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JVM源码分析之G1垃圾收集器分析一

对 Java 的 gc 实现比较感兴趣,原先一般都是看周志明的书,但其实并没有讲具体的 gc 源码,而是把整个思路和流程讲解了一下
特别是 G1 的具体实现
一般对 G1 的理解其实就是把原先整块的新生代老年代分成了以 region 为单位的小块内存,简而言之,就是原先对新生代老年代的收集会涉及到整个代的堆内存空间,而G1 把它变成了更细致的小块内存
这带来了一个很明显的好处和一个很明显的坏处,好处是内存收集可以更灵活,耗时会变短,但整个收集的处理复杂度就变高了
目前看了一点点关于 G1 收集的预期时间相关的代码

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HeapWord* G1CollectedHeap::do_collection_pause(size_t word_size,
uint gc_count_before,
bool* succeeded,
GCCause::Cause gc_cause) {
assert_heap_not_locked_and_not_at_safepoint();
VM_G1CollectForAllocation op(word_size,
gc_count_before,
gc_cause,
false, /* should_initiate_conc_mark */
g1_policy()->max_pause_time_ms());
VMThread::execute(&op);

HeapWord* result = op.result();
bool ret_succeeded = op.prologue_succeeded() && op.pause_succeeded();
assert(result == NULL || ret_succeeded,
"the result should be NULL if the VM did not succeed");
*succeeded = ret_succeeded;

assert_heap_not_locked();
return result;
}

这里就是收集时需要停顿的,其中VMThread::execute(&op);是具体执行的,真正执行的是VM_G1CollectForAllocation::doit方法

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void VM_G1CollectForAllocation::doit() {
G1CollectedHeap* g1h = G1CollectedHeap::heap();
assert(!_should_initiate_conc_mark || g1h->should_do_concurrent_full_gc(_gc_cause),
"only a GC locker, a System.gc(), stats update, whitebox, or a hum allocation induced GC should start a cycle");

if (_word_size > 0) {
// An allocation has been requested. So, try to do that first.
_result = g1h->attempt_allocation_at_safepoint(_word_size,
false /* expect_null_cur_alloc_region */);
if (_result != NULL) {
// If we can successfully allocate before we actually do the
// pause then we will consider this pause successful.
_pause_succeeded = true;
return;
}
}

GCCauseSetter x(g1h, _gc_cause);
if (_should_initiate_conc_mark) {
// It's safer to read old_marking_cycles_completed() here, given
// that noone else will be updating it concurrently. Since we'll
// only need it if we're initiating a marking cycle, no point in
// setting it earlier.
_old_marking_cycles_completed_before = g1h->old_marking_cycles_completed();

// At this point we are supposed to start a concurrent cycle. We
// will do so if one is not already in progress.
bool res = g1h->g1_policy()->force_initial_mark_if_outside_cycle(_gc_cause);

// The above routine returns true if we were able to force the
// next GC pause to be an initial mark; it returns false if a
// marking cycle is already in progress.
//
// If a marking cycle is already in progress just return and skip the
// pause below - if the reason for requesting this initial mark pause
// was due to a System.gc() then the requesting thread should block in
// doit_epilogue() until the marking cycle is complete.
//
// If this initial mark pause was requested as part of a humongous
// allocation then we know that the marking cycle must just have
// been started by another thread (possibly also allocating a humongous
// object) as there was no active marking cycle when the requesting
// thread checked before calling collect() in
// attempt_allocation_humongous(). Retrying the GC, in this case,
// will cause the requesting thread to spin inside collect() until the
// just started marking cycle is complete - which may be a while. So
// we do NOT retry the GC.
if (!res) {
assert(_word_size == 0, "Concurrent Full GC/Humongous Object IM shouldn't be allocating");
if (_gc_cause != GCCause::_g1_humongous_allocation) {
_should_retry_gc = true;
}
return;
}
}

// Try a partial collection of some kind.
_pause_succeeded = g1h->do_collection_pause_at_safepoint(_target_pause_time_ms);

if (_pause_succeeded) {
if (_word_size > 0) {
// An allocation had been requested. Do it, eventually trying a stronger
// kind of GC.
_result = g1h->satisfy_failed_allocation(_word_size, &_pause_succeeded);
} else {
bool should_upgrade_to_full = !g1h->should_do_concurrent_full_gc(_gc_cause) &&
!g1h->has_regions_left_for_allocation();
if (should_upgrade_to_full) {
// There has been a request to perform a GC to free some space. We have no
// information on how much memory has been asked for. In case there are
// absolutely no regions left to allocate into, do a maximally compacting full GC.
log_info(gc, ergo)("Attempting maximally compacting collection");
_pause_succeeded = g1h->do_full_collection(false, /* explicit gc */
true /* clear_all_soft_refs */);
}
}
guarantee(_pause_succeeded, "Elevated collections during the safepoint must always succeed.");
} else {
assert(_result == NULL, "invariant");
// The only reason for the pause to not be successful is that, the GC locker is
// active (or has become active since the prologue was executed). In this case
// we should retry the pause after waiting for the GC locker to become inactive.
_should_retry_gc = true;
}
}

这里可以看到核心的是G1CollectedHeap::do_collection_pause_at_safepoint这个方法,它带上了目标暂停时间的值

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G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
assert_at_safepoint_on_vm_thread();
guarantee(!is_gc_active(), "collection is not reentrant");

if (GCLocker::check_active_before_gc()) {
return false;
}

_gc_timer_stw->register_gc_start();

GCIdMark gc_id_mark;
_gc_tracer_stw->report_gc_start(gc_cause(), _gc_timer_stw->gc_start());

SvcGCMarker sgcm(SvcGCMarker::MINOR);
ResourceMark rm;

g1_policy()->note_gc_start();

wait_for_root_region_scanning();

print_heap_before_gc();
print_heap_regions();
trace_heap_before_gc(_gc_tracer_stw);

_verifier->verify_region_sets_optional();
_verifier->verify_dirty_young_regions();

// We should not be doing initial mark unless the conc mark thread is running
if (!_cm_thread->should_terminate()) {
// This call will decide whether this pause is an initial-mark
// pause. If it is, in_initial_mark_gc() will return true
// for the duration of this pause.
g1_policy()->decide_on_conc_mark_initiation();
}

// We do not allow initial-mark to be piggy-backed on a mixed GC.
assert(!collector_state()->in_initial_mark_gc() ||
collector_state()->in_young_only_phase(), "sanity");

// We also do not allow mixed GCs during marking.
assert(!collector_state()->mark_or_rebuild_in_progress() || collector_state()->in_young_only_phase(), "sanity");

// Record whether this pause is an initial mark. When the current
// thread has completed its logging output and it's safe to signal
// the CM thread, the flag's value in the policy has been reset.
bool should_start_conc_mark = collector_state()->in_initial_mark_gc();

// Inner scope for scope based logging, timers, and stats collection
{
EvacuationInfo evacuation_info;

if (collector_state()->in_initial_mark_gc()) {
// We are about to start a marking cycle, so we increment the
// full collection counter.
increment_old_marking_cycles_started();
_cm->gc_tracer_cm()->set_gc_cause(gc_cause());
}

_gc_tracer_stw->report_yc_type(collector_state()->yc_type());

GCTraceCPUTime tcpu;

G1HeapVerifier::G1VerifyType verify_type;
FormatBuffer<> gc_string("Pause Young ");
if (collector_state()->in_initial_mark_gc()) {
gc_string.append("(Concurrent Start)");
verify_type = G1HeapVerifier::G1VerifyConcurrentStart;
} else if (collector_state()->in_young_only_phase()) {
if (collector_state()->in_young_gc_before_mixed()) {
gc_string.append("(Prepare Mixed)");
} else {
gc_string.append("(Normal)");
}
verify_type = G1HeapVerifier::G1VerifyYoungNormal;
} else {
gc_string.append("(Mixed)");
verify_type = G1HeapVerifier::G1VerifyMixed;
}
GCTraceTime(Info, gc) tm(gc_string, NULL, gc_cause(), true);

uint active_workers = AdaptiveSizePolicy::calc_active_workers(workers()->total_workers(),
workers()->active_workers(),
Threads::number_of_non_daemon_threads());
active_workers = workers()->update_active_workers(active_workers);
log_info(gc,task)("Using %u workers of %u for evacuation", active_workers, workers()->total_workers());

TraceCollectorStats tcs(g1mm()->incremental_collection_counters());
TraceMemoryManagerStats tms(&_memory_manager, gc_cause(),
collector_state()->yc_type() == Mixed /* allMemoryPoolsAffected */);

G1HeapTransition heap_transition(this);
size_t heap_used_bytes_before_gc = used();

// Don't dynamically change the number of GC threads this early. A value of
// 0 is used to indicate serial work. When parallel work is done,
// it will be set.

{ // Call to jvmpi::post_class_unload_events must occur outside of active GC
IsGCActiveMark x;

gc_prologue(false);

if (VerifyRememberedSets) {
log_info(gc, verify)("[Verifying RemSets before GC]");
VerifyRegionRemSetClosure v_cl;
heap_region_iterate(&v_cl);
}

_verifier->verify_before_gc(verify_type);

_verifier->check_bitmaps("GC Start");

#if COMPILER2_OR_JVMCI
DerivedPointerTable::clear();
#endif

// Please see comment in g1CollectedHeap.hpp and
// G1CollectedHeap::ref_processing_init() to see how
// reference processing currently works in G1.

// Enable discovery in the STW reference processor
_ref_processor_stw->enable_discovery();

{
// We want to temporarily turn off discovery by the
// CM ref processor, if necessary, and turn it back on
// on again later if we do. Using a scoped
// NoRefDiscovery object will do this.
NoRefDiscovery no_cm_discovery(_ref_processor_cm);

// Forget the current alloc region (we might even choose it to be part
// of the collection set!).
_allocator->release_mutator_alloc_region();

// This timing is only used by the ergonomics to handle our pause target.
// It is unclear why this should not include the full pause. We will
// investigate this in CR 7178365.
//
// Preserving the old comment here if that helps the investigation:
//
// The elapsed time induced by the start time below deliberately elides
// the possible verification above.
double sample_start_time_sec = os::elapsedTime();

g1_policy()->record_collection_pause_start(sample_start_time_sec);

if (collector_state()->in_initial_mark_gc()) {
concurrent_mark()->pre_initial_mark();
}

g1_policy()->finalize_collection_set(target_pause_time_ms, &_survivor);

evacuation_info.set_collectionset_regions(collection_set()->region_length());

// Make sure the remembered sets are up to date. This needs to be
// done before register_humongous_regions_with_cset(), because the
// remembered sets are used there to choose eager reclaim candidates.
// If the remembered sets are not up to date we might miss some
// entries that need to be handled.
g1_rem_set()->cleanupHRRS();

register_humongous_regions_with_cset();

assert(_verifier->check_cset_fast_test(), "Inconsistency in the InCSetState table.");

// We call this after finalize_cset() to
// ensure that the CSet has been finalized.
_cm->verify_no_cset_oops();

if (_hr_printer.is_active()) {
G1PrintCollectionSetClosure cl(&_hr_printer);
_collection_set.iterate(&cl);
}

// Initialize the GC alloc regions.
_allocator->init_gc_alloc_regions(evacuation_info);

G1ParScanThreadStateSet per_thread_states(this, workers()->active_workers(), collection_set()->young_region_length());
pre_evacuate_collection_set();

// Actually do the work...
evacuate_collection_set(&per_thread_states);

post_evacuate_collection_set(evacuation_info, &per_thread_states);

const size_t* surviving_young_words = per_thread_states.surviving_young_words();
free_collection_set(&_collection_set, evacuation_info, surviving_young_words);

eagerly_reclaim_humongous_regions();

record_obj_copy_mem_stats();
_survivor_evac_stats.adjust_desired_plab_sz();
_old_evac_stats.adjust_desired_plab_sz();

double start = os::elapsedTime();
start_new_collection_set();
g1_policy()->phase_times()->record_start_new_cset_time_ms((os::elapsedTime() - start) * 1000.0);

if (evacuation_failed()) {
set_used(recalculate_used());
if (_archive_allocator != NULL) {
_archive_allocator->clear_used();
}
for (uint i = 0; i < ParallelGCThreads; i++) {
if (_evacuation_failed_info_array[i].has_failed()) {
_gc_tracer_stw->report_evacuation_failed(_evacuation_failed_info_array[i]);
}
}
} else {
// The "used" of the the collection set have already been subtracted
// when they were freed. Add in the bytes evacuated.
increase_used(g1_policy()->bytes_copied_during_gc());
}

if (collector_state()->in_initial_mark_gc()) {
// We have to do this before we notify the CM threads that
// they can start working to make sure that all the
// appropriate initialization is done on the CM object.
concurrent_mark()->post_initial_mark();
// Note that we don't actually trigger the CM thread at
// this point. We do that later when we're sure that
// the current thread has completed its logging output.
}

allocate_dummy_regions();

_allocator->init_mutator_alloc_region();

{
size_t expand_bytes = _heap_sizing_policy->expansion_amount();
if (expand_bytes > 0) {
size_t bytes_before = capacity();
// No need for an ergo logging here,
// expansion_amount() does this when it returns a value > 0.
double expand_ms;
if (!expand(expand_bytes, _workers, &expand_ms)) {
// We failed to expand the heap. Cannot do anything about it.
}
g1_policy()->phase_times()->record_expand_heap_time(expand_ms);
}
}

// We redo the verification but now wrt to the new CSet which
// has just got initialized after the previous CSet was freed.
_cm->verify_no_cset_oops();

// This timing is only used by the ergonomics to handle our pause target.
// It is unclear why this should not include the full pause. We will
// investigate this in CR 7178365.
double sample_end_time_sec = os::elapsedTime();
double pause_time_ms = (sample_end_time_sec - sample_start_time_sec) * MILLIUNITS;
size_t total_cards_scanned = g1_policy()->phase_times()->sum_thread_work_items(G1GCPhaseTimes::ScanRS, G1GCPhaseTimes::ScanRSScannedCards);
g1_policy()->record_collection_pause_end(pause_time_ms, total_cards_scanned, heap_used_bytes_before_gc);

evacuation_info.set_collectionset_used_before(collection_set()->bytes_used_before());
evacuation_info.set_bytes_copied(g1_policy()->bytes_copied_during_gc());

if (VerifyRememberedSets) {
log_info(gc, verify)("[Verifying RemSets after GC]");
VerifyRegionRemSetClosure v_cl;
heap_region_iterate(&v_cl);
}

_verifier->verify_after_gc(verify_type);
_verifier->check_bitmaps("GC End");

assert(!_ref_processor_stw->discovery_enabled(), "Postcondition");
_ref_processor_stw->verify_no_references_recorded();

// CM reference discovery will be re-enabled if necessary.
}

#ifdef TRACESPINNING
ParallelTaskTerminator::print_termination_counts();
#endif

gc_epilogue(false);
}

// Print the remainder of the GC log output.
if (evacuation_failed()) {
log_info(gc)("To-space exhausted");
}

g1_policy()->print_phases();
heap_transition.print();

// It is not yet to safe to tell the concurrent mark to
// start as we have some optional output below. We don't want the
// output from the concurrent mark thread interfering with this
// logging output either.

_hrm.verify_optional();
_verifier->verify_region_sets_optional();

TASKQUEUE_STATS_ONLY(print_taskqueue_stats());
TASKQUEUE_STATS_ONLY(reset_taskqueue_stats());

print_heap_after_gc();
print_heap_regions();
trace_heap_after_gc(_gc_tracer_stw);

// We must call G1MonitoringSupport::update_sizes() in the same scoping level
// as an active TraceMemoryManagerStats object (i.e. before the destructor for the
// TraceMemoryManagerStats is called) so that the G1 memory pools are updated
// before any GC notifications are raised.
g1mm()->update_sizes();

_gc_tracer_stw->report_evacuation_info(&evacuation_info);
_gc_tracer_stw->report_tenuring_threshold(_g1_policy->tenuring_threshold());
_gc_timer_stw->register_gc_end();
_gc_tracer_stw->report_gc_end(_gc_timer_stw->gc_end(), _gc_timer_stw->time_partitions());
}
// It should now be safe to tell the concurrent mark thread to start
// without its logging output interfering with the logging output
// that came from the pause.

if (should_start_conc_mark) {
// CAUTION: after the doConcurrentMark() call below,
// the concurrent marking thread(s) could be running
// concurrently with us. Make sure that anything after
// this point does not assume that we are the only GC thread
// running. Note: of course, the actual marking work will
// not start until the safepoint itself is released in
// SuspendibleThreadSet::desynchronize().
do_concurrent_mark();
}

return true;
}

往下走就是这一步G1Policy::finalize_collection_set,去处理新生代和老年代

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void G1Policy::finalize_collection_set(double target_pause_time_ms, G1SurvivorRegions* survivor) {
double time_remaining_ms = _collection_set->finalize_young_part(target_pause_time_ms, survivor);
_collection_set->finalize_old_part(time_remaining_ms);
}

这里分别调用了两个方法,可以看到剩余时间是往下传的,来看一下具体的方法

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double G1CollectionSet::finalize_young_part(double target_pause_time_ms, G1SurvivorRegions* survivors) {
double young_start_time_sec = os::elapsedTime();

finalize_incremental_building();

guarantee(target_pause_time_ms > 0.0,
"target_pause_time_ms = %1.6lf should be positive", target_pause_time_ms);

size_t pending_cards = _policy->pending_cards();
double base_time_ms = _policy->predict_base_elapsed_time_ms(pending_cards);
double time_remaining_ms = MAX2(target_pause_time_ms - base_time_ms, 0.0);

log_trace(gc, ergo, cset)("Start choosing CSet. pending cards: " SIZE_FORMAT " predicted base time: %1.2fms remaining time: %1.2fms target pause time: %1.2fms",
pending_cards, base_time_ms, time_remaining_ms, target_pause_time_ms);

// The young list is laid with the survivor regions from the previous
// pause are appended to the RHS of the young list, i.e.
// [Newly Young Regions ++ Survivors from last pause].

uint survivor_region_length = survivors->length();
uint eden_region_length = _g1h->eden_regions_count();
init_region_lengths(eden_region_length, survivor_region_length);

verify_young_cset_indices();

// Clear the fields that point to the survivor list - they are all young now.
survivors->convert_to_eden();

_bytes_used_before = _inc_bytes_used_before;
time_remaining_ms = MAX2(time_remaining_ms - _inc_predicted_elapsed_time_ms, 0.0);

log_trace(gc, ergo, cset)("Add young regions to CSet. eden: %u regions, survivors: %u regions, predicted young region time: %1.2fms, target pause time: %1.2fms",
eden_region_length, survivor_region_length, _inc_predicted_elapsed_time_ms, target_pause_time_ms);

// The number of recorded young regions is the incremental
// collection set's current size
set_recorded_rs_lengths(_inc_recorded_rs_lengths);

double young_end_time_sec = os::elapsedTime();
phase_times()->record_young_cset_choice_time_ms((young_end_time_sec - young_start_time_sec) * 1000.0);

return time_remaining_ms;
}

下面是老年代的部分

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void G1CollectionSet::finalize_old_part(double time_remaining_ms) {
double non_young_start_time_sec = os::elapsedTime();
double predicted_old_time_ms = 0.0;

if (collector_state()->in_mixed_phase()) {
cset_chooser()->verify();
const uint min_old_cset_length = _policy->calc_min_old_cset_length();
const uint max_old_cset_length = _policy->calc_max_old_cset_length();

uint expensive_region_num = 0;
bool check_time_remaining = _policy->adaptive_young_list_length();

HeapRegion* hr = cset_chooser()->peek();
while (hr != NULL) {
if (old_region_length() >= max_old_cset_length) {
// Added maximum number of old regions to the CSet.
log_debug(gc, ergo, cset)("Finish adding old regions to CSet (old CSet region num reached max). old %u regions, max %u regions",
old_region_length(), max_old_cset_length);
break;
}

// Stop adding regions if the remaining reclaimable space is
// not above G1HeapWastePercent.
size_t reclaimable_bytes = cset_chooser()->remaining_reclaimable_bytes();
double reclaimable_percent = _policy->reclaimable_bytes_percent(reclaimable_bytes);
double threshold = (double) G1HeapWastePercent;
if (reclaimable_percent <= threshold) {
// We've added enough old regions that the amount of uncollected
// reclaimable space is at or below the waste threshold. Stop
// adding old regions to the CSet.
log_debug(gc, ergo, cset)("Finish adding old regions to CSet (reclaimable percentage not over threshold). "
"old %u regions, max %u regions, reclaimable: " SIZE_FORMAT "B (%1.2f%%) threshold: " UINTX_FORMAT "%%",
old_region_length(), max_old_cset_length, reclaimable_bytes, reclaimable_percent, G1HeapWastePercent);
break;
}

double predicted_time_ms = predict_region_elapsed_time_ms(hr);
if (check_time_remaining) {
if (predicted_time_ms > time_remaining_ms) {
// Too expensive for the current CSet.

if (old_region_length() >= min_old_cset_length) {
// We have added the minimum number of old regions to the CSet,
// we are done with this CSet.
log_debug(gc, ergo, cset)("Finish adding old regions to CSet (predicted time is too high). "
"predicted time: %1.2fms, remaining time: %1.2fms old %u regions, min %u regions",
predicted_time_ms, time_remaining_ms, old_region_length(), min_old_cset_length);
break;
}

// We'll add it anyway given that we haven't reached the
// minimum number of old regions.
expensive_region_num += 1;
}
} else {
if (old_region_length() >= min_old_cset_length) {
// In the non-auto-tuning case, we'll finish adding regions
// to the CSet if we reach the minimum.

log_debug(gc, ergo, cset)("Finish adding old regions to CSet (old CSet region num reached min). old %u regions, min %u regions",
old_region_length(), min_old_cset_length);
break;
}
}

// We will add this region to the CSet.
time_remaining_ms = MAX2(time_remaining_ms - predicted_time_ms, 0.0);
predicted_old_time_ms += predicted_time_ms;
cset_chooser()->pop(); // already have region via peek()
_g1h->old_set_remove(hr);
add_old_region(hr);

hr = cset_chooser()->peek();
}
if (hr == NULL) {
log_debug(gc, ergo, cset)("Finish adding old regions to CSet (candidate old regions not available)");
}

if (expensive_region_num > 0) {
// We print the information once here at the end, predicated on
// whether we added any apparently expensive regions or not, to
// avoid generating output per region.
log_debug(gc, ergo, cset)("Added expensive regions to CSet (old CSet region num not reached min)."
"old: %u regions, expensive: %u regions, min: %u regions, remaining time: %1.2fms",
old_region_length(), expensive_region_num, min_old_cset_length, time_remaining_ms);
}

cset_chooser()->verify();
}

stop_incremental_building();

log_debug(gc, ergo, cset)("Finish choosing CSet. old: %u regions, predicted old region time: %1.2fms, time remaining: %1.2f",
old_region_length(), predicted_old_time_ms, time_remaining_ms);

double non_young_end_time_sec = os::elapsedTime();
phase_times()->record_non_young_cset_choice_time_ms((non_young_end_time_sec - non_young_start_time_sec) * 1000.0);

QuickSort::sort(_collection_set_regions, _collection_set_cur_length, compare_region_idx, true);
}

上面第三行是个判断,当前是否是 mixed 回收阶段,如果不是的话其实是没有老年代什么事的,所以可以看到代码基本是从这个 if 判断
if (collector_state()->in_mixed_phase()) {开始往下走的
先写到这,偏向于做笔记用,有错轻拍

请我喝杯咖啡