTime * GCTime
+    //     (See High School physics discussions on constant acceleration: D = v0 * t + 1/2 * a * t^2)
+    //  7. if Consumption exceeds headroom, trigger now
+    //
+    // Though larger sample size may improve quality of predictor, it also delays trigger response.  Smaller sample sizes
+    // are more susceptible to false triggers based on random noise.  The default configuration uses a sample size of 8 and
+    // a sample period of roughly 15 ms, spanning approximately 120 ms of execution.
+    if (consumption_accelerated > allocatable_words) {
+      size_t size_t_alloc_rate = (size_t) current_rate_by_acceleration * HeapWordSize;
+      if (acceleration > 0) {
+        size_t size_t_acceleration = (size_t) acceleration * HeapWordSize;
+        log_trigger("Accelerated consumption (" PROPERFMT ") exceeds free headroom (" PROPERFMT ") at "
+                    "current rate (" PROPERFMT "/s) with acceleration (" PROPERFMT "/s/s) for planned %s GC time (%.2f ms)",
+                    PROPERFMTARGS(consumption_accelerated * HeapWordSize),
+                    PROPERFMTARGS(allocatable_words * HeapWordSize),
+                    PROPERFMTARGS(size_t_alloc_rate),
+                    PROPERFMTARGS(size_t_acceleration),
+                    future_accelerated_planned_gc_time_is_average? "(from average)": "(by linear prediction)",
+                    future_accelerated_planned_gc_time * 1000);
+      } else {
+        log_trigger("Momentary spike consumption (" PROPERFMT ") exceeds free headroom (" PROPERFMT ") at "
+                    "current rate (" PROPERFMT "/s) for planned %s GC time (%.2f ms) (spike threshold = %.2f)",
+                    PROPERFMTARGS(consumption_accelerated * HeapWordSize),
+                    PROPERFMTARGS(allocatable_words * HeapWordSize),
+                    PROPERFMTARGS(size_t_alloc_rate),
+                    future_accelerated_planned_gc_time_is_average? "(from average)": "(by linear prediction)",
+                    future_accelerated_planned_gc_time * 1000, _spike_threshold_sd);
+
+
+      }
+      _spike_acceleration_num_samples = 0;
+      _spike_acceleration_first_sample_index = 0;
+
+      // Count this as a form of RATE trigger for purposes of adjusting heuristic triggering configuration because this
+      // trigger is influenced more by margin_of_error_sd than by spike_threshold_sd.
+      accept_trigger_with_type(RATE);
+      return true;
+    }
+  }
+
+  // Suppose we don't trigger now, but decide to trigger in the next regulator cycle.  What will be the GC time then?
+  predicted_future_gc_time = predict_gc_time(now + get_planned_sleep_interval());
+  if (predicted_future_gc_time > avg_cycle_time) {
+    future_planned_gc_time = predicted_future_gc_time;
+    future_planned_gc_time_is_average = false;
+  } else {
+    future_planned_gc_time = avg_cycle_time;
+    future_planned_gc_time_is_average = true;
+  }
+
+  log_debug(gc)("%s: average GC time: %.2f ms, predicted GC time: %.2f ms, allocation rate: %.0f %s/s",
+                _space_info->name(), avg_cycle_time * 1000, predicted_future_gc_time * 1000,
+                byte_size_in_proper_unit(avg_alloc_rate), proper_unit_for_byte_size(avg_alloc_rate));
+  size_t allocatable_bytes = allocatable_words * HeapWordSize;
+  avg_time_to_deplete_available = allocatable_bytes / avg_alloc_rate;
+
+  if (future_planned_gc_time > avg_time_to_deplete_available) {
+    log_trigger("%s GC time (%.2f ms) is above the time for average allocation rate (%.0f %sB/s)"
+                " to deplete free headroom (%zu%s) (margin of error = %.2f)",
+                future_planned_gc_time_is_average? "Average": "Linear prediction of", future_planned_gc_time * 1000,
+                byte_size_in_proper_unit(avg_alloc_rate),    proper_unit_for_byte_size(avg_alloc_rate),
+                byte_size_in_proper_unit(allocatable_bytes), proper_unit_for_byte_size(allocatable_bytes),
+                _margin_of_error_sd);
+
+    size_t spike_headroom = capacity / 100 * ShenandoahAllocSpikeFactor;
+    size_t penalties      = capacity / 100 * _gc_time_penalties;
+    size_t allocation_headroom = available;
+    allocation_headroom -= MIN2(allocation_headroom, spike_headroom);
+    allocation_headroom -= MIN2(allocation_headroom, penalties);
+    log_info(gc, ergo)("Free headroom: " PROPERFMT " (free) - " PROPERFMT "(spike) - " PROPERFMT " (penalties) = " PROPERFMT,
+                       PROPERFMTARGS(available),
+                       PROPERFMTARGS(spike_headroom),
+                       PROPERFMTARGS(penalties),
+                       PROPERFMTARGS(allocation_headroom));
     accept_trigger_with_type(RATE);
     return true;
   }
 
-  bool is_spiking = _allocation_rate.is_spiking(rate, _spike_threshold_sd);
-  if (is_spiking && avg_cycle_time > allocation_headroom / rate) {
-    log_trigger("Average GC time (%.2f ms) is above the time for instantaneous allocation rate (%.0f %sB/s) to deplete free headroom (%zu%s) (spike threshold = %.2f)",
-                 avg_cycle_time * 1000,
-                 byte_size_in_proper_unit(rate), proper_unit_for_byte_size(rate),
-                 byte_size_in_proper_unit(allocation_headroom), proper_unit_for_byte_size(allocation_headroom),
-                 _spike_threshold_sd);
+  is_spiking = _allocation_rate.is_spiking(rate, _spike_threshold_sd);
+  spike_time_to_deplete_available = (rate == 0)? 0: allocatable_bytes / rate;
+  if (is_spiking && (rate != 0) && (future_planned_gc_time > spike_time_to_deplete_available)) {
+    log_trigger("%s GC time (%.2f ms) is above the time for instantaneous allocation rate (%.0f %sB/s)"
+                " to deplete free headroom (%zu%s) (spike threshold = %.2f)",
+                future_planned_gc_time_is_average? "Average": "Linear prediction of", future_planned_gc_time * 1000,
+                byte_size_in_proper_unit(rate),        proper_unit_for_byte_size(rate),
+                byte_size_in_proper_unit(allocatable_bytes), proper_unit_for_byte_size(allocatable_bytes),
+                _spike_threshold_sd);
     accept_trigger_with_type(SPIKE);
     return true;
   }
-
-  if (ShenandoahHeuristics::should_start_gc()) {
-    _start_gc_is_pending = true;
-    return true;
-  } else {
-    return false;
-  }
+  return ShenandoahHeuristics::should_start_gc();
 }
 
 void ShenandoahAdaptiveHeuristics::adjust_last_trigger_parameters(double amount) {
@@ -352,6 +711,112 @@ size_t ShenandoahAdaptiveHeuristics::min_free_threshold() {
   return ShenandoahHeap::heap()->soft_max_capacity() / 100 * ShenandoahMinFreeThreshold;
 }
 
+// This is called each time a new rate sample has been gathered, as governed by ShenandoahAccelerationSamplePeriod.
+// Unlike traditional calculation of average allocation rate, there is no adjustment for standard deviation of the
+// accelerated rate prediction.
+size_t ShenandoahAdaptiveHeuristics::accelerated_consumption(double& acceleration, double& current_rate,
+                                                             double avg_alloc_rate_words_per_second,
+                                                             double predicted_cycle_time) const
+{
+  double *x_array = (double *) alloca(ShenandoahRateAccelerationSampleSize * sizeof(double));
+  double *y_array = (double *) alloca(ShenandoahRateAccelerationSampleSize * sizeof(double));
+  double x_sum = 0.0;
+  double y_sum = 0.0;
+
+  assert(_spike_acceleration_num_samples > 0, "At minimum, we should have sample from this period");
+
+  double weighted_average_alloc;
+  if (_spike_acceleration_num_samples >= ShenandoahRateAccelerationSampleSize) {
+    double weighted_y_sum = 0;
+    double total_weight = 0;
+    double previous_x = 0;
+    uint delta = _spike_acceleration_num_samples - ShenandoahRateAccelerationSampleSize;
+    for (uint i = 0; i < ShenandoahRateAccelerationSampleSize; i++) {
+      uint index = (_spike_acceleration_first_sample_index + delta + i) % _spike_acceleration_buffer_size;
+      x_array[i] = _spike_acceleration_rate_timestamps[index];
+      x_sum += x_array[i];
+      y_array[i] = _spike_acceleration_rate_samples[index];
+      if (i > 0) {
+        // first sample not included in weighted average because it has no weight.
+        double sample_weight = x_array[i] - x_array[i-1];
+        weighted_y_sum = y_array[i] * sample_weight;
+        total_weight += sample_weight;
+      }
+      y_sum += y_array[i];
+    }
+    weighted_average_alloc = (total_weight > 0)? weighted_y_sum / total_weight: 0;
+  } else {
+    weighted_average_alloc = 0;
+  }
+
+  double momentary_rate;
+  if (_spike_acceleration_num_samples > ShenandoahMomentaryAllocationRateSpikeSampleSize) {
+    // Num samples must be strictly greater than sample size, because we need one extra sample to compute rate and weights
+    // In this context, the weight of a y value (an allocation rate) is the duration for which this allocation rate was
+    // active (the time since previous y value was reported).  An allocation rate measured over a span of 300 ms (e.g. during
+    // concurrent GC) has much more "weight" than an allocation rate measured over a span of 15 s.
+    double weighted_y_sum = 0;
+    double total_weight = 0;
+    double sum_for_average = 0.0;
+    uint delta = _spike_acceleration_num_samples - ShenandoahMomentaryAllocationRateSpikeSampleSize;
+    for (uint i = 0; i < ShenandoahMomentaryAllocationRateSpikeSampleSize; i++) {
+      uint sample_index = (_spike_acceleration_first_sample_index + delta + i) % _spike_acceleration_buffer_size;
+      uint preceding_index = (sample_index == 0)? _spike_acceleration_buffer_size - 1: sample_index - 1;
+      double sample_weight = (_spike_acceleration_rate_timestamps[sample_index]
+                              - _spike_acceleration_rate_timestamps[preceding_index]);
+      weighted_y_sum += _spike_acceleration_rate_samples[sample_index] * sample_weight;
+      total_weight += sample_weight;
+    }
+    momentary_rate = weighted_y_sum / total_weight;
+    bool is_spiking = _allocation_rate.is_spiking(momentary_rate, _spike_threshold_sd);
+    if (!is_spiking) {
+      // Disable momentary spike trigger unless allocation rate delta from average exceeds sd
+      momentary_rate = 0.0;
+    }
+  } else {
+    momentary_rate = 0.0;
+  }
+
+  // By default, use momentary_rate for current rate and zero acceleration. Overwrite iff best-fit line has positive slope.
+  current_rate = momentary_rate;
+  acceleration = 0.0;
+  if ((_spike_acceleration_num_samples >= ShenandoahRateAccelerationSampleSize)
+      && (weighted_average_alloc >= avg_alloc_rate_words_per_second))  {
+    // If the average rate across the acceleration samples is below the overall average, this sample is not eligible to
+    //  represent acceleration of allocation rate.  We may just be catching up with allocations after a lull.
+
+    double *xy_array = (double *) alloca(ShenandoahRateAccelerationSampleSize * sizeof(double));
+    double *x2_array = (double *) alloca(ShenandoahRateAccelerationSampleSize * sizeof(double));
+    double xy_sum = 0.0;
+    double x2_sum = 0.0;
+    for (uint i = 0; i < ShenandoahRateAccelerationSampleSize; i++) {
+      xy_array[i] = x_array[i] * y_array[i];
+      xy_sum += xy_array[i];
+      x2_array[i] = x_array[i] * x_array[i];
+      x2_sum += x2_array[i];
+    }
+    // Find the best-fit least-squares linear representation of rate vs time
+    double m;                 /* slope */
+    double b;                 /* y-intercept */
+
+    m = ((ShenandoahRateAccelerationSampleSize * xy_sum - x_sum * y_sum)
+         / (ShenandoahRateAccelerationSampleSize * x2_sum - x_sum * x_sum));
+    b = (y_sum - m * x_sum) / ShenandoahRateAccelerationSampleSize;
+
+    if (m > 0) {
+      double proposed_current_rate = m * x_array[ShenandoahRateAccelerationSampleSize - 1] + b;
+      acceleration = m;
+      current_rate = proposed_current_rate;
+    }
+    // else, leave current_rate = momentary_rate, acceleration = 0
+  }
+  // and here also, leave current_rate = momentary_rate, acceleration = 0
+
+  double time_delta = get_planned_sleep_interval() + predicted_cycle_time;
+  size_t words_to_be_consumed = (size_t) (current_rate * time_delta + 0.5 * acceleration * time_delta * time_delta);
+  return words_to_be_consumed;
+}
+
 ShenandoahAllocationRate::ShenandoahAllocationRate() :
   _last_sample_time(os::elapsedTime()),
   _last_sample_value(0),
@@ -363,7 +828,7 @@ ShenandoahAllocationRate::ShenandoahAllocationRate() :
 double ShenandoahAllocationRate::force_sample(size_t allocated, size_t &unaccounted_bytes_allocated) {
   const double MinSampleTime = 0.002;    // Do not sample if time since last update is less than 2 ms
   double now = os::elapsedTime();
-  double time_since_last_update = now -_last_sample_time;
+  double time_since_last_update = now - _last_sample_time;
   if (time_since_last_update < MinSampleTime) {
     unaccounted_bytes_allocated = allocated - _last_sample_value;
     _last_sample_value = 0;
@@ -412,8 +877,10 @@ bool ShenandoahAllocationRate::is_spiking(double rate, double threshold) const {
 
   double sd = _rate.sd();
   if (sd > 0) {
-    // There is a small chance that that rate has already been sampled, but it
-    // seems not to matter in practice.
+    // There is a small chance that that rate has already been sampled, but it seems not to matter in practice.
+    // Note that z_score reports how close the rate is to the average.  A value between -1 and 1 means we are within one
+    // standard deviation.  A value between -3 and +3 means we are within 3 standard deviations.  We only check for z_score
+    // greater than threshold because we are looking for an allocation spike which is greater than the mean.
     double z_score = (rate - _rate.avg()) / sd;
     if (z_score > threshold) {
       return true;
diff --git a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.hpp b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.hpp
index 9b7824a50d7..c761f2a82f3 100644
--- a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.hpp
+++ b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.hpp
@@ -27,7 +27,9 @@
 #define SHARE_GC_SHENANDOAH_HEURISTICS_SHENANDOAHADAPTIVEHEURISTICS_HPP
 
 #include "gc/shenandoah/heuristics/shenandoahHeuristics.hpp"
+#include "gc/shenandoah/shenandoahFreeSet.hpp"
 #include "gc/shenandoah/shenandoahPhaseTimings.hpp"
+#include "gc/shenandoah/shenandoahRegulatorThread.hpp"
 #include "gc/shenandoah/shenandoahSharedVariables.hpp"
 #include "memory/allocation.hpp"
 #include "utilities/numberSeq.hpp"
@@ -108,6 +110,26 @@ public:
 
   virtual ~ShenandoahAdaptiveHeuristics();
 
+  virtual void initialize() override;
+
+  virtual void post_initialize() override;
+
+  virtual void adjust_penalty(intx step) override;
+
+  // At the end of GC(N), we idle GC until necessary to start the next GC.  Compute the threshold of memory that can be allocated
+  // before we need to start the next GC.
+  void start_idle_span() override;
+
+  // Having observed a new allocation rate sample, add this to the acceleration history so that we can determine if allocation
+  // rate is accelerating.
+  void add_rate_to_acceleration_history(double timestamp, double rate);
+
+  // Compute and return the current allocation rate, the current rate of acceleration, and the amount of memory that we expect
+  // to consume if we start GC right now and gc takes predicted_cycle_time to complete.
+  size_t accelerated_consumption(double& acceleration, double& current_rate,
+                                 double avg_rate_words_per_sec, double predicted_cycle_time) const;
+
+
   void choose_collection_set_from_regiondata(ShenandoahCollectionSet* cset,
                                                        RegionData* data, size_t size,
                                                        size_t actual_free) override;
@@ -136,6 +158,8 @@ public:
   const static double LOWEST_EXPECTED_AVAILABLE_AT_END;
   const static double HIGHEST_EXPECTED_AVAILABLE_AT_END;
 
+  const static size_t GC_TIME_SAMPLE_SIZE;
+
   friend class ShenandoahAllocationRate;
 
   // Used to record the last trigger that signaled to start a GC.
@@ -150,9 +174,19 @@ public:
   void adjust_margin_of_error(double amount);
   void adjust_spike_threshold(double amount);
 
+  // Returns number of words that can be allocated before we need to trigger next GC, given available in bytes.
+  inline size_t allocatable(size_t available) const {
+    return (available > _headroom_adjustment)? (available - _headroom_adjustment) / HeapWordSize: 0;
+  }
+
 protected:
   ShenandoahAllocationRate _allocation_rate;
 
+  // Invocations of should_start_gc() happen approximately once per ms.  Queries of allocation rate only happen if a
+  // a certain amount of time has passed since the previous query.
+  size_t _allocated_at_previous_query;
+  double _time_of_previous_allocation_query;
+
   // The margin of error expressed in standard deviations to add to our
   // average cycle time and allocation rate. As this value increases we
   // tend to overestimate the rate at which mutators will deplete the
@@ -179,6 +213,48 @@ protected:
   // source of feedback to adjust trigger parameters.
   TruncatedSeq _available;
 
+  ShenandoahFreeSet* _free_set;
+
+  // This represents the time at which the allocation rate was most recently sampled for the purpose of detecting acceleration.
+  double _previous_acceleration_sample_timestamp;
+  size_t _total_allocations_at_start_of_idle;
+
+  // bytes of headroom at which we should trigger GC
+  size_t _headroom_adjustment;
+
+  // Keep track of GC_TIME_SAMPLE_SIZE most recent concurrent GC cycle times
+  uint _gc_time_first_sample_index;
+  uint _gc_time_num_samples;
+  double* const _gc_time_timestamps;
+  double* const _gc_time_samples;
+  double* const _gc_time_xy;    // timestamp * sample
+  double* const _gc_time_xx;    // timestamp squared
+  double _gc_time_sum_of_timestamps;
+  double _gc_time_sum_of_samples;
+  double _gc_time_sum_of_xy;
+  double _gc_time_sum_of_xx;
+
+  double _gc_time_m;            // slope
+  double _gc_time_b;            // y-intercept
+  double _gc_time_sd;           // sd on deviance from prediction
+
+  // In preparation for a span during which GC will be idle, compute the headroom adjustment that will be used to
+  // detect when GC needs to trigger.
+  void compute_headroom_adjustment() override;
+
+  void add_gc_time(double timestamp_at_start, double duration);
+  void add_degenerated_gc_time(double timestamp_at_start, double duration);
+  double predict_gc_time(double timestamp_at_start);
+
+  // Keep track of SPIKE_ACCELERATION_SAMPLE_SIZE most recent spike allocation rate measurements. Note that it is
+  // typical to experience a small spike following end of GC cycle, as mutator threads refresh their TLABs.  But
+  // there is generally an abundance of memory at this time as well, so this will not generally trigger GC.
+  uint _spike_acceleration_buffer_size;
+  uint _spike_acceleration_first_sample_index;
+  uint _spike_acceleration_num_samples;
+  double* const _spike_acceleration_rate_samples; // holds rates in words/second
+  double* const _spike_acceleration_rate_timestamps;
+
   // A conservative minimum threshold of free space that we'll try to maintain when possible.
   // For example, we might trigger a concurrent gc if we are likely to drop below
   // this threshold, or we might consider this when dynamically resizing generations
diff --git a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahGenerationalHeuristics.cpp b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahGenerationalHeuristics.cpp
index 029b917deab..f3d31b8d0e1 100644
--- a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahGenerationalHeuristics.cpp
+++ b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahGenerationalHeuristics.cpp
@@ -52,6 +52,12 @@ static int compare_by_aged_live(AgedRegionData a, AgedRegionData b) {
   return 0;
 }
 
+void ShenandoahGenerationalHeuristics::post_initialize() {
+  ShenandoahHeuristics::post_initialize();
+  _free_set = ShenandoahHeap::heap()->free_set();
+  compute_headroom_adjustment();
+}
+
 inline void assert_no_in_place_promotions() {
 #ifdef ASSERT
   class ShenandoahNoInPlacePromotions : public ShenandoahHeapRegionClosure {
diff --git a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahGenerationalHeuristics.hpp b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahGenerationalHeuristics.hpp
index 74d657feab7..da883d0d26f 100644
--- a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahGenerationalHeuristics.hpp
+++ b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahGenerationalHeuristics.hpp
@@ -49,6 +49,8 @@ public:
 
   void choose_collection_set(ShenandoahCollectionSet* collection_set) override;
 
+  virtual void post_initialize() override;
+
 private:
   // Compute evacuation budgets prior to choosing collection set.
   void compute_evacuation_budgets(ShenandoahHeap* const heap);
diff --git a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.cpp b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.cpp
index 8fc744112bf..603e00c401d 100644
--- a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.cpp
+++ b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.cpp
@@ -46,13 +46,16 @@ int ShenandoahHeuristics::compare_by_garbage(RegionData a, RegionData b) {
 }
 
 ShenandoahHeuristics::ShenandoahHeuristics(ShenandoahSpaceInfo* space_info) :
+  _most_recent_trigger_evaluation_time(os::elapsedTime()),
+  _most_recent_planned_sleep_interval(0.0),
   _start_gc_is_pending(false),
   _declined_trigger_count(0),
   _most_recent_declined_trigger_count(0),
   _space_info(space_info),
   _region_data(nullptr),
   _guaranteed_gc_interval(0),
-  _cycle_start(os::elapsedTime()),
+  _precursor_cycle_start(os::elapsedTime()),
+  _cycle_start(_precursor_cycle_start),
   _last_cycle_end(0),
   _gc_times_learned(0),
   _gc_time_penalties(0),
@@ -156,6 +159,19 @@ void ShenandoahHeuristics::choose_collection_set(ShenandoahCollectionSet* collec
   collection_set->summarize(total_garbage, immediate_garbage, immediate_regions);
 }
 
+void ShenandoahHeuristics::start_idle_span() {
+  // do nothing
+}
+
+void ShenandoahHeuristics::record_degenerated_cycle_start(bool out_of_cycle) {
+  if (out_of_cycle) {
+    _precursor_cycle_start = _cycle_start = os::elapsedTime();
+  } else {
+    _precursor_cycle_start = _cycle_start;
+    _cycle_start = os::elapsedTime();
+  }
+}
+
 void ShenandoahHeuristics::record_cycle_start() {
   _cycle_start = os::elapsedTime();
 }
@@ -197,7 +213,6 @@ bool ShenandoahHeuristics::should_degenerate_cycle() {
 void ShenandoahHeuristics::adjust_penalty(intx step) {
   assert(0 <= _gc_time_penalties && _gc_time_penalties <= 100,
          "In range before adjustment: %zd", _gc_time_penalties);
-
   if ((_most_recent_declined_trigger_count <= Penalty_Free_Declinations) && (step > 0)) {
     // Don't penalize if heuristics are not responsible for a negative outcome.  Allow Penalty_Free_Declinations following
     // previous GC for self calibration without penalty.
@@ -274,6 +289,17 @@ void ShenandoahHeuristics::initialize() {
   // Nothing to do by default.
 }
 
+void ShenandoahHeuristics::post_initialize() {
+  // Nothing to do by default.
+}
+
 double ShenandoahHeuristics::elapsed_cycle_time() const {
   return os::elapsedTime() - _cycle_start;
 }
+
+
+// Includes the time spent in abandoned concurrent GC cycle that may have triggered this degenerated cycle.
+double ShenandoahHeuristics::elapsed_degenerated_cycle_time() const {
+  double now = os::elapsedTime();
+  return now - _precursor_cycle_start;
+}
diff --git a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.hpp b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.hpp
index 633c4e87126..5bfba4f52d5 100644
--- a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.hpp
+++ b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.hpp
@@ -78,6 +78,10 @@ class ShenandoahHeuristics : public CHeapObj<mtGC> {
   };
 #endif
 
+private:
+  double _most_recent_trigger_evaluation_time;
+  double _most_recent_planned_sleep_interval;
+
 protected:
   static const uint Moving_Average_Samples = 10; // Number of samples to store in moving averages
 
@@ -85,14 +89,13 @@ protected:
   size_t _declined_trigger_count;         // This counts how many times since previous GC finished that this
                                           //  heuristic has answered false to should_start_gc().
   size_t _most_recent_declined_trigger_count;
-                                       ;  // This represents the value of _declined_trigger_count as captured at the
+                                          // This represents the value of _declined_trigger_count as captured at the
                                           //  moment the most recent GC effort was triggered.  In case the most recent
                                           //  concurrent GC effort degenerates, the value of this variable allows us to
                                           //  differentiate between degeneration because heuristic was overly optimistic
                                           //  in delaying the trigger vs. degeneration for other reasons (such as the
                                           //  most recent GC triggered "immediately" after previous GC finished, but the
                                           //  free headroom has already been depleted).
-
   class RegionData {
     private:
     ShenandoahHeapRegion* _region;
@@ -103,6 +106,7 @@ protected:
 #ifdef ASSERT
     UnionTag _union_tag;
 #endif
+
     public:
 
     inline void clear() {
@@ -171,6 +175,7 @@ protected:
 
   size_t _guaranteed_gc_interval;
 
+  double _precursor_cycle_start;
   double _cycle_start;
   double _last_cycle_end;
 
@@ -188,7 +193,7 @@ protected:
                                                      RegionData* data, size_t data_size,
                                                      size_t free) = 0;
 
-  void adjust_penalty(intx step);
+  virtual void adjust_penalty(intx step);
 
   inline void accept_trigger() {
     _most_recent_declined_trigger_count = _declined_trigger_count;
@@ -200,6 +205,14 @@ protected:
     _declined_trigger_count++;
   }
 
+  inline double get_most_recent_wake_time() const {
+    return _most_recent_trigger_evaluation_time;
+  }
+
+  inline double get_planned_sleep_interval() const {
+    return _most_recent_planned_sleep_interval;
+  }
+
 public:
   ShenandoahHeuristics(ShenandoahSpaceInfo* space_info);
   virtual ~ShenandoahHeuristics();
@@ -212,10 +225,22 @@ public:
     _guaranteed_gc_interval = guaranteed_gc_interval;
   }
 
+  virtual void start_idle_span();
+  virtual void compute_headroom_adjustment() {
+    // Default implementation does nothing.
+  }
+
   virtual void record_cycle_start();
 
+  void record_degenerated_cycle_start(bool out_of_cycle);
+
   virtual void record_cycle_end();
 
+  void update_should_start_query_times(double now, double planned_sleep_interval) {
+    _most_recent_trigger_evaluation_time = now;
+    _most_recent_planned_sleep_interval = planned_sleep_interval;
+  }
+
   virtual bool should_start_gc();
 
   inline void cancel_trigger_request() {
@@ -248,8 +273,10 @@ public:
   virtual bool is_diagnostic() = 0;
   virtual bool is_experimental() = 0;
   virtual void initialize();
+  virtual void post_initialize();
 
   double elapsed_cycle_time() const;
+  double elapsed_degenerated_cycle_time() const;
 
   virtual size_t force_alloc_rate_sample(size_t bytes_allocated) {
     // do nothing
diff --git a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahYoungHeuristics.cpp b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahYoungHeuristics.cpp
index beff2200d90..09508a1163f 100644
--- a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahYoungHeuristics.cpp
+++ b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahYoungHeuristics.cpp
@@ -137,6 +137,7 @@ bool ShenandoahYoungHeuristics::should_start_gc() {
 
   // inherited triggers have already decided to start a cycle, so no further evaluation is required
   if (ShenandoahAdaptiveHeuristics::should_start_gc()) {
+    // ShenandoahAdaptiveHeuristics::should_start_gc() has already accepted trigger, or declined it.
     return true;
   }
 
@@ -178,7 +179,7 @@ size_t ShenandoahYoungHeuristics::bytes_of_allocation_runway_before_gc_trigger(s
   size_t capacity = _space_info->max_capacity();
   size_t usage = _space_info->used();
   size_t available = (capacity > usage)? capacity - usage: 0;
-  size_t allocated = _space_info->bytes_allocated_since_gc_start();
+  size_t allocated = _free_set->get_bytes_allocated_since_gc_start();
 
   size_t available_young_collected = ShenandoahHeap::heap()->collection_set()->get_young_available_bytes_collected();
   size_t anticipated_available =
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahConcurrentGC.cpp b/src/hotspot/share/gc/shenandoah/shenandoahConcurrentGC.cpp
index 5206a0558e8..f0125c38cae 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahConcurrentGC.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahConcurrentGC.cpp
@@ -1215,6 +1215,7 @@ void ShenandoahConcurrentGC::op_final_update_refs() {
   }
 
   heap->rebuild_free_set(true /*concurrent*/);
+  _generation->heuristics()->start_idle_span();
 
   {
     ShenandoahTimingsTracker timing(ShenandoahPhaseTimings::final_update_refs_propagate_gc_state);
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp b/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp
index bc11659c5e5..c5607421265 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp
@@ -59,6 +59,7 @@ void ShenandoahControlThread::run_service() {
 
   ShenandoahCollectorPolicy* const policy = heap->shenandoah_policy();
   ShenandoahHeuristics* const heuristics = heap->heuristics();
+  double most_recent_wake_time = os::elapsedTime();
   while (!should_terminate()) {
     const GCCause::Cause cancelled_cause = heap->cancelled_cause();
     if (cancelled_cause == GCCause::_shenandoah_stop_vm) {
@@ -222,16 +223,26 @@ void ShenandoahControlThread::run_service() {
     // Wait before performing the next action. If allocation happened during this wait,
     // we exit sooner, to let heuristics re-evaluate new conditions. If we are at idle,
     // back off exponentially.
-    const double current = os::elapsedTime();
+    const double before_sleep = most_recent_wake_time;
     if (heap->has_changed()) {
       sleep = ShenandoahControlIntervalMin;
-    } else if ((current - last_sleep_adjust_time) * 1000 > ShenandoahControlIntervalAdjustPeriod){
+    } else if ((before_sleep - last_sleep_adjust_time) * 1000 > ShenandoahControlIntervalAdjustPeriod){
       sleep = MIN2<int>(ShenandoahControlIntervalMax, MAX2(1, sleep * 2));
-      last_sleep_adjust_time = current;
+      last_sleep_adjust_time = before_sleep;
     }
-
     MonitorLocker ml(&_control_lock, Mutex::_no_safepoint_check_flag);
     ml.wait(sleep);
+    // Record a conservative estimate of the longest anticipated sleep duration until we sample again.
+    double planned_sleep_interval = MIN2<int>(ShenandoahControlIntervalMax, MAX2(1, sleep * 2)) / 1000.0;
+    most_recent_wake_time = os::elapsedTime();
+    heuristics->update_should_start_query_times(most_recent_wake_time, planned_sleep_interval);
+    if (LogTarget(Debug, gc, thread)::is_enabled()) {
+      double elapsed = most_recent_wake_time - before_sleep;
+      double hiccup = elapsed - double(sleep);
+      if (hiccup > 0.001) {
+        log_debug(gc, thread)("Control Thread hiccup time: %.3fs", hiccup);
+      }
+    }
   }
 }
 
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp b/src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp
index 99776e38bfe..8cd8a390c4a 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp
@@ -314,6 +314,7 @@ void ShenandoahDegenGC::op_degenerated() {
   if (progress) {
     heap->notify_gc_progress();
     _generation->heuristics()->record_degenerated();
+    heap->start_idle_span();
   } else if (policy->should_upgrade_degenerated_gc()) {
     // Upgrade to full GC, register full-GC impact on heuristics.
     op_degenerated_futile();
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp b/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp
index 961800f20d9..c39e2e7bb79 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp
@@ -287,9 +287,25 @@ void ShenandoahFreeSet::resize_old_collector_capacity(size_t regions) {
   // else, old generation is already appropriately sized
 }
 
+
 void ShenandoahFreeSet::reset_bytes_allocated_since_gc_start(size_t initial_bytes_allocated) {
   shenandoah_assert_heaplocked();
+  // Future inquiries of get_total_bytes_allocated() will return the sum of
+  //    _total_bytes_previously_allocated and _mutator_bytes_allocated_since_gc_start.
+  // Since _mutator_bytes_allocated_since_gc_start does not start at zero, we subtract initial_bytes_allocated so as
+  // to not double count these allocated bytes.
+  size_t original_mutator_bytes_allocated_since_gc_start = _mutator_bytes_allocated_since_gc_start;
+
+  // Setting _mutator_bytes_allocated_since_gc_start before _total_bytes_previously_allocated reduces the damage
+  // in the case that the control or regulator thread queries get_bytes_allocated_since_previous_sample() between
+  // the two assignments.
+  //
+  // These are not declared as volatile so the compiler or hardware may reorder the assignments.  The implementation of
+  // get_bytes_allocated_since_previous_cycle() is robust to this possibility, as are triggering heuristics.  The current
+  // implementation assumes we are better off to tolerate the very rare race rather than impose a synchronization penalty
+  // on every update and fetch.  (Perhaps it would be better to make the opposite tradeoff for improved maintainability.)
   _mutator_bytes_allocated_since_gc_start = initial_bytes_allocated;
+  _total_bytes_previously_allocated += original_mutator_bytes_allocated_since_gc_start - initial_bytes_allocated;
 }
 
 void ShenandoahFreeSet::increase_bytes_allocated(size_t bytes) {
@@ -1211,6 +1227,8 @@ inline void ShenandoahRegionPartitions::assert_bounds_sanity() {
 ShenandoahFreeSet::ShenandoahFreeSet(ShenandoahHeap* heap, size_t max_regions) :
   _heap(heap),
   _partitions(max_regions, this),
+  _total_bytes_previously_allocated(0),
+  _mutator_bytes_at_last_sample(0),
   _total_humongous_waste(0),
   _alloc_bias_weight(0),
   _total_young_used(0),
@@ -1676,9 +1694,6 @@ HeapWord* ShenandoahFreeSet::try_allocate_in(ShenandoahHeapRegion* r, Shenandoah
     // Regardless of whether this allocation succeeded, if the remaining memory is less than PLAB:min_size(), retire this region.
     // Note that retire_from_partition() increases used to account for waste.
 
-    // Also, if this allocation request failed and the consumed within this region * ShenandoahEvacWaste > region size,
-    // then retire the region so that subsequent searches can find available memory more quickly.
-
     size_t idx = r->index();
     size_t waste_bytes = _partitions.retire_from_partition(orig_partition, idx, r->used());
     DEBUG_ONLY(boundary_changed = true;)
@@ -1796,7 +1811,6 @@ HeapWord* ShenandoahFreeSet::allocate_contiguous(ShenandoahAllocRequest& req, bo
       // found the match
       break;
     }
-
     end++;
   }
 
@@ -2036,7 +2050,8 @@ void ShenandoahFreeSet::clear_internal() {
   _partitions.set_bias_from_left_to_right(ShenandoahFreeSetPartitionId::OldCollector, false);
 }
 
-void ShenandoahFreeSet::find_regions_with_alloc_capacity(size_t &young_trashed_regions, size_t &old_trashed_regions,
+// Returns total allocatable words in Mutator partition
+size_t ShenandoahFreeSet::find_regions_with_alloc_capacity(size_t &young_trashed_regions, size_t &old_trashed_regions,
                                                          size_t &first_old_region, size_t &last_old_region,
                                                          size_t &old_region_count) {
   // This resets all state information, removing all regions from all sets.
@@ -2054,6 +2069,8 @@ void ShenandoahFreeSet::find_regions_with_alloc_capacity(size_t &young_trashed_r
   size_t region_size_bytes = _partitions.region_size_bytes();
   size_t max_regions = _partitions.max();
 
+  size_t mutator_alloc_capacity_in_words = 0;
+
   size_t mutator_leftmost = max_regions;
   size_t mutator_rightmost = 0;
   size_t mutator_leftmost_empty = max_regions;
@@ -2123,6 +2140,7 @@ void ShenandoahFreeSet::find_regions_with_alloc_capacity(size_t &young_trashed_r
         if (region->is_trash() || !region->is_old()) {
           // Both young and old (possibly immediately) collected regions (trashed) are placed into the Mutator set
           _partitions.raw_assign_membership(idx, ShenandoahFreeSetPartitionId::Mutator);
+          mutator_alloc_capacity_in_words += ac / HeapWordSize;
           if (idx < mutator_leftmost) {
             mutator_leftmost = idx;
           }
@@ -2279,6 +2297,7 @@ void ShenandoahFreeSet::find_regions_with_alloc_capacity(size_t &young_trashed_r
                       _partitions.rightmost(ShenandoahFreeSetPartitionId::Mutator),
                       _partitions.leftmost(ShenandoahFreeSetPartitionId::OldCollector),
                       _partitions.rightmost(ShenandoahFreeSetPartitionId::OldCollector));
+  return mutator_alloc_capacity_in_words;
 }
 
 void ShenandoahFreeSet::transfer_humongous_regions_from_mutator_to_old_collector(size_t xfer_regions,
@@ -2583,19 +2602,20 @@ void ShenandoahFreeSet::prepare_to_rebuild(size_t &young_trashed_regions, size_t
   clear();
   log_debug(gc, free)("Rebuilding FreeSet");
 
-  // This places regions that have alloc_capacity into the old_collector set if they identify as is_old() or the
-  // mutator set otherwise.  All trashed (cset) regions are affiliated young and placed in mutator set.
-  find_regions_with_alloc_capacity(young_trashed_regions, old_trashed_regions,
-                                   first_old_region, last_old_region, old_region_count);
+  // Place regions that have alloc_capacity into the old_collector set if they identify as is_old() or the
+  // mutator set otherwise.  All trashed (cset) regions are affiliated young and placed in mutator set.  Save the
+  // allocatable words in mutator partition in state variable.
+  _prepare_to_rebuild_mutator_free = find_regions_with_alloc_capacity(young_trashed_regions, old_trashed_regions,
+                                                                      first_old_region, last_old_region, old_region_count);
 }
 
-
-void ShenandoahFreeSet::finish_rebuild(size_t young_cset_regions, size_t old_cset_regions, size_t old_region_count) {
+// Return mutator free
+void ShenandoahFreeSet::finish_rebuild(size_t young_trashed_regions, size_t old_trashed_regions, size_t old_region_count) {
   shenandoah_assert_heaplocked();
   size_t young_reserve(0), old_reserve(0);
 
   if (_heap->mode()->is_generational()) {
-    compute_young_and_old_reserves(young_cset_regions, old_cset_regions, young_reserve, old_reserve);
+    compute_young_and_old_reserves(young_trashed_regions, old_trashed_regions, young_reserve, old_reserve);
   } else {
     young_reserve = (_heap->max_capacity() / 100) * ShenandoahEvacReserve;
     old_reserve = 0;
@@ -2744,10 +2764,13 @@ void ShenandoahFreeSet::compute_young_and_old_reserves(size_t young_trashed_regi
 // into the collector set or old collector set in order to assure that the memory available for allocations within
 // the collector set is at least to_reserve and the memory available for allocations within the old collector set
 // is at least to_reserve_old.
-void ShenandoahFreeSet::reserve_regions(size_t to_reserve, size_t to_reserve_old, size_t &old_region_count,
-                                        size_t &young_used_regions, size_t &old_used_regions,
-                                        size_t &young_used_bytes, size_t &old_used_bytes) {
+//
+// Returns total mutator alloc capacity, in words.
+size_t ShenandoahFreeSet::reserve_regions(size_t to_reserve, size_t to_reserve_old, size_t &old_region_count,
+                                          size_t &young_used_regions, size_t &old_used_regions,
+                                          size_t &young_used_bytes, size_t &old_used_bytes) {
   const size_t region_size_bytes = ShenandoahHeapRegion::region_size_bytes();
+  size_t mutator_allocatable_words = _prepare_to_rebuild_mutator_free;
 
   young_used_regions = 0;
   old_used_regions = 0;
@@ -2825,6 +2848,8 @@ void ShenandoahFreeSet::reserve_regions(size_t to_reserve, size_t to_reserve_old
                               _partitions.leftmost(ShenandoahFreeSetPartitionId::OldCollector),
                               _partitions.rightmost(ShenandoahFreeSetPartitionId::OldCollector));
           old_region_count++;
+          assert(ac = ShenandoahHeapRegion::region_size_bytes(), "Cannot move to old unless entire region is in alloc capacity");
+          mutator_allocatable_words -= ShenandoahHeapRegion::region_size_words();
           continue;
         }
       }
@@ -2868,8 +2893,10 @@ void ShenandoahFreeSet::reserve_regions(size_t to_reserve, size_t to_reserve_old
                             "  Collector range [%zd, %zd]",
                             _partitions.leftmost(ShenandoahFreeSetPartitionId::Mutator),
                             _partitions.rightmost(ShenandoahFreeSetPartitionId::Mutator),
-                            _partitions.leftmost(ShenandoahFreeSetPartitionId::Collector),
-                            _partitions.rightmost(ShenandoahFreeSetPartitionId::Collector));
+                            _partitions.leftmost(ShenandoahFreeSetPartitionId::OldCollector),
+                            _partitions.rightmost(ShenandoahFreeSetPartitionId::OldCollector));
+
+        mutator_allocatable_words -= ac / HeapWordSize;
         continue;
       }
 
@@ -2977,6 +3004,7 @@ void ShenandoahFreeSet::reserve_regions(size_t to_reserve, size_t to_reserve_old
                          PROPERFMTARGS(to_reserve), PROPERFMTARGS(reserve));
     }
   }
+  return mutator_allocatable_words;
 }
 
 void ShenandoahFreeSet::establish_old_collector_alloc_bias() {
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp b/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp
index d55a06d5713..2df06432bd2 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp
@@ -437,6 +437,12 @@ private:
   ShenandoahHeap* const _heap;
   ShenandoahRegionPartitions _partitions;
 
+  size_t _total_bytes_previously_allocated;
+  size_t _mutator_bytes_at_last_sample;
+
+  // Temporarily holds mutator_Free allocatable bytes between prepare_to_rebuild() and finish_rebuild()
+  size_t _prepare_to_rebuild_mutator_free;
+
   // This locks the rebuild process (in combination with the global heap lock).  Whenever we rebuild the free set,
   // we first acquire the global heap lock and then we acquire this _rebuild_lock in a nested context.  Threads that
   // need to check available, acquire only the _rebuild_lock to make sure that they are not obtaining the value of
@@ -446,10 +452,10 @@ private:
   // locks will acquire them in the same order: first the global heap lock and then the rebuild lock.
   ShenandoahRebuildLock _rebuild_lock;
 
-  size_t _total_humongous_waste;
-
   HeapWord* allocate_aligned_plab(size_t size, ShenandoahAllocRequest& req, ShenandoahHeapRegion* r);
 
+  size_t _total_humongous_waste;
+
   // We re-evaluate the left-to-right allocation bias whenever _alloc_bias_weight is less than zero.  Each time
   // we allocate an object, we decrement the count of this value.  Each time we re-evaluate whether to allocate
   // from right-to-left or left-to-right, we reset the value of this counter to _InitialAllocBiasWeight.
@@ -662,10 +668,47 @@ public:
 
   void increase_bytes_allocated(size_t bytes);
 
+  // Return an approximation of the bytes allocated since GC start.  The value returned is monotonically non-decreasing
+  // in time within each GC cycle.  For certain GC cycles, the value returned may include some bytes allocated before
+  // the start of the current GC cycle.
   inline size_t get_bytes_allocated_since_gc_start() const {
     return _mutator_bytes_allocated_since_gc_start;
   }
 
+  inline size_t get_total_bytes_allocated() {
+    return  _mutator_bytes_allocated_since_gc_start + _total_bytes_previously_allocated;
+  }
+
+  inline size_t get_bytes_allocated_since_previous_sample() {
+    size_t total_bytes = get_total_bytes_allocated();
+    size_t result;
+    if (total_bytes < _mutator_bytes_at_last_sample) {
+      // This rare condition may occur if bytes allocated overflows (wraps around) size_t tally of allocations.
+      // This may also occur in the very rare situation that get_total_bytes_allocated() is queried in the middle of
+      // reset_bytes_allocated_since_gc_start().  Note that there is no lock to assure that the two global variables
+      // it modifies are modified atomically (_total_bytes_previously_allocated and _mutator_byts_allocated_since_gc_start)
+      // This has been observed to occur when an out-of-cycle degenerated cycle is starting (and thus calls
+      // reset_bytes_allocated_since_gc_start()) at the same time that the control (non-generational mode) or
+      // regulator (generational-mode) thread calls should_start_gc() (which invokes get_bytes_allocated_since_previous_sample()).
+      //
+      // Handle this rare situation by responding with the "innocent" value 0 and resetting internal state so that the
+      // the next query can recalibrate.
+      result = 0;
+    } else {
+      // Note: there's always the possibility that the tally of total allocations exceeds the 64-bit capacity of our size_t
+      // counter.  We assume that the difference between relevant samples does not exceed this count.  Example:
+      //   Suppose _mutator_words_at_last_sample is 0xffff_ffff_ffff_fff0 (18,446,744,073,709,551,600 Decimal)
+      //                        and _total_words is 0x0000_0000_0000_0800 (                    32,768 Decimal)
+      // Then, total_words - _mutator_words_at_last_sample can be done adding 1's complement of subtrahend:
+      //   1's complement of _mutator_words_at_last_sample is: 0x0000_0000_0000_0010 (    16 Decimal))
+      //                                     plus total_words: 0x0000_0000_0000_0800 (32,768 Decimal)
+      //                                                  sum: 0x0000_0000_0000_0810 (32,784 Decimal)
+      result = total_bytes - _mutator_bytes_at_last_sample;
+    }
+    _mutator_bytes_at_last_sample = total_bytes;
+    return result;
+  }
+
   // Public because ShenandoahRegionPartitions assertions require access.
   inline size_t alloc_capacity(ShenandoahHeapRegion *r) const;
   inline size_t alloc_capacity(size_t idx) const;
@@ -781,15 +824,15 @@ public:
   // Acquire heap lock and log status, assuming heap lock is not acquired by the caller.
   void log_status_under_lock();
 
-  // Note that capacity is the number of regions that had available memory at most recent rebuild.  It is not the
-  // entire size of the young or global generation.  (Regions within the generation that were fully utilized at time of
-  // rebuild are not counted as part of capacity.)
-
-  // All three of the following functions may produce stale data if called without owning the global heap lock.
+  // All four of the following functions may produce stale data if called without owning the global heap lock.
   // Changes to the values of these variables are performed with a lock.  A change to capacity or used "atomically"
   // adjusts available with respect to lock holders.  However, sequential calls to these three functions may produce
   // inconsistent data: available may not equal capacity - used because the intermediate states of any "atomic"
   // locked action can be seen by these unlocked functions.
+
+  // Note that capacity is the number of regions that had available memory at most recent rebuild.  It is not the
+  // entire size of the young or global generation.  (Regions within the generation that were fully utilized at time of
+  // rebuild are not counted as part of capacity.)
   inline size_t capacity_holding_lock() const {
     shenandoah_assert_heaplocked();
     return _partitions.capacity_of(ShenandoahFreeSetPartitionId::Mutator);
@@ -808,11 +851,14 @@ public:
     ShenandoahRebuildLocker locker(rebuild_lock());
     return _partitions.used_by(ShenandoahFreeSetPartitionId::Mutator);
   }
+  inline size_t reserved()  const { return _partitions.capacity_of(ShenandoahFreeSetPartitionId::Collector);           }
   inline size_t available() {
     shenandoah_assert_not_heaplocked();
     ShenandoahRebuildLocker locker(rebuild_lock());
     return _partitions.available_in_locked_for_rebuild(ShenandoahFreeSetPartitionId::Mutator);
   }
+  inline size_t available_holding_lock() const
+                                  { return _partitions.available_in(ShenandoahFreeSetPartitionId::Mutator); }
 
   // Use this version of available() if the heap lock is held.
   inline size_t available_locked() const {
@@ -880,13 +926,17 @@ public:
   //   first_old_region is the index of the first region that is part of the OldCollector set
   //    last_old_region is the index of the last region that is part of the OldCollector set
   //   old_region_count is the number of regions in the OldCollector set that have memory available to be allocated
-  void find_regions_with_alloc_capacity(size_t &young_cset_regions, size_t &old_cset_regions,
-                                        size_t &first_old_region, size_t &last_old_region, size_t &old_region_count);
+  //
+  // Returns allocatable memory within Mutator partition, in words.
+  size_t find_regions_with_alloc_capacity(size_t &young_cset_regions, size_t &old_cset_regions,
+                                          size_t &first_old_region, size_t &last_old_region, size_t &old_region_count);
 
   // Ensure that Collector has at least to_reserve bytes of available memory, and OldCollector has at least old_reserve
   // bytes of available memory.  On input, old_region_count holds the number of regions already present in the
   // OldCollector partition.  Upon return, old_region_count holds the updated number of regions in the OldCollector partition.
-  void reserve_regions(size_t to_reserve, size_t old_reserve, size_t &old_region_count,
+  //
+  // Returns allocatable memory within Mutator partition, in words.
+  size_t reserve_regions(size_t to_reserve, size_t old_reserve, size_t &old_region_count,
                        size_t &young_used_regions, size_t &old_used_regions, size_t &young_used_bytes, size_t &old_used_bytes);
 
   // Reserve space for evacuations, with regions reserved for old evacuations placed to the right
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahFullGC.cpp b/src/hotspot/share/gc/shenandoah/shenandoahFullGC.cpp
index 3c92750cc0c..750f7e9122d 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahFullGC.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahFullGC.cpp
@@ -252,6 +252,7 @@ void ShenandoahFullGC::do_it(GCCause::Cause gc_cause) {
 
     phase5_epilog();
   }
+  heap->start_idle_span();
 
   // Resize metaspace
   MetaspaceGC::compute_new_size();
@@ -1124,8 +1125,9 @@ void ShenandoahFullGC::phase5_epilog() {
       if (heap->mode()->is_generational()) {
         ShenandoahGenerationalFullGC::compute_balances();
       }
-      free_set->finish_rebuild(young_trashed_regions, old_trashed_regions, num_old);
+      heap->free_set()->finish_rebuild(young_trashed_regions, old_trashed_regions, num_old);
     }
+
     // Set mark incomplete because the marking bitmaps have been reset except pinned regions.
     _generation->set_mark_incomplete();
 
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahGeneration.cpp b/src/hotspot/share/gc/shenandoah/shenandoahGeneration.cpp
index ddb50ee0020..b2d5e5423dd 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahGeneration.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahGeneration.cpp
@@ -151,6 +151,10 @@ ShenandoahHeuristics* ShenandoahGeneration::initialize_heuristics(ShenandoahMode
   return _heuristics;
 }
 
+void ShenandoahGeneration::post_initialize_heuristics() {
+  _heuristics->post_initialize();
+}
+
 void ShenandoahGeneration::set_evacuation_reserve(size_t new_val) {
   shenandoah_assert_heaplocked();
   _evacuation_reserve = new_val;
@@ -358,8 +362,7 @@ void ShenandoahGeneration::cancel_marking() {
   set_concurrent_mark_in_progress(false);
 }
 
-ShenandoahGeneration::ShenandoahGeneration(ShenandoahGenerationType type,
-                                           uint max_workers) :
+ShenandoahGeneration::ShenandoahGeneration(ShenandoahGenerationType type, uint max_workers) :
   _type(type),
   _task_queues(new ShenandoahObjToScanQueueSet(max_workers)),
   _ref_processor(new ShenandoahReferenceProcessor(this, MAX2(max_workers, 1U))),
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahGeneration.hpp b/src/hotspot/share/gc/shenandoah/shenandoahGeneration.hpp
index 946f2b91520..1a549be8988 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahGeneration.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahGeneration.hpp
@@ -83,10 +83,10 @@ private:
   ShenandoahReferenceProcessor* ref_processor() { return _ref_processor; }
 
   virtual ShenandoahHeuristics* initialize_heuristics(ShenandoahMode* gc_mode);
+  virtual void post_initialize_heuristics();
 
   virtual void post_initialize(ShenandoahHeap* heap);
 
-  virtual size_t bytes_allocated_since_gc_start() const override = 0;
   virtual size_t used() const override = 0;
   virtual size_t used_regions() const = 0;
   virtual size_t used_regions_size() const = 0;
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp b/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp
index 3b57190cc75..cc7547b8ac1 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp
@@ -622,10 +622,11 @@ void ShenandoahGenerationalControlThread::service_stw_full_cycle(GCCause::Cause
 
 void ShenandoahGenerationalControlThread::service_stw_degenerated_cycle(const ShenandoahGCRequest& request) {
   assert(_degen_point != ShenandoahGC::_degenerated_unset, "Degenerated point should be set");
+  request.generation->heuristics()->record_degenerated_cycle_start(ShenandoahGC::ShenandoahDegenPoint::_degenerated_outside_cycle
+                                                                  == _degen_point);
   _heap->increment_total_collections(false);
 
   ShenandoahGCSession session(request.cause, request.generation);
-
   ShenandoahDegenGC gc(_degen_point, request.generation);
   gc.collect(request.cause);
   _degen_point = ShenandoahGC::_degenerated_unset;
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.cpp b/src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.cpp
index 2c2e5533c01..b302bde8510 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.cpp
@@ -90,11 +90,23 @@ ShenandoahGenerationalHeap::ShenandoahGenerationalHeap(ShenandoahCollectorPolicy
   assert(is_aligned(_max_plab_size, CardTable::card_size_in_words()), "max_plab_size must be aligned");
 }
 
+void ShenandoahGenerationalHeap::initialize_generations() {
+  ShenandoahHeap::initialize_generations();
+  _young_generation->post_initialize(this);
+  _old_generation->post_initialize(this);
+}
+
 void ShenandoahGenerationalHeap::post_initialize() {
   ShenandoahHeap::post_initialize();
   _age_census = new ShenandoahAgeCensus();
 }
 
+void ShenandoahGenerationalHeap::post_initialize_heuristics() {
+  ShenandoahHeap::post_initialize_heuristics();
+  _young_generation->post_initialize_heuristics();
+  _old_generation->post_initialize_heuristics();
+}
+
 void ShenandoahGenerationalHeap::print_init_logger() const {
   ShenandoahGenerationalInitLogger logger;
   logger.print_all();
@@ -110,12 +122,6 @@ void ShenandoahGenerationalHeap::initialize_heuristics() {
   _old_generation->initialize_heuristics(mode());
 }
 
-void ShenandoahGenerationalHeap::post_initialize_heuristics() {
-  ShenandoahHeap::post_initialize_heuristics();
-  _young_generation->post_initialize(this);
-  _old_generation->post_initialize(this);
-}
-
 void ShenandoahGenerationalHeap::initialize_serviceability() {
   assert(mode()->is_generational(), "Only for the generational mode");
   _young_gen_memory_pool = new ShenandoahYoungGenMemoryPool(this);
@@ -152,6 +158,10 @@ void ShenandoahGenerationalHeap::stop() {
   regulator_thread()->stop();
 }
 
+void ShenandoahGenerationalHeap::start_idle_span() {
+  young_generation()->heuristics()->start_idle_span();
+}
+
 bool ShenandoahGenerationalHeap::requires_barriers(stackChunkOop obj) const {
   if (is_idle()) {
     return false;
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.hpp b/src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.hpp
index 719bae52a83..7fe0362aa3f 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.hpp
@@ -40,6 +40,7 @@ class ShenandoahGenerationalHeap : public ShenandoahHeap {
 public:
   explicit ShenandoahGenerationalHeap(ShenandoahCollectorPolicy* policy);
   void post_initialize() override;
+  void initialize_generations() override;
   void initialize_heuristics() override;
   void post_initialize_heuristics() override;
 
@@ -82,6 +83,8 @@ public:
 
   inline bool is_tenurable(const ShenandoahHeapRegion* r) const;
 
+  void start_idle_span() override;
+
   // Ages regions that haven't been used for allocations in the current cycle.
   // Resets ages for regions that have been used for allocations.
   void update_region_ages(ShenandoahMarkingContext* ctx);
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp b/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp
index d78bdae6a51..c6889351161 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp
@@ -435,7 +435,7 @@ jint ShenandoahHeap::initialize() {
     }
 
     _free_set = new ShenandoahFreeSet(this, _num_regions);
-    post_initialize_heuristics();
+    initialize_generations();
 
     // We are initializing free set.  We ignore cset region tallies.
     size_t young_trashed_regions, old_trashed_regions, first_old, last_old, num_old;
@@ -492,16 +492,17 @@ jint ShenandoahHeap::initialize() {
   _phase_timings = new ShenandoahPhaseTimings(max_workers());
   ShenandoahCodeRoots::initialize();
 
+  // Initialization of controller makes use of variables established by initialize_heuristics.
   initialize_controller();
 
+  // Certain initialization of heuristics must be deferred until after controller is initialized.
+  post_initialize_heuristics();
+  start_idle_span();
   if (ShenandoahUncommit) {
     _uncommit_thread = new ShenandoahUncommitThread(this);
   }
-
   print_init_logger();
-
   FullGCForwarding::initialize(_heap_region);
-
   return JNI_OK;
 }
 
@@ -545,10 +546,6 @@ void ShenandoahHeap::initialize_heuristics() {
   _global_generation->initialize_heuristics(mode());
 }
 
-void ShenandoahHeap::post_initialize_heuristics() {
-  _global_generation->post_initialize(this);
-}
-
 #ifdef _MSC_VER
 #pragma warning( push )
 #pragma warning( disable:4355 ) // 'this' : used in base member initializer list
@@ -690,6 +687,11 @@ public:
   }
 };
 
+void ShenandoahHeap::initialize_generations() {
+  _global_generation->post_initialize(this);
+}
+
+// We do not call this explicitly  It is called by Hotspot infrastructure.
 void ShenandoahHeap::post_initialize() {
   CollectedHeap::post_initialize();
 
@@ -717,6 +719,10 @@ void ShenandoahHeap::post_initialize() {
   JFR_ONLY(ShenandoahJFRSupport::register_jfr_type_serializers();)
 }
 
+void ShenandoahHeap::post_initialize_heuristics() {
+  _global_generation->post_initialize_heuristics();
+}
+
 ShenandoahHeuristics* ShenandoahHeap::heuristics() {
   return _global_generation->heuristics();
 }
@@ -760,6 +766,7 @@ void ShenandoahHeap::set_soft_max_capacity(size_t v) {
          "Should be in bounds: %zu <= %zu <= %zu",
          min_capacity(), v, max_capacity());
   _soft_max_size.store_relaxed(v);
+  heuristics()->compute_headroom_adjustment();
 }
 
 size_t ShenandoahHeap::min_capacity() const {
@@ -835,6 +842,10 @@ void ShenandoahHeap::notify_heap_changed() {
   _heap_changed.try_set();
 }
 
+void ShenandoahHeap::start_idle_span() {
+  heuristics()->start_idle_span();
+}
+
 void ShenandoahHeap::set_forced_counters_update(bool value) {
   monitoring_support()->set_forced_counters_update(value);
 }
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp b/src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp
index 85ad339469d..d4604be0aec 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp
@@ -195,6 +195,7 @@ public:
   ShenandoahHeap(ShenandoahCollectorPolicy* policy);
   jint initialize() override;
   void post_initialize() override;
+  virtual void initialize_generations();
   void initialize_mode();
   virtual void initialize_heuristics();
   virtual void post_initialize_heuristics();
@@ -393,6 +394,8 @@ public:
     return _heap_changed.try_unset();
   }
 
+  virtual void start_idle_span();
+
   void set_concurrent_young_mark_in_progress(bool in_progress);
   void set_concurrent_old_mark_in_progress(bool in_progress);
   void set_evacuation_in_progress(bool in_progress);
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahOldGeneration.cpp b/src/hotspot/share/gc/shenandoah/shenandoahOldGeneration.cpp
index 4fda65b4030..1b12909bcaf 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahOldGeneration.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahOldGeneration.cpp
@@ -422,6 +422,7 @@ void ShenandoahOldGeneration::prepare_regions_and_collection_set(bool concurrent
     // At the end of old-gen, we may find that we have reclaimed immediate garbage, allowing a longer allocation runway.
     // We may also find that we have accumulated canddiate regions for mixed evacuation.  If so, we will want to expand
     // the OldCollector reserve in order to make room for these mixed evacuations.
+
     assert(ShenandoahHeap::heap()->mode()->is_generational(), "sanity");
     assert(young_trash_regions == 0, "sanity");
     ShenandoahGenerationalHeap* gen_heap = ShenandoahGenerationalHeap::heap();
@@ -765,6 +766,7 @@ size_t ShenandoahOldGeneration::used_regions_size() const {
   return used_regions * ShenandoahHeapRegion::region_size_bytes();
 }
 
+// For the old generation, max_capacity() equals soft_max_capacity()
 size_t ShenandoahOldGeneration::max_capacity() const {
   size_t total_regions = _free_set->total_old_regions();
   return total_regions * ShenandoahHeapRegion::region_size_bytes();
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahRegulatorThread.cpp b/src/hotspot/share/gc/shenandoah/shenandoahRegulatorThread.cpp
index ec4b7c7217c..fe92a3a3e08 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahRegulatorThread.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahRegulatorThread.cpp
@@ -36,7 +36,8 @@ ShenandoahRegulatorThread::ShenandoahRegulatorThread(ShenandoahGenerationalContr
   _heap(ShenandoahHeap::heap()),
   _control_thread(control_thread),
   _sleep(ShenandoahControlIntervalMin),
-  _last_sleep_adjust_time(os::elapsedTime()) {
+  _most_recent_wake_time(os::elapsedTime()),
+  _last_sleep_adjust_time(_most_recent_wake_time) {
   shenandoah_assert_generational();
   _old_heuristics = _heap->old_generation()->heuristics();
   _young_heuristics = _heap->young_generation()->heuristics();
@@ -115,19 +116,22 @@ void ShenandoahRegulatorThread::regulator_sleep() {
   // Wait before performing the next action. If allocation happened during this wait,
   // we exit sooner, to let heuristics re-evaluate new conditions. If we are at idle,
   // back off exponentially.
-  double current = os::elapsedTime();
-
+  double before_sleep_time = _most_recent_wake_time;
   if (ShenandoahHeap::heap()->has_changed()) {
     _sleep = ShenandoahControlIntervalMin;
-  } else if ((current - _last_sleep_adjust_time) * 1000 > ShenandoahControlIntervalAdjustPeriod){
+  } else if ((before_sleep_time - _last_sleep_adjust_time) * 1000 > ShenandoahControlIntervalAdjustPeriod){
     _sleep = MIN2<uint>(ShenandoahControlIntervalMax, MAX2(1u, _sleep * 2));
-    _last_sleep_adjust_time = current;
+    _last_sleep_adjust_time = before_sleep_time;
   }
 
   SuspendibleThreadSetLeaver leaver;
   os::naked_short_sleep(_sleep);
+  double wake_time = os::elapsedTime();
+  _most_recent_period = wake_time - _most_recent_wake_time;
+  _most_recent_wake_time = wake_time;
+  _young_heuristics->update_should_start_query_times(_most_recent_wake_time, double(_sleep) / 1000.0);
   if (LogTarget(Debug, gc, thread)::is_enabled()) {
-    double elapsed = os::elapsedTime() - current;
+    double elapsed = _most_recent_wake_time - before_sleep_time;
     double hiccup = elapsed - double(_sleep);
     if (hiccup > 0.001) {
       log_debug(gc, thread)("Regulator hiccup time: %.3fs", hiccup);
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahRegulatorThread.hpp b/src/hotspot/share/gc/shenandoah/shenandoahRegulatorThread.hpp
index 2519025b6fb..cc41bc2c65b 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahRegulatorThread.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahRegulatorThread.hpp
@@ -79,7 +79,10 @@ class ShenandoahRegulatorThread: public ConcurrentGCThread {
   ShenandoahOldHeuristics* _old_heuristics;
   ShenandoahHeuristics* _global_heuristics;
 
+  // duration of planned regulator sleep period, in ms
   uint _sleep;
+  double _most_recent_wake_time;
+  double _most_recent_period;
   double _last_sleep_adjust_time;
 };
 
diff --git a/src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp b/src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp
index 3eb1a06a911..d3e9a1f9fae 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp
@@ -34,6 +34,59 @@
                             range,                                          \
                             constraint)                                     \
                                                                             \
+  product(uint, ShenandoahAccelerationSamplePeriod, 15, EXPERIMENTAL,       \
+          "When at least this much time (measured in ms) has passed "       \
+          "since the acceleration allocation rate was most recently "       \
+          "sampled, capture another allocation rate sample for the purpose "\
+          "of detecting acceleration or momentary spikes in allocation "    \
+          "rate. A smaller value allows quicker response to changes in "    \
+          "allocation rates but is more vulnerable to noise and requires "  \
+          "more monitoring effort.")                                        \
+          range(1, 1000)                                                    \
+                                                                            \
+  product(uint, ShenandoahRateAccelerationSampleSize, 8, EXPERIMENTAL,      \
+          "In selected ShenandoahControlIntervals "                         \
+          "(if ShenandoahAccelerationSamplePeriod ms have passed "          \
+          "since previous allocation rate sample), "                        \
+          "we compute the allocation rate since the previous rate was "     \
+          "sampled.  This many samples are analyzed to determine whether "  \
+          "allocation rates are accelerating.  Acceleration may occur "     \
+          "due to increasing client demand or due to phase changes in "     \
+          "an application.  A larger value reduces sensitivity to "         \
+          "noise and delays recognition of the accelerating trend.  A "     \
+          "larger value may also cause the heuristic to miss detection "    \
+          "of very quick accelerations.  Smaller values may cause random "  \
+          "noise to be perceived as acceleration of allocation rate, "      \
+          "triggering excess collections.  Note that the acceleration "     \
+          "need not last the entire span of the sampled duration to be "    \
+          "detected.  If the last several of all samples are signficantly " \
+          "larger than the other samples, the best fit line through all "   \
+          "sampled values will have an upward slope, manifesting as "       \
+          "acceleration.")                                                  \
+          range(1,64)                                                       \
+                                                                            \
+  product(uint, ShenandoahMomentaryAllocationRateSpikeSampleSize,           \
+          2, EXPERIMENTAL,                                                  \
+          "In selected ShenandoahControlIntervals "                         \
+          "(if ShenandoahAccelerationSamplePeriod ms have passed "          \
+          "since previous allocation rate sample), we compute "             \
+          "the allocation rate since the previous rate was sampled. "       \
+          "The weighted average of this "                                   \
+          "many most recent momentary allocation rate samples is compared " \
+          "against current allocation runway and anticipated GC time to "   \
+          "determine whether a spike in momentary allocation rate "         \
+          "justifies an early GC trigger.  Momentary allocation spike "     \
+          "detection is in addition to previously implemented "             \
+          "ShenandoahAdaptiveInitialSpikeThreshold, the latter of which "   \
+          "is more effective at detecting slower spikes.  The latter "      \
+          "spike detection samples at the rate specifieid by "              \
+          "ShenandoahAdaptiveSampleFrequencyHz.  The value of this "        \
+          "parameter must be less than the value of "                       \
+          "ShenandoahRateAccelerationSampleSize.  A larger value makes "    \
+          "momentary spike detection less sensitive.  A smaller value "     \
+          "may result in excessive GC triggers.")                           \
+          range(1,64)                                                       \
+                                                                            \
   product(uintx, ShenandoahGenerationalMinPIPUsage, 30, EXPERIMENTAL,       \
           "(Generational mode only) What percent of a heap region "         \
           "should be used before we consider promoting a region in "        \