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8367708: GenShen: Reduce total evacuation burden

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Reviewed-by: wkemper
This commit is contained in:
Kelvin Nilsen 2025-09-18 16:06:59 +00:00
parent b8f2c7387f
commit c597384ad6
4 changed files with 28 additions and 8 deletions
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2
src/hotspot/share/gc/shenandoah/heuristics/shenandoahOldHeuristics.cpp
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@ -412,7 +412,7 @@ void ShenandoahOldHeuristics::prepare_for_old_collections() {
size_t defrag_count = 0;
size_t total_uncollected_old_regions = _last_old_region - _last_old_collection_candidate;
if (cand_idx > _last_old_collection_candidate) {
if ((ShenandoahGenerationalHumongousReserve > 0) && (cand_idx > _last_old_collection_candidate)) {
// Above, we have added into the set of mixed-evacuation candidates all old-gen regions for which the live memory
// that they contain is below a particular old-garbage threshold. Regions that were not selected for the collection
// set hold enough live memory that it is not considered efficient (by "garbage-first standards") to compact these

10
src/hotspot/share/gc/shenandoah/shenandoahGeneration.cpp
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@ -535,6 +535,8 @@ size_t ShenandoahGeneration::select_aged_regions(size_t old_available) {
const size_t old_garbage_threshold = (ShenandoahHeapRegion::region_size_bytes() * ShenandoahOldGarbageThreshold) / 100;
const size_t pip_used_threshold = (ShenandoahHeapRegion::region_size_bytes() * ShenandoahGenerationalMinPIPUsage) / 100;
size_t old_consumed = 0;
size_t promo_potential = 0;
size_t candidates = 0;
@ -557,10 +559,8 @@ size_t ShenandoahGeneration::select_aged_regions(size_t old_available) {
continue;
}
if (heap->is_tenurable(r)) {
if ((r->garbage() < old_garbage_threshold)) {
// This tenure-worthy region has too little garbage, so we do not want to expend the copying effort to
// reclaim the garbage; instead this region may be eligible for promotion-in-place to the
// old generation.
if ((r->garbage() < old_garbage_threshold) && (r->used() > pip_used_threshold)) {
// We prefer to promote this region in place because is has a small amount of garbage and a large usage.
HeapWord* tams = ctx->top_at_mark_start(r);
HeapWord* original_top = r->top();
if (!heap->is_concurrent_old_mark_in_progress() && tams == original_top) {
@ -586,7 +586,7 @@ size_t ShenandoahGeneration::select_aged_regions(size_t old_available) {
// Else, we do not promote this region (either in place or by copy) because it has received new allocations.
// During evacuation, we exclude from promotion regions for which age > tenure threshold, garbage < garbage-threshold,
// and get_top_before_promote() != tams
// used > pip_used_threshold, and get_top_before_promote() != tams
} else {
// Record this promotion-eligible candidate region. After sorting and selecting the best candidates below,
// we may still decide to exclude this promotion-eligible region from the current collection set. If this

8
src/hotspot/share/gc/shenandoah/shenandoahGenerationalEvacuationTask.cpp
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@ -146,7 +146,13 @@ void ShenandoahGenerationalEvacuationTask::maybe_promote_region(ShenandoahHeapRe
// more garbage than ShenandoahOldGarbageThreshold, we'll promote by evacuation. If there is room for evacuation
// in this cycle, the region will be in the collection set. If there is not room, the region will be promoted
// by evacuation in some future GC cycle.
promote_humongous(r);
// We do not promote primitive arrays because there's no performance penalty keeping them in young. When/if they
// become garbage, reclaiming the memory from young is much quicker and more efficient than reclaiming them from old.
oop obj = cast_to_oop(r->bottom());
if (!obj->is_typeArray()) {
promote_humongous(r);
}
} else if (r->is_regular() && (r->get_top_before_promote() != nullptr)) {
// Likewise, we cannot put promote-in-place regions into the collection set because that would also trigger
// the LRB to copy on reference fetch.

16
src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp
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@ -34,6 +34,20 @@
range, \
constraint) \
\
product(uintx, ShenandoahGenerationalMinPIPUsage, 30, EXPERIMENTAL, \
"(Generational mode only) What percent of a heap region " \
"should be used before we consider promoting a region in " \
"place? Regions with less than this amount of used will " \
"promoted by evacuation. A benefit of promoting in place " \
"is that less work is required by the GC at the time the " \
"region is promoted. A disadvantage of promoting in place " \
"is that this introduces fragmentation of old-gen memory, " \
"with old-gen regions scattered throughout the heap. Regions " \
"that have been promoted in place may need to be evacuated at " \
"a later time in order to compact old-gen memory to enable " \
"future humongous allocations.") \
range(0,100) \
\
product(uintx, ShenandoahGenerationalHumongousReserve, 0, EXPERIMENTAL, \
"(Generational mode only) What percent of the heap should be " \
"reserved for humongous objects if possible. Old-generation " \
@ -165,7 +179,7 @@
"collector accepts. In percents of heap region size.") \
range(0,100) \
\
product(uintx, ShenandoahOldGarbageThreshold, 15, EXPERIMENTAL, \
product(uintx, ShenandoahOldGarbageThreshold, 25, EXPERIMENTAL, \
"How much garbage an old region has to contain before it would " \
"be taken for collection.") \
range(0,100) \