概述
collect_in_background由ConcurrentMarkSweepThread线程执行，默认每2秒钟检查一次是否要进行bacgroud gc，如果满足如下条件之一，则进行gc:
- 请求full gc(_full_gc_requested=true)
- UseCMSInitiatingOccupancyOnly=false：统计数据有效且完成一次cms gc的时间小于cms generation变满的时间或统计数据无效且cms的使用率大于CMSBootstrapOccupancy（默认50%）
- 老年代内存使用率大于_initiating_occupancy （默认为92%）
- UseCMSInitiatingOccupancyOnly＝false，且是因为分配对象时内存不足导致的扩容
JVM启动时会计算_initiating_occupancy：
void ConcurrentMarkSweepGeneration::init_initiating_occupancy(intx io, uintx tr) {
assert(io <= 100 && tr <= 100, "Check the arguments");
if (io >= 0) {
_initiating_occupancy = (double)io / 100.0;
} else {
_initiating_occupancy = ((100 - MinHeapFreeRatio) +
(double)(tr * MinHeapFreeRatio) / 100.0)
/ 100.0;
}
}
此处的io为CMSInitiatingOccupancyFraction(默认－1)，tr为CMSTriggerRatio(80)，MinHeapFreeRatio默认为40，因此默认_initiating_occupancy＝92%
CMS GC流程如下：
- 如果正在进行foregroud gc或UseAsyncConcMarkSweepGC＝false,则放弃执行；
- 判断是否需要卸载类:
- System.gc()并开启ExplicitGCInvokesConcurrentAndUnloadsClasses，则需要卸载类;
- 开启CMSClassUnloadingEnabled，且自上次卸载类之后发生gc的次数大于CMSClassUnloadingMaxInterval（默认0）或老年代内存使用率大于CMSIsTooFullPercentage（默认98）
从上面可以看到，默认情况下，一旦开启CMSClassUnloadingEnabled,必然会卸载类
- 正式开始执行cms gc，分步执行下列步骤:
- InitialMarking:
- Marking
- Precleaning
- AbortablePreclean
- FinalMarking
- Sweeping
- Resizing
- Resetting
InitialMarking
通过vm线程执行VM_CMS_Initial_Mark，由于必须到达安全点，因此需要暂停业务线程，最终调用的逻辑为：
void CMSParInitialMarkTask::work(uint worker_id) {
elapsedTimer _timer;
ResourceMark rm;
HandleMark hm;
// ---------- scan from roots --------------
_timer.start();
GenCollectedHeap* gch = GenCollectedHeap::heap();
Par_MarkRefsIntoClosure par_mri_cl(_collector->_span, &(_collector->_markBitMap));
CMKlassClosure klass_closure(&par_mri_cl);
// ---------- young gen roots --------------
{
work_on_young_gen_roots(worker_id, &par_mri_cl);
_timer.stop();
if (PrintCMSStatistics != 0) {
gclog_or_tty->print_cr(
"Finished young gen initial mark scan work in %dth thread: %3.3f sec",
worker_id, _timer.seconds());
}
}
// ---------- remaining roots --------------
_timer.reset();
_timer.start();
gch->gen_process_strong_roots(_collector->_cmsGen->level(),
false, // yg was scanned above
false, // this is parallel code
false, // not scavenging
SharedHeap::ScanningOption(_collector->CMSCollector::roots_scanning_options()),
&par_mri_cl,
true, // walk all of code cache if (so & SO_CodeCache)
NULL,
&klass_closure);
assert(_collector->should_unload_classes()
|| (_collector->CMSCollector::roots_scanning_options() & SharedHeap::SO_CodeCache),
"if we didn't scan the code cache, we have to be ready to drop nmethods with expired weak oops");
_timer.stop();
if (PrintCMSStatistics != 0) {
gclog_or_tty->print_cr(
"Finished remaining root initial mark scan work in %dth thread: %3.3f sec",
worker_id, _timer.seconds());
}
}
Marking
并发标记在ConcurrentMarkSweepThread线程执行,和业务线程并行，最终的执行逻辑为:
bool CMSCollector::do_marking_st(bool asynch) {
ResourceMark rm;
HandleMark hm;
// Temporarily make refs discovery single threaded (non-MT)
ReferenceProcessorMTDiscoveryMutator rp_mut_discovery(ref_processor(), false);
MarkFromRootsClosure markFromRootsClosure(this, _span, &_markBitMap,
&_markStack, CMSYield && asynch);
// the last argument to iterate indicates whether the iteration
// should be incremental with periodic yields.
_markBitMap.iterate(&markFromRootsClosure);
// If _restart_addr is non-NULL, a marking stack overflow
// occurred; we need to do a fresh iteration from the
// indicated restart address.
while (_restart_addr != NULL) {
if (_foregroundGCIsActive && asynch) {
// We may be running into repeated stack overflows, having
// reached the limit of the stack size, while making very
// slow forward progress. It may be best to bail out and
// let the foreground collector do its job.
// Clear _restart_addr, so that foreground GC
// works from scratch. This avoids the headache of
// a "rescan" which would otherwise be needed because
// of the dirty mod union table & card table.
_restart_addr = NULL;
return false; // indicating failure to complete marking
}
// Deal with stack overflow:
// we restart marking from _restart_addr
HeapWord* ra = _restart_addr;
markFromRootsClosure.reset(ra);
_restart_addr = NULL;
_markBitMap.iterate(&markFromRootsClosure, ra, _span.end());
}
return true;
}
Precleaning
预清理在ConcurrentMarkSweepThread线程执行,和业务线程并行，最终的执行逻辑为:
``
## AbortablePreclean
可取消的预清理在ConcurrentMarkSweepThread线程执行,和业务线程并行，最终的执行逻辑为:
```c
FinalMarking
最终标记在vm线程执行,会暂停业务线程，最终执行逻辑为:
if (CMSScavengeBeforeRemark) {
GenCollectedHeap* gch = GenCollectedHeap::heap();
// Temporarily set flag to false, GCH->do_collection will
// expect it to be false and set to true
FlagSetting fl(gch->_is_gc_active, false);
NOT_PRODUCT(GCTraceTime t("Scavenge-Before-Remark",
PrintGCDetails && Verbose, true, _gc_timer_cm);)
int level = _cmsGen->level() - 1;
if (level >= 0) {
gch->do_collection(true, // full (i.e. force, see below)
false, // !clear_all_soft_refs
0, // size
false, // is_tlab
level // max_level
);
}
}
FreelistLocker x(this);
MutexLockerEx y(bitMapLock(),
Mutex::_no_safepoint_check_flag);
assert(!init_mark_was_synchronous, "but that's impossible!");
checkpointRootsFinalWork(asynch, clear_all_soft_refs, false);
void CMSCollector::checkpointRootsFinalWork(bool asynch,
bool clear_all_soft_refs, bool init_mark_was_synchronous) {
NOT_PRODUCT(GCTraceTime tr("checkpointRootsFinalWork", PrintGCDetails, false, _gc_timer_cm);)
assert(haveFreelistLocks(), "must have free list locks");
assert_lock_strong(bitMapLock());
if (UseAdaptiveSizePolicy) {
size_policy()->checkpoint_roots_final_begin();
}
ResourceMark rm;
HandleMark hm;
GenCollectedHeap* gch = GenCollectedHeap::heap();
if (should_unload_classes()) {
CodeCache::gc_prologue();
}
assert(haveFreelistLocks(), "must have free list locks");
assert_lock_strong(bitMapLock());
if (!init_mark_was_synchronous) {
// We might assume that we need not fill TLAB's when
// CMSScavengeBeforeRemark is set, because we may have just done
// a scavenge which would have filled all TLAB's -- and besides
// Eden would be empty. This however may not always be the case --
// for instance although we asked for a scavenge, it may not have
// happened because of a JNI critical section. We probably need
// a policy for deciding whether we can in that case wait until
// the critical section releases and then do the remark following
// the scavenge, and skip it here. In the absence of that policy,
// or of an indication of whether the scavenge did indeed occur,
// we cannot rely on TLAB's having been filled and must do
// so here just in case a scavenge did not happen.
gch->ensure_parsability(false); // fill TLAB's, but no need to retire them
// Update the saved marks which may affect the root scans.
gch->save_marks();
if (CMSPrintEdenSurvivorChunks) {
print_eden_and_survivor_chunk_arrays();
}
{
COMPILER2_PRESENT(DerivedPointerTableDeactivate dpt_deact;)
// Note on the role of the mod union table:
// Since the marker in "markFromRoots" marks concurrently with
// mutators, it is possible for some reachable objects not to have been
// scanned. For instance, an only reference to an object A was
// placed in object B after the marker scanned B. Unless B is rescanned,
// A would be collected. Such updates to references in marked objects
// are detected via the mod union table which is the set of all cards
// dirtied since the first checkpoint in this GC cycle and prior to
// the most recent young generation GC, minus those cleaned up by the
// concurrent precleaning.
if (CMSParallelRemarkEnabled && CollectedHeap::use_parallel_gc_threads()) {
GCTraceTime t("Rescan (parallel) ", PrintGCDetails, false, _gc_timer_cm);
do_remark_parallel();
} else {
GCTraceTime t("Rescan (non-parallel) ", PrintGCDetails, false,
_gc_timer_cm);
do_remark_non_parallel();
}
}
} else {
assert(!asynch, "Can't have init_mark_was_synchronous in asynch mode");
// The initial mark was stop-world, so there's no rescanning to
// do; go straight on to the next step below.
}
verify_work_stacks_empty();
verify_overflow_empty();
{
NOT_PRODUCT(GCTraceTime ts("refProcessingWork", PrintGCDetails, false, _gc_timer_cm);)
refProcessingWork(asynch, clear_all_soft_refs);
}
verify_work_stacks_empty();
verify_overflow_empty();
if (should_unload_classes()) {
CodeCache::gc_epilogue();
}
JvmtiExport::gc_epilogue();
// If we encountered any (marking stack / work queue) overflow
// events during the current CMS cycle, take appropriate
// remedial measures, where possible, so as to try and avoid
// recurrence of that condition.
assert(_markStack.isEmpty(), "No grey objects");
size_t ser_ovflw = _ser_pmc_remark_ovflw + _ser_pmc_preclean_ovflw +
_ser_kac_ovflw + _ser_kac_preclean_ovflw;
if (ser_ovflw > 0) {
if (PrintCMSStatistics != 0) {
gclog_or_tty->print_cr("Marking stack overflow (benign) "
"(pmc_pc="SIZE_FORMAT", pmc_rm="SIZE_FORMAT", kac="SIZE_FORMAT
", kac_preclean="SIZE_FORMAT")",
_ser_pmc_preclean_ovflw, _ser_pmc_remark_ovflw,
_ser_kac_ovflw, _ser_kac_preclean_ovflw);
}
_markStack.expand();
_ser_pmc_remark_ovflw = 0;
_ser_pmc_preclean_ovflw = 0;
_ser_kac_preclean_ovflw = 0;
_ser_kac_ovflw = 0;
}
if (_par_pmc_remark_ovflw > 0 || _par_kac_ovflw > 0) {
if (PrintCMSStatistics != 0) {
gclog_or_tty->print_cr("Work queue overflow (benign) "
"(pmc_rm="SIZE_FORMAT", kac="SIZE_FORMAT")",
_par_pmc_remark_ovflw, _par_kac_ovflw);
}
_par_pmc_remark_ovflw = 0;
_par_kac_ovflw = 0;
}
if (PrintCMSStatistics != 0) {
if (_markStack._hit_limit > 0) {
gclog_or_tty->print_cr(" (benign) Hit max stack size limit ("SIZE_FORMAT")",
_markStack._hit_limit);
}
if (_markStack._failed_double > 0) {
gclog_or_tty->print_cr(" (benign) Failed stack doubling ("SIZE_FORMAT"),"
" current capacity "SIZE_FORMAT,
_markStack._failed_double,
_markStack.capacity());
}
}
_markStack._hit_limit = 0;
_markStack._failed_double = 0;
if ((VerifyAfterGC || VerifyDuringGC) &&
GenCollectedHeap::heap()->total_collections() >= VerifyGCStartAt) {
verify_after_remark();
}
_gc_tracer_cm->report_object_count_after_gc(&_is_alive_closure);
// Change under the freelistLocks.
_collectorState = Sweeping;
// Call isAllClear() under bitMapLock
assert(_modUnionTable.isAllClear(),
"Should be clear by end of the final marking");
assert(_ct->klass_rem_set()->mod_union_is_clear(),
"Should be clear by end of the final marking");
if (UseAdaptiveSizePolicy) {
size_policy()->checkpoint_roots_final_end(gch->gc_cause());
}
}
Sweeping
清理在ConcurrentMarkSweepThread线程执行,和业务线程并行，最终的执行逻辑为:
void CMSCollector::sweepWork(ConcurrentMarkSweepGeneration* gen,
bool asynch) {
// We iterate over the space(s) underlying this generation,
// checking the mark bit map to see if the bits corresponding
// to specific blocks are marked or not. Blocks that are
// marked are live and are not swept up. All remaining blocks
// are swept up, with coalescing on-the-fly as we sweep up
// contiguous free and/or garbage blocks:
// We need to ensure that the sweeper synchronizes with allocators
// and stop-the-world collectors. In particular, the following
// locks are used:
// . CMS token: if this is held, a stop the world collection cannot occur
// . freelistLock: if this is held no allocation can occur from this
// generation by another thread
// . bitMapLock: if this is held, no other thread can access or update
//
// Note that we need to hold the freelistLock if we use
// block iterate below; else the iterator might go awry if
// a mutator (or promotion) causes block contents to change
// (for instance if the allocator divvies up a block).
// If we hold the free list lock, for all practical purposes
// young generation GC's can't occur (they'll usually need to
// promote), so we might as well prevent all young generation
// GC's while we do a sweeping step. For the same reason, we might
// as well take the bit map lock for the entire duration
// check that we hold the requisite locks
assert(have_cms_token(), "Should hold cms token");
assert( (asynch && ConcurrentMarkSweepThread::cms_thread_has_cms_token())
|| (!asynch && ConcurrentMarkSweepThread::vm_thread_has_cms_token()),
"Should possess CMS token to sweep");
assert_lock_strong(gen->freelistLock());
assert_lock_strong(bitMapLock());
assert(!_inter_sweep_timer.is_active(), "Was switched off in an outer context");
assert(_intra_sweep_timer.is_active(), "Was switched on in an outer context");
gen->cmsSpace()->beginSweepFLCensus((float)(_inter_sweep_timer.seconds()),
_inter_sweep_estimate.padded_average(),
_intra_sweep_estimate.padded_average());
gen->setNearLargestChunk();
{
SweepClosure sweepClosure(this, gen, &_markBitMap,
CMSYield && asynch);
gen->cmsSpace()->blk_iterate_careful(&sweepClosure);
// We need to free-up/coalesce garbage/blocks from a
// co-terminal free run. This is done in the SweepClosure
// destructor; so, do not remove this scope, else the
// end-of-sweep-census below will be off by a little bit.
}
gen->cmsSpace()->sweep_completed();
gen->cmsSpace()->endSweepFLCensus(sweep_count());
if (should_unload_classes()) { // unloaded classes this cycle,
_concurrent_cycles_since_last_unload = 0; // ... reset count
} else { // did not unload classes,
_concurrent_cycles_since_last_unload++; // ... increment count
}
}
Resizing
重新计算容量在ConcurrentMarkSweepThread线程执行,和业务线程并行，最终的执行逻辑为:
void CMSCollector::compute_new_size() {
assert_locked_or_safepoint(Heap_lock);
FreelistLocker z(this);
//重新计算Metaspace的大小，默认情况下，gc之后Metaspace的空闲比例必须大于等于40%否则需要扩容；空闲比例大于70%则需要减小容量
MetaspaceGC::compute_new_size();
//重新计算老年代的大小，默认情况下，gc之后老年代的空闲比例必须大于等于40%否则需要扩容；空闲比例大于70%则需要减小容量
_cmsGen->compute_new_size_free_list();
}
Resetting
重置在ConcurrentMarkSweepThread线程执行,和业务线程并行:
CMS 日志解析
-
2018-02-09T09:57:40.697+0800: 40.746: [GC (CMS Initial Mark) [1 CMS-initial-mark: 435454K(1585152K)] 600948K(2972160K), 0.0357643 secs] [Times: user=0.13 sys=0.01, real=0.04 secs]
- 1表示level，老年代的level为1
- 435454K为老年代已使用内存，1585152K表示老年代容量
- 600948K表示堆已使用内存，2972160K表示堆容量
-
2018-02-09T09:57:40.733+0800: 40.783: [CMS-concurrent-mark-start]
-
2018-02-09T09:57:42.109+0800: 42.159: [CMS-concurrent-mark: 1.155/1.376 secs] [Times: user=4.77 sys=0.24, real=1.37 secs]
- 1.155表示并行标记耗费时间，1.376表示并行标记开始到结束的系统时间
-
2018-02-09T09:57:42.109+0800: 42.159: [CMS-concurrent-preclean-start]
-
2018-02-09T09:57:42.119+0800: 42.169: [CMS-concurrent-preclean: 0.010/0.010 secs] [Times: user=0.03 sys=0.00, real=0.01 secs]
-
2018-02-09T09:57:42.120+0800: 42.169: [CMS-concurrent-abortable-preclean-start]
CMS: abort preclean due to time -
2018-02-09T09:57:47.419+0800: 47.469: [CMS-concurrent-abortable-preclean: 5.024/5.299 secs] [Times: user=8.06 sys=0.16, real=5.30 secs]
-
2018-02-09T09:57:47.420+0800: 47.470: [GC (CMS Final Remark) [YG occupancy: 575396 K (1387008 K)]
-
2018-02-09T09:57:47.420+0800: 47.470: [Rescan (parallel) , 0.1041595 secs]
-
2018-02-09T09:57:47.524+0800: 47.574: [weak refs processing, 0.0000340 secs]
-
2018-02-09T09:57:47.525+0800: 47.574: [class unloading, 0.0216172 secs]
-
2018-02-09T09:57:47.546+0800: 47.596: [scrub symbol table, 0.0288155 secs]
-
2018-02-09T09:57:47.575+0800: 47.624: [scrub string table, 0.0024586 secs][1 CMS-remark: 435454K(1585152K)] 1010850K(2972160K), 0.1618950 secs] [Times: user=0.47 sys=0.00, real=0.17 secs]
-
2018-02-09T09:57:47.582+0800: 47.632: [CMS-concurrent-sweep-start]
-
2018-02-09T09:57:47.843+0800: 47.892: [CMS-concurrent-sweep: 0.260/0.260 secs] [Times: user=0.28 sys=0.00, real=0.26 secs]
-
2018-02-09T09:57:47.843+0800: 47.892: [CMS-concurrent-reset-start]
-
2018-02-09T09:57:47.846+0800: 47.895: [CMS-concurrent-reset: 0.003/0.003 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
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