介绍 G1GC 的文章很多，但是读完之后都感觉写得不够透彻，所以笔者决定直接读 G1 的源代码，这一系列文章就是 G1 代码的阅读笔记。
由于 gdb/lldb 的功力不足，所以选择了日志打印比较详细的 openjdk-jdk10。
先从最常见的 Pause Young (G1 Evacuation Pause) 开始吧。
0. 封装类
Evacuation Pause 的逻辑封装在 VM_G1IncCollectionPause 类中，其中的处理逻辑包含了 Young GC，Mixed GC 和 Initial Mark。
// share/gc/g1/vm_operations_g1.hpp#L80
class VM_G1IncCollectionPause: public VM_G1OperationWithAllocRequest {
private:
bool _should_initiate_conc_mark;
bool _should_retry_gc;
double _target_pause_time_ms;
uint _old_marking_cycles_completed_before;
public:
VM_G1IncCollectionPause(uint gc_count_before,
size_t word_size,
bool should_initiate_conc_mark,
double target_pause_time_ms,
GCCause::Cause gc_cause);
virtual VMOp_Type type() const { return VMOp_G1IncCollectionPause; }
virtual bool doit_prologue();
virtual void doit();
virtual void doit_epilogue();
virtual const char* name() const {
return "garbage-first incremental collection pause";
}
bool should_retry_gc() const { return _should_retry_gc; }
};
1. Pause Young (G1 Evacuation Pause) 触发流程
我们暂且忽略其它情况，只是按照 Pause Young (G1 Evacuation Pause) 追踪代码执行。
触发 Pause Young (G1 Evacuation Pause) 的大致流程如下：
Universe::heap()->mem_allocate(size, &gc_overhead_limit_was_exceeded);
attempt_allocation(word_size, &gc_count_before, &gclocker_retry_count);
attempt_allocation_slow(word_size, context, gc_count_before_ret, gclocker_retry_count_ret);
do_collection_pause(word_size, gc_count_before, &succeeded, GCCause::_g1_inc_collection_pause);
如果两次尝试分配内存失败，并且满足 should_try_gc 的条件，那么就会执行 VM_G1IncCollectionPause。
G1CollectedHeap::do_collection_pause
// share/gc/g1/g1CollectedHeap.cpp#L2617
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_G1IncCollectionPause op(gc_count_before,
word_size,
false, /* should_initiate_conc_mark */
g1_policy()->max_pause_time_ms(),
gc_cause);
op.set_allocation_context(AllocationContext::current());
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;
}
具体逻辑在 doit 方法中：
VM_G1IncCollectionPause::doit
// share/gc/g1/vm_operations_g1.cpp#L90
void VM_G1IncCollectionPause::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,
allocation_context(),
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;
}
}
_pause_succeeded =
g1h->do_collection_pause_at_safepoint(_target_pause_time_ms);
if (_pause_succeeded && _word_size > 0) {
// An allocation had been requested.
_result = g1h->attempt_allocation_at_safepoint(_word_size,
allocation_context(),
true /* expect_null_cur_alloc_region */);
} else {
assert(_result == NULL, "invariant");
if (!_pause_succeeded) {
// Another possible reason reason for the pause to not be successful
// is that, again, the GC locker is active (and 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;
}
}
}
首先又尝试分配了一次，如果还申请不到内存，那么就开始进行垃圾回收了。
G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms)
下面我们对照 GC 日志一步一步跟踪 Young GC 的逻辑。
3. Phase 1 -- Pre Evacuate Collection Set
GC(6) Pre Evacuate Collection Set: 0.2ms
GC(6) Prepare TLABs: 0.3ms
GC(6) Choose Collection Set: 0.0ms
GC(6) Humongous Register: 0.2ms
GC(6) Humongous Total: 128
GC(6) Humongous Candidate: 128
a. Prepare TLAB
// share/gc/g1/g1CollectedHeap.cpp#L2581
double start = os::elapsedTime();
accumulate_statistics_all_tlabs();
ensure_parsability(true);
g1_policy()->phase_times()->record_prepare_tlab_time_ms((os::elapsedTime() - start) * 1000.0);
将 TLAB 处理为可 parse 的
b. 释放 mutator 分配的 region
// share/gc/g1/g1CollectedHeap.cpp#L5207
void G1CollectedHeap::retire_mutator_alloc_region(HeapRegion* alloc_region,
size_t allocated_bytes) {
assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);
assert(alloc_region->is_eden(), "all mutator alloc regions should be eden");
collection_set()->add_eden_region(alloc_region);
increase_used(allocated_bytes);
_hr_printer.retire(alloc_region);
// We update the eden sizes here, when the region is retired,
// instead of when it's allocated, since this is the point that its
// used space has been recored in _summary_bytes_used.
g1mm()->update_eden_size();
}
核心代码是将 region 放到 CSet 中
c. 选择 CSet
// share/gc/g1/g1CollectionSet.cpp#L354
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);
collector_state()->set_last_gc_was_young(collector_state()->gcs_are_young());
// 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 = _g1->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;
}
Young GC 的 CSet 是 Eden 和 Survivor 区，实际已经在 CSet 中了，所以这部分基本不耗时。
这部分还有一块核心的代码，基于 dirtyCard 和 RememberSet 预估 pause time。
d. 筛选满足条件的 Humongous Region
// share/gc/g1/g1CollectedHeap.cpp#L2663
bool humongous_region_is_candidate(G1CollectedHeap* heap, HeapRegion* region) const {
assert(region->is_starts_humongous(), "Must start a humongous object");
oop obj = oop(region->bottom());
// Dead objects cannot be eager reclaim candidates. Due to class
// unloading it is unsafe to query their classes so we return early.
if (heap->is_obj_dead(obj, region)) {
return false;
}
// Candidate selection must satisfy the following constraints
// while concurrent marking is in progress:
//
// * In order to maintain SATB invariants, an object must not be
// reclaimed if it was allocated before the start of marking and
// has not had its references scanned. Such an object must have
// its references (including type metadata) scanned to ensure no
// live objects are missed by the marking process. Objects
// allocated after the start of concurrent marking don't need to
// be scanned.
//
// * An object must not be reclaimed if it is on the concurrent
// mark stack. Objects allocated after the start of concurrent
// marking are never pushed on the mark stack.
//
// Nominating only objects allocated after the start of concurrent
// marking is sufficient to meet both constraints. This may miss
// some objects that satisfy the constraints, but the marking data
// structures don't support efficiently performing the needed
// additional tests or scrubbing of the mark stack.
//
// However, we presently only nominate is_typeArray() objects.
// A humongous object containing references induces remembered
// set entries on other regions. In order to reclaim such an
// object, those remembered sets would need to be cleaned up.
//
// We also treat is_typeArray() objects specially, allowing them
// to be reclaimed even if allocated before the start of
// concurrent mark. For this we rely on mark stack insertion to
// exclude is_typeArray() objects, preventing reclaiming an object
// that is in the mark stack. We also rely on the metadata for
// such objects to be built-in and so ensured to be kept live.
// Frequent allocation and drop of large binary blobs is an
// important use case for eager reclaim, and this special handling
// may reduce needed headroom.
return obj->is_typeArray() && is_remset_small(region);
}
也就是选择 array 类型并且 RemSet 小于等于 G1RSetSparseRegionEntries 的 region，加入 CSet (_humongous_reclaim_candidates) 中
4. Phase 2 -- Evacuate Collection Set
GC(6) Evacuate Collection Set: 17.2ms
GC(6) GC Worker Start (ms): Min: 73651.3, Avg: 73651.3, Max: 73651.3, Diff: 0.0, Workers: 2
GC(6) 73651.3 73651.3
GC(6) Ext Root Scanning (ms): Min: 0.9, Avg: 0.9, Max: 0.9, Diff: 0.0, Sum: 1.9, Workers: 2
GC(6) 0.9 0.9
GC(6) Thread Roots (ms): Min: 0.1, Avg: 0.3, Max: 0.4, Diff: 0.3, Sum: 0.5, Workers: 2
GC(6) 0.1 0.4
GC(6) StringTable Roots (ms): Min: 0.6, Avg: 0.6, Max: 0.6, Diff: 0.1, Sum: 1.2, Workers: 2
GC(6) 0.6 0.6
GC(6) Universe Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) JNI Handles Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) ObjectSynchronizer Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) Management Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) SystemDictionary Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.1, Workers: 2
GC(6) 0.1 0.0
GC(6) CLDG Roots (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.2, Workers: 2
GC(6) 0.2 0.0
GC(6) JVMTI Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) CM RefProcessor Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) Wait For Strong CLD (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) Weak CLD Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) SATB Filtering (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) Update RS (ms): Min: 2.1, Avg: 2.4, Max: 2.7, Diff: 0.6, Sum: 4.9, Workers: 2
GC(6) 2.1 2.7
GC(6) Processed Buffers: Min: 9, Avg: 10.0, Max: 11, Diff: 2, Sum: 20, Workers: 2
GC(6) 9 11
GC(6) Scanned Cards: Min: 1684, Avg: 1809.5, Max: 1935, Diff: 251, Sum: 3619, Workers: 2
GC(6) 1935 1684
GC(6) Skipped Cards: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0, Workers: 2
GC(6) 0 0
GC(6) Scan HCC (ms): Min: 0.7, Avg: 0.7, Max: 0.7, Diff: 0.0, Sum: 1.4, Workers: 2
GC(6) 0.7 0.7
GC(6) Scan RS (ms): Min: 5.1, Avg: 5.1, Max: 5.2, Diff: 0.1, Sum: 10.3, Workers: 2
GC(6) 5.2 5.1
GC(6) Scanned Cards: Min: 4019, Avg: 4362.0, Max: 4705, Diff: 686, Sum: 8724, Workers: 2
GC(6) 4705 4019
GC(6) Claimed Cards: Min: 19702, Avg: 23614.5, Max: 27527, Diff: 7825, Sum: 47229, Workers: 2
GC(6) 19702 27527
GC(6) Skipped Cards: Min: 5000, Avg: 5125.0, Max: 5250, Diff: 250, Sum: 10250, Workers: 2
GC(6) 5000 5250
GC(6) Code Root Scanning (ms): Min: 0.1, Avg: 0.4, Max: 0.7, Diff: 0.6, Sum: 0.8, Workers: 2
GC(6) 0.7 0.1
GC(6) AOT Root Scanning (ms): skipped
GC(6) - -
GC(6) Object Copy (ms): Min: 7.9, Avg: 7.9, Max: 7.9, Diff: 0.0, Sum: 15.7, Workers: 2
GC(6) 7.9 7.9
GC(6) Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 2, Workers: 2
GC(6) 1 1
GC(6) GC Worker Other (ms): Min: 0.1, Avg: 0.1, Max: 0.2, Diff: 0.1, Sum: 0.3, Workers: 2
GC(6) 0.1 0.2
GC(6) GC Worker Total (ms): Min: 16.9, Avg: 16.9, Max: 17.0, Diff: 0.1, Sum: 33.8, Workers: 2
GC(6) 16.9 17.0
GC(6) GC Worker End (ms): Min: 73668.2, Avg: 73668.2, Max: 73668.3, Diff: 0.1, Workers: 2
GC(6) 73668.2 73668.3
下面开始执行垃圾收集了，这部分逻辑封装在 G1ParTask 和 G1RootProcessor 中，由 ParallelGCThreads 个 "GC Thread #" 线程并行执行。(或者通过 UseDynamicNumberOfGCThreads 设置动态线程数)
a. Ext Root Scanning
// share/gc/g1/g1RootProcessor.cpp#L75
void G1RootProcessor::evacuate_roots(G1EvacuationRootClosures* closures, uint worker_i) {
double ext_roots_start = os::elapsedTime();
G1GCPhaseTimes* phase_times = _g1h->g1_policy()->phase_times();
process_java_roots(closures, phase_times, worker_i);
// This is the point where this worker thread will not find more strong CLDs/nmethods.
// Report this so G1 can synchronize the strong and weak CLDs/nmethods processing.
if (closures->trace_metadata()) {
worker_has_discovered_all_strong_classes();
}
process_vm_roots(closures, phase_times, worker_i);
process_string_table_roots(closures, phase_times, worker_i);
{
// Now the CM ref_processor roots.
G1GCParPhaseTimesTracker x(phase_times, G1GCPhaseTimes::CMRefRoots, worker_i);
if (!_process_strong_tasks.is_task_claimed(G1RP_PS_refProcessor_oops_do)) {
// We need to treat the discovered reference lists of the
// concurrent mark ref processor as roots and keep entries
// (which are added by the marking threads) on them live
// until they can be processed at the end of marking.
_g1h->ref_processor_cm()->weak_oops_do(closures->strong_oops());
}
}
if (closures->trace_metadata()) {
{
G1GCParPhaseTimesTracker x(phase_times, G1GCPhaseTimes::WaitForStrongCLD, worker_i);
// Barrier to make sure all workers passed
// the strong CLD and strong nmethods phases.
wait_until_all_strong_classes_discovered();
}
// Now take the complement of the strong CLDs.
G1GCParPhaseTimesTracker x(phase_times, G1GCPhaseTimes::WeakCLDRoots, worker_i);
assert(closures->second_pass_weak_clds() != NULL, "Should be non-null if we are tracing metadata.");
ClassLoaderDataGraph::roots_cld_do(NULL, closures->second_pass_weak_clds());
} else {
phase_times->record_time_secs(G1GCPhaseTimes::WaitForStrongCLD, worker_i, 0.0);
phase_times->record_time_secs(G1GCPhaseTimes::WeakCLDRoots, worker_i, 0.0);
assert(closures->second_pass_weak_clds() == NULL, "Should be null if not tracing metadata.");
}
// Finish up any enqueued closure apps (attributed as object copy time).
closures->flush();
double obj_copy_time_sec = closures->closure_app_seconds();
phase_times->record_time_secs(G1GCPhaseTimes::ObjCopy, worker_i, obj_copy_time_sec);
double ext_root_time_sec = os::elapsedTime() - ext_roots_start - obj_copy_time_sec;
phase_times->record_time_secs(G1GCPhaseTimes::ExtRootScan, worker_i, ext_root_time_sec);
// During conc marking we have to filter the per-thread SATB buffers
// to make sure we remove any oops into the CSet (which will show up
// as implicitly live).
{
G1GCParPhaseTimesTracker x(phase_times, G1GCPhaseTimes::SATBFiltering, worker_i);
if (!_process_strong_tasks.is_task_claimed(G1RP_PS_filter_satb_buffers) && _g1h->collector_state()->mark_in_progress()) {
JavaThread::satb_mark_queue_set().filter_thread_buffers();
}
}
_process_strong_tasks.all_tasks_completed(n_workers());
}
// share/gc/g1/g1OopClosures.inline.hpp#L220
template <G1Barrier barrier, G1Mark do_mark_object, bool use_ext>
template <class T>
void G1ParCopyClosure<barrier, do_mark_object, use_ext>::do_oop_work(T* p) {
T heap_oop = oopDesc::load_heap_oop(p);
if (oopDesc::is_null(heap_oop)) {
return;
}
oop obj = oopDesc::decode_heap_oop_not_null(heap_oop);
assert(_worker_id == _par_scan_state->worker_id(), "sanity");
const InCSetState state = _g1->in_cset_state(obj);
if (state.is_in_cset()) {
oop forwardee;
markOop m = obj->mark();
if (m->is_marked()) {
forwardee = (oop) m->decode_pointer();
} else {
forwardee = _par_scan_state->copy_to_survivor_space(state, obj, m);
}
assert(forwardee != NULL, "forwardee should not be NULL");
oopDesc::encode_store_heap_oop(p, forwardee);
if (do_mark_object != G1MarkNone && forwardee != obj) {
// If the object is self-forwarded we don't need to explicitly
// mark it, the evacuation failure protocol will do so.
mark_forwarded_object(obj, forwardee);
}
if (barrier == G1BarrierCLD) {
do_cld_barrier(forwardee);
}
} else {
if (state.is_humongous()) {
_g1->set_humongous_is_live(obj);
}
if (use_ext && state.is_ext()) {
_par_scan_state->do_oop_ext(p);
}
// The object is not in collection set. If we're a root scanning
// closure during an initial mark pause then attempt to mark the object.
if (do_mark_object == G1MarkFromRoot) {
mark_object(obj);
}
}
}
首先从 root 进行扫描、复制 obj
b. Update RS 和 Scan RS
// share/gc/g1/g1RemSet.cpp#L494
void G1RemSet::oops_into_collection_set_do(G1ParScanThreadState* pss,
CodeBlobClosure* heap_region_codeblobs,
uint worker_i) {
update_rem_set(pss, worker_i);
scan_rem_set(pss, heap_region_codeblobs, worker_i);;
}
更新扫描 Remember Set，将待处理的引用加入队列。
c. Object Copy
// share/gc/g1/g1ParScanThreadState.inline.hpp#L117
template <class T> inline void G1ParScanThreadState::deal_with_reference(T* ref_to_scan) {
if (!has_partial_array_mask(ref_to_scan)) {
HeapRegion* r = _g1h->heap_region_containing(ref_to_scan);
do_oop_evac(ref_to_scan, r);
} else {
do_oop_partial_array((oop*)ref_to_scan);
}
}
从 RefToScanQueue 中取任务来处理
// share/gc/g1/g1ParScanThreadState.inline.hpp#L32
template <class T> void G1ParScanThreadState::do_oop_evac(T* p, HeapRegion* from) {
assert(!oopDesc::is_null(oopDesc::load_decode_heap_oop(p)),
"Reference should not be NULL here as such are never pushed to the task queue.");
oop obj = oopDesc::load_decode_heap_oop_not_null(p);
// Although we never intentionally push references outside of the collection
// set, due to (benign) races in the claim mechanism during RSet scanning more
// than one thread might claim the same card. So the same card may be
// processed multiple times. So redo this check.
const InCSetState in_cset_state = _g1h->in_cset_state(obj);
if (in_cset_state.is_in_cset()) {
markOop m = obj->mark();
if (m->is_marked()) {
obj = (oop) m->decode_pointer();
} else {
obj = copy_to_survivor_space(in_cset_state, obj, m);
}
oopDesc::encode_store_heap_oop(p, obj);
} else if (in_cset_state.is_humongous()) {
_g1h->set_humongous_is_live(obj);
} else {
assert(in_cset_state.is_default() || in_cset_state.is_ext(),
"In_cset_state must be NotInCSet or Ext here, but is " CSETSTATE_FORMAT, in_cset_state.value())
}
assert(obj != NULL, "Must be");
if (!HeapRegion::is_in_same_region(p, obj)) {
update_rs(from, p, obj);
}
}
如果在 CSet 中并且没有被 mark，那么就复制到 survivor 区，或者 promote 到 old 区。
5. Phase 3 -- Post Evacuate Collection Set
GC(6) Post Evacuate Collection Set: 3.7ms
GC(6) Code Roots Fixup: 0.0ms
GC(6) Preserve CM Refs: 0.0ms
GC(6) Parallel Preserve CM Refs (ms): skipped
GC(6) - -
GC(6) Reference Processing: 0.0ms
GC(6) SoftReference: 0.5ms
GC(6) Balance queues: 0.0ms
GC(6) Phase1: 0.2ms
GC(6) Process lists (ms) Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) Phase2: 0.1ms
GC(6) Process lists (ms) Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) Phase3: 0.1ms
GC(6) Process lists (ms) Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) Discovered: 0
GC(6) Cleared: 0
GC(6) WeakReference: 0.3ms
GC(6) Balance queues: 0.0ms
GC(6) Phase2: 0.1ms
GC(6) Process lists (ms) Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) Phase3: 0.1ms
GC(6) Process lists (ms) Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) Discovered: 23
GC(6) Cleared: 11
GC(6) FinalReference: 0.3ms
GC(6) Balance queues: 0.0ms
GC(6) Phase2: 0.2ms
GC(6) Process lists (ms) Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) Phase3: 0.1ms
GC(6) Process lists (ms) Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) Discovered: 0
GC(6) Cleared: 0
GC(6) PhantomReference: 0.3ms
GC(6) Balance queues: 0.0ms
GC(6) Phase2: 0.2ms
GC(6) Process lists (ms) Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) Phase3: 0.1ms
GC(6) Process lists (ms) Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) Discovered: 24
GC(6) Cleared: 15
GC(6) Clear Card Table: 0.2ms
GC(6) Reference Enqueuing: 0.1ms
GC(6) Reference Counts: Soft: 0 Weak: 12 Final: 0 Phantom: 9
GC(6) Merge Per-Thread State: 0.0ms
GC(6) Code Roots Purge: 0.0ms
GC(6) Redirty Cards: 0.2ms
GC(6) Parallel Redirty (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 2
GC(6) 0.0 0.0
GC(6) Redirtied Cards: Min: 0, Avg: 1843.0, Max: 3686, Diff: 3686, Sum: 3686, Workers: 2
GC(6) 3686 0
GC(6) DerivedPointerTable Update: 0.0ms
GC(6) Free Collection Set: 1.1ms
GC(6) Free Collection Set Serial: 0.1ms
GC(6) Young Free Collection Set (ms): Min: 0.2, Avg: 0.2, Max: 0.3, Diff: 0.1, Sum: 0.5, Workers: 2
GC(6) 0.2 0.3
GC(6) Non-Young Free Collection Set (ms): skipped
GC(6) - -
GC(6) Humongous Reclaim: 2.0ms
GC(6) Humongous Reclaimed: 0
GC(6) Start New Collection Set: 0.0ms
GC(6) Resize TLABs: 0.3ms
GC(6) Expand Heap After Collection: 0.0ms
a. Reference Processing
// share/gc/g1/g1CollectedHeap.cpp#L4371
void G1CollectedHeap::post_evacuate_collection_set(EvacuationInfo& evacuation_info, G1ParScanThreadStateSet* per_thread_states) {
// Process any discovered reference objects - we have
// to do this _before_ we retire the GC alloc regions
// as we may have to copy some 'reachable' referent
// objects (and their reachable sub-graphs) that were
// not copied during the pause.
if (g1_policy()->should_process_references()) {
preserve_cm_referents(per_thread_states);
process_discovered_references(per_thread_states);
} else {
ref_processor_stw()->verify_no_references_recorded();
}
G1STWIsAliveClosure is_alive(this);
G1KeepAliveClosure keep_alive(this);
{
double start = os::elapsedTime();
WeakProcessor::weak_oops_do(&is_alive, &keep_alive);
double time_ms = (os::elapsedTime() - start) * 1000.0;
g1_policy()->phase_times()->record_ref_proc_time(time_ms);
}
if (G1StringDedup::is_enabled()) {
double fixup_start = os::elapsedTime();
G1StringDedup::unlink_or_oops_do(&is_alive, &keep_alive, true, g1_policy()->phase_times());
double fixup_time_ms = (os::elapsedTime() - fixup_start) * 1000.0;
g1_policy()->phase_times()->record_string_dedup_fixup_time(fixup_time_ms);
}
g1_rem_set()->cleanup_after_oops_into_collection_set_do();
if (evacuation_failed()) {
restore_after_evac_failure();
// Reset the G1EvacuationFailureALot counters and flags
// Note: the values are reset only when an actual
// evacuation failure occurs.
NOT_PRODUCT(reset_evacuation_should_fail();)
}
_preserved_marks_set.assert_empty();
// Enqueue any remaining references remaining on the STW
// reference processor's discovered lists. We need to do
// this after the card table is cleaned (and verified) as
// the act of enqueueing entries on to the pending list
// will log these updates (and dirty their associated
// cards). We need these updates logged to update any
// RSets.
if (g1_policy()->should_process_references()) {
enqueue_discovered_references(per_thread_states);
} else {
g1_policy()->phase_times()->record_ref_enq_time(0);
}
_allocator->release_gc_alloc_regions(evacuation_info);
merge_per_thread_state_info(per_thread_states);
// Reset and re-enable the hot card cache.
// Note the counts for the cards in the regions in the
// collection set are reset when the collection set is freed.
_hot_card_cache->reset_hot_cache();
_hot_card_cache->set_use_cache(true);
purge_code_root_memory();
redirty_logged_cards();
#if COMPILER2_OR_JVMCI
double start = os::elapsedTime();
DerivedPointerTable::update_pointers();
g1_policy()->phase_times()->record_derived_pointer_table_update_time((os::elapsedTime() - start) * 1000.0);
#endif
g1_policy()->print_age_table();
}
处理弱引用
b. Free Collection Set
// share/gc/g1/g1CollectedHeap.cpp#L4794
void G1CollectedHeap::free_collection_set(G1CollectionSet* collection_set, EvacuationInfo& evacuation_info, const size_t* surviving_young_words) {
_eden.clear();
double free_cset_start_time = os::elapsedTime();
{
uint const num_chunks = MAX2(_collection_set.region_length() / G1FreeCollectionSetTask::chunk_size(), 1U);
uint const num_workers = MIN2(workers()->active_workers(), num_chunks);
G1FreeCollectionSetTask cl(collection_set, &evacuation_info, surviving_young_words);
log_debug(gc, ergo)("Running %s using %u workers for collection set length %u",
cl.name(),
num_workers,
_collection_set.region_length());
workers()->run_task(&cl, num_workers);
}
g1_policy()->phase_times()->record_total_free_cset_time_ms((os::elapsedTime() - free_cset_start_time) * 1000.0);
collection_set->clear();
}
c. Humongous Reclaim
// share/gc/g1/g1CollectedHeap.cpp#L4928
void G1CollectedHeap::eagerly_reclaim_humongous_regions() {
assert_at_safepoint(true);
if (!G1EagerReclaimHumongousObjects ||
(!_has_humongous_reclaim_candidates && !log_is_enabled(Debug, gc, humongous))) {
g1_policy()->phase_times()->record_fast_reclaim_humongous_time_ms(0.0, 0);
return;
}
double start_time = os::elapsedTime();
FreeRegionList local_cleanup_list("Local Humongous Cleanup List");
G1FreeHumongousRegionClosure cl(&local_cleanup_list);
heap_region_iterate(&cl);
remove_from_old_sets(0, cl.humongous_regions_reclaimed());
G1HRPrinter* hrp = hr_printer();
if (hrp->is_active()) {
FreeRegionListIterator iter(&local_cleanup_list);
while (iter.more_available()) {
HeapRegion* hr = iter.get_next();
hrp->cleanup(hr);
}
}
prepend_to_freelist(&local_cleanup_list);
decrement_summary_bytes(cl.bytes_freed());
g1_policy()->phase_times()->record_fast_reclaim_humongous_time_ms((os::elapsedTime() - start_time) * 1000.0,
cl.humongous_objects_reclaimed());
}
d. Start New Collection Set
// share/gc/g1/g1CollectedHeap.cpp#L2888
void G1CollectedHeap::start_new_collection_set() {
collection_set()->start_incremental_building();
clear_cset_fast_test();
guarantee(_eden.length() == 0, "eden should have been cleared");
g1_policy()->transfer_survivors_to_cset(survivor());
}
异常情况 -- To-space exhausted
// share/gc/g1/g1ParScanThreadState.cpp#L220
oop G1ParScanThreadState::copy_to_survivor_space(InCSetState const state,
oop const old,
markOop const old_mark) {
const size_t word_sz = old->size();
HeapRegion* const from_region = _g1h->heap_region_containing(old);
// +1 to make the -1 indexes valid...
const int young_index = from_region->young_index_in_cset()+1;
assert( (from_region->is_young() && young_index > 0) ||
(!from_region->is_young() && young_index == 0), "invariant" );
const AllocationContext_t context = from_region->allocation_context();
uint age = 0;
InCSetState dest_state = next_state(state, old_mark, age);
// The second clause is to prevent premature evacuation failure in case there
// is still space in survivor, but old gen is full.
if (_old_gen_is_full && dest_state.is_old()) {
return handle_evacuation_failure_par(old, old_mark);
}
HeapWord* obj_ptr = _plab_allocator->plab_allocate(dest_state, word_sz, context);
// PLAB allocations should succeed most of the time, so we'll
// normally check against NULL once and that's it.
if (obj_ptr == NULL) {
bool plab_refill_failed = false;
obj_ptr = _plab_allocator->allocate_direct_or_new_plab(dest_state, word_sz, context, &plab_refill_failed);
if (obj_ptr == NULL) {
obj_ptr = allocate_in_next_plab(state, &dest_state, word_sz, context, plab_refill_failed);
if (obj_ptr == NULL) {
// This will either forward-to-self, or detect that someone else has
// installed a forwarding pointer.
return handle_evacuation_failure_par(old, old_mark);
}
}
if (_g1h->_gc_tracer_stw->should_report_promotion_events()) {
// The events are checked individually as part of the actual commit
report_promotion_event(dest_state, old, word_sz, age, obj_ptr, context);
}
}
assert(obj_ptr != NULL, "when we get here, allocation should have succeeded");
assert(_g1h->is_in_reserved(obj_ptr), "Allocated memory should be in the heap");
#ifndef PRODUCT
// Should this evacuation fail?
if (_g1h->evacuation_should_fail()) {
// Doing this after all the allocation attempts also tests the
// undo_allocation() method too.
_plab_allocator->undo_allocation(dest_state, obj_ptr, word_sz, context);
return handle_evacuation_failure_par(old, old_mark);
}
#endif // !PRODUCT
// We're going to allocate linearly, so might as well prefetch ahead.
Prefetch::write(obj_ptr, PrefetchCopyIntervalInBytes);
const oop obj = oop(obj_ptr);
const oop forward_ptr = old->forward_to_atomic(obj);
if (forward_ptr == NULL) {
Copy::aligned_disjoint_words((HeapWord*) old, obj_ptr, word_sz);
if (dest_state.is_young()) {
if (age < markOopDesc::max_age) {
age++;
}
if (old_mark->has_displaced_mark_helper()) {
// In this case, we have to install the mark word first,
// otherwise obj looks to be forwarded (the old mark word,
// which contains the forward pointer, was copied)
obj->set_mark(old_mark);
markOop new_mark = old_mark->displaced_mark_helper()->set_age(age);
old_mark->set_displaced_mark_helper(new_mark);
} else {
obj->set_mark(old_mark->set_age(age));
}
_age_table.add(age, word_sz);
} else {
obj->set_mark(old_mark);
}
if (G1StringDedup::is_enabled()) {
const bool is_from_young = state.is_young();
const bool is_to_young = dest_state.is_young();
assert(is_from_young == _g1h->heap_region_containing(old)->is_young(),
"sanity");
assert(is_to_young == _g1h->heap_region_containing(obj)->is_young(),
"sanity");
G1StringDedup::enqueue_from_evacuation(is_from_young,
is_to_young,
_worker_id,
obj);
}
_surviving_young_words[young_index] += word_sz;
if (obj->is_objArray() && arrayOop(obj)->length() >= ParGCArrayScanChunk) {
// We keep track of the next start index in the length field of
// the to-space object. The actual length can be found in the
// length field of the from-space object.
arrayOop(obj)->set_length(0);
oop* old_p = set_partial_array_mask(old);
push_on_queue(old_p);
} else {
HeapRegion* const to_region = _g1h->heap_region_containing(obj_ptr);
_scanner.set_region(to_region);
obj->oop_iterate_backwards(&_scanner);
}
return obj;
} else {
_plab_allocator->undo_allocation(dest_state, obj_ptr, word_sz, context);
return forward_ptr;
}
}
To-space exhausted 产生的原因只有一个：无法为复制对象申请空间。
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