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jdk/src/hotspot/share/gc/shared/referenceProcessor.cpp
Albert Mingkun Yang 4d383b9fcd 8275298: Remove unnecessary weak_oops_do call in adjust weak roots phase 
Reviewed-by: tschatzl, sjohanss
2021-10-18 16:02:22 +00:00

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/*
* Copyright (c) 2001, 2021, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
#include "precompiled.hpp"
#include "classfile/javaClasses.inline.hpp"
#include "gc/shared/collectedHeap.hpp"
#include "gc/shared/collectedHeap.inline.hpp"
#include "gc/shared/gc_globals.hpp"
#include "gc/shared/gcTimer.hpp"
#include "gc/shared/gcTraceTime.inline.hpp"
#include "gc/shared/referencePolicy.hpp"
#include "gc/shared/referenceProcessor.inline.hpp"
#include "gc/shared/referenceProcessorPhaseTimes.hpp"
#include "logging/log.hpp"
#include "memory/allocation.inline.hpp"
#include "memory/resourceArea.hpp"
#include "memory/universe.hpp"
#include "oops/access.inline.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/java.hpp"
#include "runtime/nonJavaThread.hpp"
#include "utilities/globalDefinitions.hpp"
ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
void referenceProcessor_init() {
ReferenceProcessor::init_statics();
}
void ReferenceProcessor::init_statics() {
// We need a monotonically non-decreasing time in ms but
// os::javaTimeMillis() does not guarantee monotonicity.
jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
// Initialize the soft ref timestamp clock.
_soft_ref_timestamp_clock = now;
// Also update the soft ref clock in j.l.r.SoftReference
java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
_always_clear_soft_ref_policy = new AlwaysClearPolicy();
if (CompilerConfig::is_c2_or_jvmci_compiler_enabled()) {
_default_soft_ref_policy = new LRUMaxHeapPolicy();
} else {
_default_soft_ref_policy = new LRUCurrentHeapPolicy();
}
guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
RefDiscoveryPolicy == ReferentBasedDiscovery,
"Unrecognized RefDiscoveryPolicy");
}
void ReferenceProcessor::enable_discovery(bool check_no_refs) {
#ifdef ASSERT
// Verify that we're not currently discovering refs
assert(!_discovering_refs, "nested call?");
if (check_no_refs) {
// Verify that the discovered lists are empty
verify_no_references_recorded();
}
#endif // ASSERT
_discovering_refs = true;
}
ReferenceProcessor::ReferenceProcessor(BoolObjectClosure* is_subject_to_discovery,
uint mt_processing_degree,
bool mt_discovery,
uint mt_discovery_degree,
bool concurrent_discovery,
BoolObjectClosure* is_alive_non_header) :
_is_subject_to_discovery(is_subject_to_discovery),
_discovering_refs(false),
_next_id(0),
_is_alive_non_header(is_alive_non_header)
{
assert(is_subject_to_discovery != NULL, "must be set");
_discovery_is_concurrent = concurrent_discovery;
_discovery_is_mt = mt_discovery;
_num_queues = MAX2(1U, mt_processing_degree);
_max_num_queues = MAX2(_num_queues, mt_discovery_degree);
_discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
_max_num_queues * number_of_subclasses_of_ref(), mtGC);
_discoveredSoftRefs = &_discovered_refs[0];
_discoveredWeakRefs = &_discoveredSoftRefs[_max_num_queues];
_discoveredFinalRefs = &_discoveredWeakRefs[_max_num_queues];
_discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_queues];
// Initialize all entries to NULL
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
_discovered_refs[i].clear();
}
setup_policy(false /* default soft ref policy */);
}
#ifndef PRODUCT
void ReferenceProcessor::verify_no_references_recorded() {
guarantee(!_discovering_refs, "Discovering refs?");
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
guarantee(_discovered_refs[i].is_empty(),
"Found non-empty discovered list at %u", i);
}
}
#endif
bool ReferenceProcessor::processing_is_mt() const {
return ParallelRefProcEnabled && _num_queues > 1;
}
void ReferenceProcessor::weak_oops_do(OopClosure* f) {
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
if (UseCompressedOops) {
f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
} else {
f->do_oop((oop*)_discovered_refs[i].adr_head());
}
}
}
void ReferenceProcessor::update_soft_ref_master_clock() {
// Update (advance) the soft ref master clock field. This must be done
// after processing the soft ref list.
// We need a monotonically non-decreasing time in ms but
// os::javaTimeMillis() does not guarantee monotonicity.
jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
NOT_PRODUCT(
if (now < _soft_ref_timestamp_clock) {
log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
_soft_ref_timestamp_clock, now);
}
)
// The values of now and _soft_ref_timestamp_clock are set using
// javaTimeNanos(), which is guaranteed to be monotonically
// non-decreasing provided the underlying platform provides such
// a time source (and it is bug free).
// In product mode, however, protect ourselves from non-monotonicity.
if (now > _soft_ref_timestamp_clock) {
_soft_ref_timestamp_clock = now;
java_lang_ref_SoftReference::set_clock(now);
}
// Else leave clock stalled at its old value until time progresses
// past clock value.
}
size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const {
size_t total = 0;
for (uint i = 0; i < _max_num_queues; ++i) {
total += lists[i].length();
}
return total;
}
#ifdef ASSERT
void ReferenceProcessor::verify_total_count_zero(DiscoveredList lists[], const char* type) {
size_t count = total_count(lists);
assert(count == 0, "%ss must be empty but has " SIZE_FORMAT " elements", type, count);
}
#endif
ReferenceProcessorStats ReferenceProcessor::process_discovered_references(RefProcProxyTask& proxy_task,
ReferenceProcessorPhaseTimes& phase_times) {
double start_time = os::elapsedTime();
// Stop treating discovered references specially.
disable_discovery();
ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
total_count(_discoveredWeakRefs),
total_count(_discoveredFinalRefs),
total_count(_discoveredPhantomRefs));
update_soft_ref_master_clock();
{
RefProcTotalPhaseTimesTracker tt(SoftWeakFinalRefsPhase, &phase_times);
process_soft_weak_final_refs(proxy_task, phase_times);
}
{
RefProcTotalPhaseTimesTracker tt(KeepAliveFinalRefsPhase, &phase_times);
process_final_keep_alive(proxy_task, phase_times);
}
{
RefProcTotalPhaseTimesTracker tt(PhantomRefsPhase, &phase_times);
process_phantom_refs(proxy_task, phase_times);
}
phase_times.set_total_time_ms((os::elapsedTime() - start_time) * 1000);
// Elements on discovered lists were pushed to the pending list.
verify_no_references_recorded();
return stats;
}
void BarrierEnqueueDiscoveredFieldClosure::enqueue(HeapWord* discovered_field_addr, oop value) {
assert(Universe::heap()->is_in(discovered_field_addr), PTR_FORMAT " not in heap", p2i(discovered_field_addr));
HeapAccess<AS_NO_KEEPALIVE>::oop_store(discovered_field_addr,
value);
}
void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
_current_discovered_addr = java_lang_ref_Reference::discovered_addr_raw(_current_discovered);
oop discovered = java_lang_ref_Reference::discovered(_current_discovered);
assert(_current_discovered_addr && oopDesc::is_oop_or_null(discovered),
"Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
_next_discovered = discovered;
_referent = java_lang_ref_Reference::unknown_referent_no_keepalive(_current_discovered);
assert(Universe::heap()->is_in_or_null(_referent),
"Wrong oop found in java.lang.Reference object");
assert(allow_null_referent ?
oopDesc::is_oop_or_null(_referent)
: oopDesc::is_oop(_referent),
"Expected an oop%s for referent field at " PTR_FORMAT,
(allow_null_referent ? " or NULL" : ""),
p2i(_referent));
}
void DiscoveredListIterator::remove() {
assert(oopDesc::is_oop(_current_discovered), "Dropping a bad reference");
RawAccess<>::oop_store(_current_discovered_addr, oop(NULL));
// First _prev_next ref actually points into DiscoveredList (gross).
oop new_next;
if (_next_discovered == _current_discovered) {
// At the end of the list, we should make _prev point to itself.
// If _ref is the first ref, then _prev_next will be in the DiscoveredList,
// and _prev will be NULL.
new_next = _prev_discovered;
} else {
new_next = _next_discovered;
}
// Remove Reference object from discovered list. We do not need barriers here,
// as we only remove. We will do the barrier when we actually advance the cursor.
RawAccess<>::oop_store(_prev_discovered_addr, new_next);
_removed++;
_refs_list.dec_length(1);
}
void DiscoveredListIterator::make_referent_alive() {
HeapWord* addr = java_lang_ref_Reference::referent_addr_raw(_current_discovered);
if (UseCompressedOops) {
_keep_alive->do_oop((narrowOop*)addr);
} else {
_keep_alive->do_oop((oop*)addr);
}
}
void DiscoveredListIterator::clear_referent() {
java_lang_ref_Reference::clear_referent(_current_discovered);
}
void DiscoveredListIterator::enqueue() {
if (_prev_discovered_addr != _refs_list.adr_head()) {
_enqueue->enqueue(_prev_discovered_addr, _current_discovered);
} else {
RawAccess<>::oop_store(_prev_discovered_addr, _current_discovered);
}
}
void DiscoveredListIterator::complete_enqueue() {
if (_prev_discovered != nullptr) {
// This is the last object.
// Swap refs_list into pending list and set obj's
// discovered to what we read from the pending list.
oop old = Universe::swap_reference_pending_list(_refs_list.head());
_enqueue->enqueue(java_lang_ref_Reference::discovered_addr_raw(_prev_discovered), old);
}
}
inline void log_preclean_ref(const DiscoveredListIterator& iter, const char* reason) {
if (log_develop_is_enabled(Trace, gc, ref)) {
ResourceMark rm;
log_develop_trace(gc, ref)("Precleaning %s reference " PTR_FORMAT ": %s",
reason, p2i(iter.obj()),
iter.obj()->klass()->internal_name());
}
}
inline void log_dropped_ref(const DiscoveredListIterator& iter, const char* reason) {
if (log_develop_is_enabled(Trace, gc, ref)) {
ResourceMark rm;
log_develop_trace(gc, ref)("Dropping %s reference " PTR_FORMAT ": %s",
reason, p2i(iter.obj()),
iter.obj()->klass()->internal_name());
}
}
inline void log_enqueued_ref(const DiscoveredListIterator& iter, const char* reason) {
if (log_develop_is_enabled(Trace, gc, ref)) {
ResourceMark rm;
log_develop_trace(gc, ref)("Enqueue %s reference (" INTPTR_FORMAT ": %s)",
reason, p2i(iter.obj()), iter.obj()->klass()->internal_name());
}
assert(oopDesc::is_oop(iter.obj()), "Adding a bad reference");
}
size_t ReferenceProcessor::process_discovered_list_work(DiscoveredList& refs_list,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
EnqueueDiscoveredFieldClosure* enqueue,
bool do_enqueue_and_clear) {
DiscoveredListIterator iter(refs_list, keep_alive, is_alive, enqueue);
while (iter.has_next()) {
iter.load_ptrs(DEBUG_ONLY(discovery_is_concurrent() /* allow_null_referent */));
if (iter.referent() == NULL) {
// Reference has been cleared since discovery; only possible if
// discovery is concurrent (checked by load_ptrs). Remove
// reference from list.
log_dropped_ref(iter, "cleared");
iter.remove();
iter.move_to_next();
} else if (iter.is_referent_alive()) {
// The referent is reachable after all.
// Remove reference from list.
log_dropped_ref(iter, "reachable");
iter.remove();
// Update the referent pointer as necessary. Note that this
// should not entail any recursive marking because the
// referent must already have been traversed.
iter.make_referent_alive();
iter.move_to_next();
} else {
if (do_enqueue_and_clear) {
iter.clear_referent();
iter.enqueue();
log_enqueued_ref(iter, "cleared");
}
// Keep in discovered list
iter.next();
}
}
if (do_enqueue_and_clear) {
iter.complete_enqueue();
refs_list.clear();
}
log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
" Refs in discovered list " INTPTR_FORMAT,
iter.removed(), iter.processed(), p2i(&refs_list));
return iter.removed();
}
size_t ReferenceProcessor::process_final_keep_alive_work(DiscoveredList& refs_list,
OopClosure* keep_alive,
EnqueueDiscoveredFieldClosure* enqueue) {
DiscoveredListIterator iter(refs_list, keep_alive, NULL, enqueue);
while (iter.has_next()) {
iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
// keep the referent and followers around
iter.make_referent_alive();
// Self-loop next, to mark the FinalReference not active.
assert(java_lang_ref_Reference::next(iter.obj()) == NULL, "enqueued FinalReference");
java_lang_ref_Reference::set_next_raw(iter.obj(), iter.obj());
iter.enqueue();
log_enqueued_ref(iter, "Final");
iter.next();
}
iter.complete_enqueue();
refs_list.clear();
assert(iter.removed() == 0, "This phase does not remove anything.");
return iter.removed();
}
void
ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
oop obj = NULL;
oop next = refs_list.head();
while (next != obj) {
obj = next;
next = java_lang_ref_Reference::discovered(obj);
java_lang_ref_Reference::set_discovered_raw(obj, NULL);
}
refs_list.clear();
}
void ReferenceProcessor::abandon_partial_discovery() {
// loop over the lists
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
if ((i % _max_num_queues) == 0) {
log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
}
clear_discovered_references(_discovered_refs[i]);
}
}
size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
DiscoveredList* list = NULL;
switch (type) {
case REF_SOFT:
list = _discoveredSoftRefs;
break;
case REF_WEAK:
list = _discoveredWeakRefs;
break;
case REF_FINAL:
list = _discoveredFinalRefs;
break;
case REF_PHANTOM:
list = _discoveredPhantomRefs;
break;
case REF_OTHER:
case REF_NONE:
default:
ShouldNotReachHere();
}
return total_count(list);
}
void RefProcTask::process_discovered_list(uint worker_id,
ReferenceType ref_type,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
EnqueueDiscoveredFieldClosure* enqueue) {
ReferenceProcessor::RefProcSubPhases subphase;
DiscoveredList* dl;
switch (ref_type) {
case ReferenceType::REF_SOFT:
subphase = ReferenceProcessor::ProcessSoftRefSubPhase;
dl = _ref_processor._discoveredSoftRefs;
break;
case ReferenceType::REF_WEAK:
subphase = ReferenceProcessor::ProcessWeakRefSubPhase;
dl = _ref_processor._discoveredWeakRefs;
break;
case ReferenceType::REF_FINAL:
subphase = ReferenceProcessor::ProcessFinalRefSubPhase;
dl = _ref_processor._discoveredFinalRefs;
break;
case ReferenceType::REF_PHANTOM:
subphase = ReferenceProcessor::ProcessPhantomRefsSubPhase;
dl = _ref_processor._discoveredPhantomRefs;
break;
default:
ShouldNotReachHere();
}
// Only Final refs are not enqueued and cleared here.
bool do_enqueue_and_clear = (ref_type != REF_FINAL);
{
RefProcSubPhasesWorkerTimeTracker tt(subphase, _phase_times, tracker_id(worker_id));
size_t const removed = _ref_processor.process_discovered_list_work(dl[worker_id],
is_alive,
keep_alive,
enqueue,
do_enqueue_and_clear);
_phase_times->add_ref_cleared(ref_type, removed);
}
}
class RefProcSoftWeakFinalPhaseTask: public RefProcTask {
public:
RefProcSoftWeakFinalPhaseTask(ReferenceProcessor& ref_processor,
ReferenceProcessorPhaseTimes* phase_times)
: RefProcTask(ref_processor,
phase_times) {}
void rp_work(uint worker_id,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
EnqueueDiscoveredFieldClosure* enqueue,
VoidClosure* complete_gc) override {
RefProcWorkerTimeTracker t(_phase_times->soft_weak_final_refs_phase_worker_time_sec(), tracker_id(worker_id));
process_discovered_list(worker_id, REF_SOFT, is_alive, keep_alive, enqueue);
process_discovered_list(worker_id, REF_WEAK, is_alive, keep_alive, enqueue);
process_discovered_list(worker_id, REF_FINAL, is_alive, keep_alive, enqueue);
// Close the reachable set; needed for collectors which keep_alive_closure do
// not immediately complete their work.
complete_gc->do_void();
}
};
class RefProcKeepAliveFinalPhaseTask: public RefProcTask {
public:
RefProcKeepAliveFinalPhaseTask(ReferenceProcessor& ref_processor,
ReferenceProcessorPhaseTimes* phase_times)
: RefProcTask(ref_processor,
phase_times) {}
void rp_work(uint worker_id,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
EnqueueDiscoveredFieldClosure* enqueue,
VoidClosure* complete_gc) override {
RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::KeepAliveFinalRefsSubPhase, _phase_times, tracker_id(worker_id));
_ref_processor.process_final_keep_alive_work(_ref_processor._discoveredFinalRefs[worker_id], keep_alive, enqueue);
// Close the reachable set
complete_gc->do_void();
}
};
class RefProcPhantomPhaseTask: public RefProcTask {
public:
RefProcPhantomPhaseTask(ReferenceProcessor& ref_processor,
ReferenceProcessorPhaseTimes* phase_times)
: RefProcTask(ref_processor,
phase_times) {}
void rp_work(uint worker_id,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
EnqueueDiscoveredFieldClosure* enqueue,
VoidClosure* complete_gc) override {
process_discovered_list(worker_id, REF_PHANTOM, is_alive, keep_alive, enqueue);
// Close the reachable set; needed for collectors which keep_alive_closure do
// not immediately complete their work.
complete_gc->do_void();
}
};
void ReferenceProcessor::log_reflist(const char* prefix, DiscoveredList list[], uint num_active_queues) {
LogTarget(Trace, gc, ref) lt;
if (!lt.is_enabled()) {
return;
}
size_t total = 0;
LogStream ls(lt);
ls.print("%s", prefix);
for (uint i = 0; i < num_active_queues; i++) {
ls.print(SIZE_FORMAT " ", list[i].length());
total += list[i].length();
}
ls.print_cr("(" SIZE_FORMAT ")", total);
}
#ifndef PRODUCT
void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint num_active_queues) {
if (!log_is_enabled(Trace, gc, ref)) {
return;
}
log_reflist("", ref_lists, num_active_queues);
#ifdef ASSERT
for (uint i = num_active_queues; i < _max_num_queues; i++) {
assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
ref_lists[i].length(), i);
}
#endif
}
#endif
void ReferenceProcessor::set_active_mt_degree(uint v) {
assert(v <= max_num_queues(), "Mt degree %u too high, maximum %u", v, max_num_queues());
_num_queues = v;
_next_id = 0;
}
bool ReferenceProcessor::need_balance_queues(DiscoveredList refs_lists[]) {
assert(processing_is_mt(), "why balance non-mt processing?");
// _num_queues is the processing degree. Only list entries up to
// _num_queues will be processed, so any non-empty lists beyond
// that must be redistributed to lists in that range. Even if not
// needed for that, balancing may be desirable to eliminate poor
// distribution of references among the lists.
if (ParallelRefProcBalancingEnabled) {
return true; // Configuration says do it.
} else {
// Configuration says don't balance, but if there are non-empty
// lists beyond the processing degree, then must ignore the
// configuration and balance anyway.
for (uint i = _num_queues; i < _max_num_queues; ++i) {
if (!refs_lists[i].is_empty()) {
return true; // Must balance despite configuration.
}
}
return false; // Safe to obey configuration and not balance.
}
}
void ReferenceProcessor::maybe_balance_queues(DiscoveredList refs_lists[]) {
assert(processing_is_mt(), "Should not call this otherwise");
if (need_balance_queues(refs_lists)) {
balance_queues(refs_lists);
}
}
// Balances reference queues.
// Move entries from all queues[0, 1, ..., _max_num_q-1] to
// queues[0, 1, ..., _num_q-1] because only the first _num_q
// corresponding to the active workers will be processed.
void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
{
// calculate total length
size_t total_refs = 0;
log_develop_trace(gc, ref)("Balance ref_lists ");
log_reflist_counts(ref_lists, _max_num_queues);
for (uint i = 0; i < _max_num_queues; ++i) {
total_refs += ref_lists[i].length();
}
size_t avg_refs = total_refs / _num_queues + 1;
uint to_idx = 0;
for (uint from_idx = 0; from_idx < _max_num_queues; from_idx++) {
bool move_all = false;
if (from_idx >= _num_queues) {
move_all = ref_lists[from_idx].length() > 0;
}
while ((ref_lists[from_idx].length() > avg_refs) ||
move_all) {
assert(to_idx < _num_queues, "Sanity Check!");
if (ref_lists[to_idx].length() < avg_refs) {
// move superfluous refs
size_t refs_to_move;
// Move all the Ref's if the from queue will not be processed.
if (move_all) {
refs_to_move = MIN2(ref_lists[from_idx].length(),
avg_refs - ref_lists[to_idx].length());
} else {
refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
avg_refs - ref_lists[to_idx].length());
}
assert(refs_to_move > 0, "otherwise the code below will fail");
oop move_head = ref_lists[from_idx].head();
oop move_tail = move_head;
oop new_head = move_head;
// find an element to split the list on
for (size_t j = 0; j < refs_to_move; ++j) {
move_tail = new_head;
new_head = java_lang_ref_Reference::discovered(new_head);
}
// Add the chain to the to list.
if (ref_lists[to_idx].head() == NULL) {
// to list is empty. Make a loop at the end.
java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
} else {
java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
}
ref_lists[to_idx].set_head(move_head);
ref_lists[to_idx].inc_length(refs_to_move);
// Remove the chain from the from list.
if (move_tail == new_head) {
// We found the end of the from list.
ref_lists[from_idx].set_head(NULL);
} else {
ref_lists[from_idx].set_head(new_head);
}
ref_lists[from_idx].dec_length(refs_to_move);
if (ref_lists[from_idx].length() == 0) {
break;
}
} else {
to_idx = (to_idx + 1) % _num_queues;
}
}
}
#ifdef ASSERT
log_reflist_counts(ref_lists, _num_queues);
size_t balanced_total_refs = 0;
for (uint i = 0; i < _num_queues; ++i) {
balanced_total_refs += ref_lists[i].length();
}
assert(total_refs == balanced_total_refs, "Balancing was incomplete");
#endif
}
void ReferenceProcessor::run_task(RefProcTask& task, RefProcProxyTask& proxy_task, bool marks_oops_alive) {
log_debug(gc, ref)("ReferenceProcessor::execute queues: %d, %s, marks_oops_alive: %s",
num_queues(),
processing_is_mt() ? "RefProcThreadModel::Multi" : "RefProcThreadModel::Single",
marks_oops_alive ? "true" : "false");
proxy_task.prepare_run_task(task, num_queues(), processing_is_mt() ? RefProcThreadModel::Multi : RefProcThreadModel::Single, marks_oops_alive);
if (processing_is_mt()) {
WorkerThreads* workers = Universe::heap()->safepoint_workers();
assert(workers != NULL, "can not dispatch multi threaded without workers");
assert(workers->active_workers() >= num_queues(),
"Ergonomically chosen workers(%u) should be less than or equal to active workers(%u)",
num_queues(), workers->active_workers());
workers->run_task(&proxy_task, num_queues());
} else {
for (unsigned i = 0; i < _max_num_queues; ++i) {
proxy_task.work(i);
}
}
}
void ReferenceProcessor::process_soft_weak_final_refs(RefProcProxyTask& proxy_task,
ReferenceProcessorPhaseTimes& phase_times) {
size_t const num_soft_refs = total_count(_discoveredSoftRefs);
size_t const num_weak_refs = total_count(_discoveredWeakRefs);
size_t const num_final_refs = total_count(_discoveredFinalRefs);
size_t const num_total_refs = num_soft_refs + num_weak_refs + num_final_refs;
phase_times.set_ref_discovered(REF_WEAK, num_weak_refs);
phase_times.set_ref_discovered(REF_FINAL, num_final_refs);
phase_times.set_processing_is_mt(processing_is_mt());
if (num_total_refs == 0) {
log_debug(gc, ref)("Skipped SoftWeakFinalRefsPhase of Reference Processing: no references");
return;
}
RefProcMTDegreeAdjuster a(this, SoftWeakFinalRefsPhase, num_total_refs);
if (processing_is_mt()) {
RefProcBalanceQueuesTimeTracker tt(SoftWeakFinalRefsPhase, &phase_times);
maybe_balance_queues(_discoveredSoftRefs);
maybe_balance_queues(_discoveredWeakRefs);
maybe_balance_queues(_discoveredFinalRefs);
}
RefProcPhaseTimeTracker tt(SoftWeakFinalRefsPhase, &phase_times);
log_reflist("SoftWeakFinalRefsPhase Soft before", _discoveredSoftRefs, _max_num_queues);
log_reflist("SoftWeakFinalRefsPhase Weak before", _discoveredWeakRefs, _max_num_queues);
log_reflist("SoftWeakFinalRefsPhase Final before", _discoveredFinalRefs, _max_num_queues);
RefProcSoftWeakFinalPhaseTask phase_task(*this, &phase_times);
run_task(phase_task, proxy_task, false);
verify_total_count_zero(_discoveredSoftRefs, "SoftReference");
verify_total_count_zero(_discoveredWeakRefs, "WeakReference");
log_reflist("SoftWeakFinalRefsPhase Final after", _discoveredFinalRefs, _max_num_queues);
}
void ReferenceProcessor::process_final_keep_alive(RefProcProxyTask& proxy_task,
ReferenceProcessorPhaseTimes& phase_times) {
size_t const num_final_refs = total_count(_discoveredFinalRefs);
phase_times.set_processing_is_mt(processing_is_mt());
if (num_final_refs == 0) {
log_debug(gc, ref)("Skipped KeepAliveFinalRefsPhase of Reference Processing: no references");
return;
}
RefProcMTDegreeAdjuster a(this, KeepAliveFinalRefsPhase, num_final_refs);
if (processing_is_mt()) {
RefProcBalanceQueuesTimeTracker tt(KeepAliveFinalRefsPhase, &phase_times);
maybe_balance_queues(_discoveredFinalRefs);
}
// Traverse referents of final references and keep them and followers alive.
RefProcPhaseTimeTracker tt(KeepAliveFinalRefsPhase, &phase_times);
RefProcKeepAliveFinalPhaseTask phase_task(*this, &phase_times);
run_task(phase_task, proxy_task, true);
verify_total_count_zero(_discoveredFinalRefs, "FinalReference");
}
void ReferenceProcessor::process_phantom_refs(RefProcProxyTask& proxy_task,
ReferenceProcessorPhaseTimes& phase_times) {
size_t const num_phantom_refs = total_count(_discoveredPhantomRefs);
phase_times.set_ref_discovered(REF_PHANTOM, num_phantom_refs);
phase_times.set_processing_is_mt(processing_is_mt());
if (num_phantom_refs == 0) {
log_debug(gc, ref)("Skipped PhantomRefsPhase of Reference Processing: no references");
return;
}
RefProcMTDegreeAdjuster a(this, PhantomRefsPhase, num_phantom_refs);
if (processing_is_mt()) {
RefProcBalanceQueuesTimeTracker tt(PhantomRefsPhase, &phase_times);
maybe_balance_queues(_discoveredPhantomRefs);
}
// Walk phantom references appropriately.
RefProcPhaseTimeTracker tt(PhantomRefsPhase, &phase_times);
log_reflist("PhantomRefsPhase Phantom before", _discoveredPhantomRefs, _max_num_queues);
RefProcPhantomPhaseTask phase_task(*this, &phase_times);
run_task(phase_task, proxy_task, false);
verify_total_count_zero(_discoveredPhantomRefs, "PhantomReference");
}
inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
uint id = 0;
// Determine the queue index to use for this object.
if (_discovery_is_mt) {
// During a multi-threaded discovery phase,
// each thread saves to its "own" list.
id = WorkerThread::current()->id();
} else {
// single-threaded discovery, we save in round-robin
// fashion to each of the lists.
if (processing_is_mt()) {
id = next_id();
}
}
assert(id < _max_num_queues, "Id is out of bounds id %u and max id %u)", id, _max_num_queues);
// Get the discovered queue to which we will add
DiscoveredList* list = NULL;
switch (rt) {
case REF_OTHER:
// Unknown reference type, no special treatment
break;
case REF_SOFT:
list = &_discoveredSoftRefs[id];
break;
case REF_WEAK:
list = &_discoveredWeakRefs[id];
break;
case REF_FINAL:
list = &_discoveredFinalRefs[id];
break;
case REF_PHANTOM:
list = &_discoveredPhantomRefs[id];
break;
case REF_NONE:
// we should not reach here if we are an InstanceRefKlass
default:
ShouldNotReachHere();
}
log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
return list;
}
inline bool ReferenceProcessor::set_discovered_link(HeapWord* discovered_addr, oop next_discovered) {
return discovery_is_mt() ? set_discovered_link_mt(discovered_addr, next_discovered)
: set_discovered_link_st(discovered_addr, next_discovered);
}
inline void ReferenceProcessor::add_to_discovered_list(DiscoveredList& refs_list,
oop obj,
HeapWord* discovered_addr) {
oop current_head = refs_list.head();
// Prepare value to put into the discovered field. The last ref must have its
// discovered field pointing to itself.
oop next_discovered = (current_head != NULL) ? current_head : obj;
bool added = set_discovered_link(discovered_addr, next_discovered);
if (added) {
// We can always add the object without synchronization: every thread has its
// own list head.
refs_list.add_as_head(obj);
log_develop_trace(gc, ref)("Discovered reference (%s) (" INTPTR_FORMAT ": %s)",
discovery_is_mt() ? "mt" : "st", p2i(obj), obj->klass()->internal_name());
} else {
log_develop_trace(gc, ref)("Already discovered reference (mt) (" INTPTR_FORMAT ": %s)",
p2i(obj), obj->klass()->internal_name());
}
}
inline bool ReferenceProcessor::set_discovered_link_st(HeapWord* discovered_addr,
oop next_discovered) {
assert(!discovery_is_mt(), "must be");
if (discovery_is_stw()) {
// Do a raw store here: the field will be visited later when processing
// the discovered references.
RawAccess<>::oop_store(discovered_addr, next_discovered);
} else {
HeapAccess<AS_NO_KEEPALIVE>::oop_store(discovered_addr, next_discovered);
}
// Always successful.
return true;
}
inline bool ReferenceProcessor::set_discovered_link_mt(HeapWord* discovered_addr,
oop next_discovered) {
assert(discovery_is_mt(), "must be");
// We must make sure this object is only enqueued once. Try to CAS into the discovered_addr.
oop retest;
if (discovery_is_stw()) {
// Try a raw store here, still making sure that we enqueue only once: the field
// will be visited later when processing the discovered references.
retest = RawAccess<>::oop_atomic_cmpxchg(discovered_addr, oop(NULL), next_discovered);
} else {
retest = HeapAccess<AS_NO_KEEPALIVE>::oop_atomic_cmpxchg(discovered_addr, oop(NULL), next_discovered);
}
return retest == NULL;
}
#ifndef PRODUCT
// Concurrent discovery might allow us to observe j.l.References with NULL
// referents, being those cleared concurrently by mutators during (or after) discovery.
void ReferenceProcessor::verify_referent(oop obj) {
bool concurrent = discovery_is_concurrent();
oop referent = java_lang_ref_Reference::unknown_referent_no_keepalive(obj);
assert(concurrent ? oopDesc::is_oop_or_null(referent) : oopDesc::is_oop(referent),
"Bad referent " INTPTR_FORMAT " found in Reference "
INTPTR_FORMAT " during %sconcurrent discovery ",
p2i(referent), p2i(obj), concurrent ? "" : "non-");
}
#endif
bool ReferenceProcessor::is_subject_to_discovery(oop const obj) const {
return _is_subject_to_discovery->do_object_b(obj);
}
// We mention two of several possible choices here:
// #0: if the reference object is not in the "originating generation"
// (or part of the heap being collected, indicated by our "span")
// we don't treat it specially (i.e. we scan it as we would
// a normal oop, treating its references as strong references).
// This means that references can't be discovered unless their
// referent is also in the same span. This is the simplest,
// most "local" and most conservative approach, albeit one
// that may cause weak references to be enqueued least promptly.
// We call this choice the "ReferenceBasedDiscovery" policy.
// #1: the reference object may be in any generation (span), but if
// the referent is in the generation (span) being currently collected
// then we can discover the reference object, provided
// the object has not already been discovered by
// a different concurrently running discoverer (as may be the
// case, for instance, if the reference object is in G1 old gen and
// the referent in G1 young gen), and provided the processing
// of this reference object by the current collector will
// appear atomically to every other discoverer in the system.
// (Thus, for instance, a concurrent discoverer may not
// discover references in other generations even if the
// referent is in its own generation). This policy may,
// in certain cases, enqueue references somewhat sooner than
// might Policy #0 above, but at marginally increased cost
// and complexity in processing these references.
// We call this choice the "ReferentBasedDiscovery" policy.
bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
// Make sure we are discovering refs (rather than processing discovered refs).
if (!_discovering_refs || !RegisterReferences) {
return false;
}
if ((rt == REF_FINAL) && (java_lang_ref_Reference::next(obj) != NULL)) {
// Don't rediscover non-active FinalReferences.
return false;
}
if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
!is_subject_to_discovery(obj)) {
// Reference is not in the originating generation;
// don't treat it specially (i.e. we want to scan it as a normal
// object with strong references).
return false;
}
// We only discover references whose referents are not (yet)
// known to be strongly reachable.
if (is_alive_non_header() != NULL) {
verify_referent(obj);
oop referent = java_lang_ref_Reference::unknown_referent_no_keepalive(obj);
if (is_alive_non_header()->do_object_b(referent)) {
return false; // referent is reachable
}
}
if (rt == REF_SOFT) {
// For soft refs we can decide now if these are not
// current candidates for clearing, in which case we
// can mark through them now, rather than delaying that
// to the reference-processing phase. Since all current
// time-stamp policies advance the soft-ref clock only
// at a full collection cycle, this is always currently
// accurate.
if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
return false;
}
}
ResourceMark rm; // Needed for tracing.
HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj);
const oop discovered = java_lang_ref_Reference::discovered(obj);
assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
if (discovered != NULL) {
// The reference has already been discovered...
log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
p2i(obj), obj->klass()->internal_name());
if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
// assumes that an object is not processed twice;
// if it's been already discovered it must be on another
// generation's discovered list; so we won't discover it.
return false;
} else {
assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
"Unrecognized policy");
// Check assumption that an object is not potentially
// discovered twice except by concurrent collectors that potentially
// trace the same Reference object twice.
assert(UseG1GC, "Only possible with a concurrent marking collector");
return true;
}
}
if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
verify_referent(obj);
// Discover if and only if EITHER:
// .. reference is in our span, OR
// .. we are a stw discoverer and referent is in our span
if (is_subject_to_discovery(obj) ||
(discovery_is_stw() &&
is_subject_to_discovery(java_lang_ref_Reference::unknown_referent_no_keepalive(obj)))) {
} else {
return false;
}
} else {
assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
is_subject_to_discovery(obj), "code inconsistency");
}
// Get the right type of discovered queue head.
DiscoveredList* list = get_discovered_list(rt);
if (list == NULL) {
return false; // nothing special needs to be done
}
add_to_discovered_list(*list, obj, discovered_addr);
assert(oopDesc::is_oop(obj), "Discovered a bad reference");
verify_referent(obj);
return true;
}
bool ReferenceProcessor::has_discovered_references() {
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
if (!_discovered_refs[i].is_empty()) {
return true;
}
}
return false;
}
void ReferenceProcessor::preclean_discovered_references(BoolObjectClosure* is_alive,
OopClosure* keep_alive,
EnqueueDiscoveredFieldClosure* enqueue,
VoidClosure* complete_gc,
YieldClosure* yield,
GCTimer* gc_timer) {
// These lists can be handled here in any order and, indeed, concurrently.
// Soft references
{
GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
log_reflist("SoftRef before: ", _discoveredSoftRefs, _max_num_queues);
for (uint i = 0; i < _max_num_queues; i++) {
if (yield->should_return()) {
return;
}
if (preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
keep_alive, enqueue, complete_gc, yield)) {
log_reflist("SoftRef abort: ", _discoveredSoftRefs, _max_num_queues);
return;
}
}
log_reflist("SoftRef after: ", _discoveredSoftRefs, _max_num_queues);
}
// Weak references
{
GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
log_reflist("WeakRef before: ", _discoveredWeakRefs, _max_num_queues);
for (uint i = 0; i < _max_num_queues; i++) {
if (yield->should_return()) {
return;
}
if (preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
keep_alive, enqueue, complete_gc, yield)) {
log_reflist("WeakRef abort: ", _discoveredWeakRefs, _max_num_queues);
return;
}
}
log_reflist("WeakRef after: ", _discoveredWeakRefs, _max_num_queues);
}
// Final references
{
GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
log_reflist("FinalRef before: ", _discoveredFinalRefs, _max_num_queues);
for (uint i = 0; i < _max_num_queues; i++) {
if (yield->should_return()) {
return;
}
if (preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
keep_alive, enqueue, complete_gc, yield)) {
log_reflist("FinalRef abort: ", _discoveredFinalRefs, _max_num_queues);
return;
}
}
log_reflist("FinalRef after: ", _discoveredFinalRefs, _max_num_queues);
}
// Phantom references
{
GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
log_reflist("PhantomRef before: ", _discoveredPhantomRefs, _max_num_queues);
for (uint i = 0; i < _max_num_queues; i++) {
if (yield->should_return()) {
return;
}
if (preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
keep_alive, enqueue, complete_gc, yield)) {
log_reflist("PhantomRef abort: ", _discoveredPhantomRefs, _max_num_queues);
return;
}
}
log_reflist("PhantomRef after: ", _discoveredPhantomRefs, _max_num_queues);
}
}
// Walk the given discovered ref list, and remove all reference objects whose
// referents are still alive or NULL. NOTE: When we are precleaning the
// ref lists, we do not disable refs discovery to honor the correct semantics of
// java.lang.Reference. Therefore, as we iterate over the discovered list (DL)
// and drop elements from it, newly discovered refs can be discovered and added
// to the DL. Because precleaning is implemented single-threaded today, for
// each per-thread DL, the insertion of refs (calling `complete_gc`) happens
// after the iteration. The clear separation means no special synchronization
// is needed.
bool ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
EnqueueDiscoveredFieldClosure* enqueue,
VoidClosure* complete_gc,
YieldClosure* yield) {
DiscoveredListIterator iter(refs_list, keep_alive, is_alive, enqueue);
while (iter.has_next()) {
if (yield->should_return_fine_grain()) {
return true;
}
iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
if (iter.referent() == nullptr) {
log_preclean_ref(iter, "cleared");
iter.remove();
iter.move_to_next();
} else if (iter.is_referent_alive()) {
log_preclean_ref(iter, "reachable");
// Remove Reference object from list
iter.remove();
// Keep alive its cohort.
iter.make_referent_alive();
iter.move_to_next();
} else {
iter.next();
}
}
// Close the reachable set
complete_gc->do_void();
if (iter.processed() > 0) {
log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
iter.removed(), iter.processed(), p2i(&refs_list));
}
return false;
}
const char* ReferenceProcessor::list_name(uint i) {
assert(i <= _max_num_queues * number_of_subclasses_of_ref(),
"Out of bounds index");
int j = i / _max_num_queues;
switch (j) {
case 0: return "SoftRef";
case 1: return "WeakRef";
case 2: return "FinalRef";
case 3: return "PhantomRef";
}
ShouldNotReachHere();
return NULL;
}
uint RefProcMTDegreeAdjuster::ergo_proc_thread_count(size_t ref_count,
uint max_threads,
RefProcPhases phase) const {
assert(0 < max_threads, "must allow at least one thread");
if (use_max_threads(phase) || (ReferencesPerThread == 0)) {
return max_threads;
}
size_t thread_count = 1 + (ref_count / ReferencesPerThread);
return (uint)MIN3(thread_count,
static_cast<size_t>(max_threads),
(size_t)os::active_processor_count());
}
bool RefProcMTDegreeAdjuster::use_max_threads(RefProcPhases phase) const {
// Even a small number of references in this phase could produce large amounts of work.
return phase == ReferenceProcessor::KeepAliveFinalRefsPhase;
}
RefProcMTDegreeAdjuster::RefProcMTDegreeAdjuster(ReferenceProcessor* rp,
RefProcPhases phase,
size_t ref_count):
_rp(rp),
_saved_num_queues(_rp->num_queues()) {
uint workers = ergo_proc_thread_count(ref_count, _rp->num_queues(), phase);
_rp->set_active_mt_degree(workers);
}
RefProcMTDegreeAdjuster::~RefProcMTDegreeAdjuster() {
// Revert to previous status.
_rp->set_active_mt_degree(_saved_num_queues);
}
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