Java 垃圾回收权威指北

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alias jflags='java -XX:+UnlockDiagnosticVMOptions -XX:+PrintFlagsFinal -version'

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-verbose:gc
-Xloggc:/data/logs/gc-%t.log
-XX:+PrintGCDetails
-XX:+PrintGCDateStamps
-XX:+PrintGCCause
-XX:+PrintTenuringDistribution
-XX:+UseGCLogFileRotation
-XX:NumberOfGCLogFiles=10
-XX:GCLogFileSize=50M
-XX:+PrintPromotionFailure

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$ jflags |  grep "Use.*GC"
     bool UseAdaptiveGCBoundary                     = false                               {product}
     bool UseAdaptiveSizeDecayMajorGCCost           = true                                {product}
     bool UseAdaptiveSizePolicyWithSystemGC         = false                               {product}
     bool UseAutoGCSelectPolicy                     = false                               {product}
     bool UseConcMarkSweepGC                        = false                               {product}
     bool UseDynamicNumberOfGCThreads               = false                               {product}
     bool UseG1GC                                   = false                               {product}
     bool UseGCLogFileRotation                      = false                               {product}
     bool UseGCOverheadLimit                        = true                                {product}
     bool UseGCTaskAffinity                         = false                               {product}
     bool UseMaximumCompactionOnSystemGC            = true                                {product}
     bool UseParNewGC                               = false                               {product}
     bool UseParallelGC                             = false                               {product}
     bool UseParallelOldGC                          = false                               {product}
     bool UseSerialGC                               = false                               {product}
java version "1.8.0_172"
Java(TM) SE Runtime Environment (build 1.8.0_172-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.172-b11, mixed mode)

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  // hotspot/src/share/vm/runtime/arguments.cpp#set_gc_specific_flags
  // Set per-collector flags
  if (UseParallelGC || UseParallelOldGC) {
    set_parallel_gc_flags();
  } else if (UseConcMarkSweepGC) { // Should be done before ParNew check below
    set_cms_and_parnew_gc_flags();
  } else if (UseParNewGC) {  // Skipped if CMS is set above
    set_parnew_gc_flags();
  } else if (UseG1GC) {
    set_g1_gc_flags();
  }

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package gc;
// 省略 import 语句
public class WhichGC {
    public static void main(String[] args) {
        try {
            List<GarbageCollectorMXBean> gcMxBeans = ManagementFactory.getGarbageCollectorMXBeans();
            for (GarbageCollectorMXBean gcMxBean : gcMxBeans) {
                System.out.println(gcMxBean.getName());
            }
        } catch (Exception exp) {
            System.err.println(exp);
        }
    }
}

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$ java gc.WhichGC  # 两个输出分别表示 young/old 代的 collector
PS Scavenge
PS MarkSweep

$ java -XX:+UseSerialGC gc.WhichGC
Copy
MarkSweepCompact

$ java -XX:+UseParNewGC gc.WhichGC # 注意提示
Java HotSpot(TM) 64-Bit Server VM warning: Using the ParNew young collector with the Serial old collector is deprecated and will likely be removed in a future release
ParNew
MarkSweepCompact

$ java -XX:+UseParallelGC gc.WhichGC
PS Scavenge
PS MarkSweep # 虽然名为 MarkSweep，但其实现是 mark-sweep-compact

$ java -XX:+UseParallelOldGC gc.WhichGC # 与上面输出一致，不加 flag 时这样同样的输出
PS Scavenge
PS MarkSweep

$ java -XX:+UseConcMarkSweepGC gc.WhichGC # ParNew 中 Par 表示 parallel，表明采用并行方式收集 young 代
ParNew
ConcurrentMarkSweep  # 注意这里没有 compact 过程，也就是说 CMS 的 old 可能会产生碎片

$ java -XX:+UseG1GC gc.WhichGC
G1 Young Generation
G1 Old Generation

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  // ParNewGeneration 构造方法
  for (uint i1 = 0; i1 < ParallelGCThreads; i1++) {
    ObjToScanQueue *q = new ObjToScanQueue();
    guarantee(q != NULL, "work_queue Allocation failure.");
    _task_queues->register_queue(i1, q);
  }
  // do_void 方法
  while (true) {

    ......
    // We have no local work, attempt to steal from other threads.

    // attempt to steal work from promoted.
    if (task_queues()->steal(par_scan_state()->thread_num(),
                             par_scan_state()->hash_seed(),
                             obj_to_scan)) {
      bool res = work_q->push(obj_to_scan);
      assert(res, "Empty queue should have room for a push.");

      //   if successful, goto Start.
      continue;

      // try global overflow list.
    } else if (par_gen()->take_from_overflow_list(par_scan_state())) {
      continue;
    }
    .......
  }
  

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-XX:YoungGenerationSizeIncrement=<Y>
-XX:TenuredGenerationSizeIncrement=<T>
-XX:AdaptiveSizeDecrementScaleFactor=<D>
# 如果增长的增量是 X，那么减少的减量则为 X/D

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2018-12-27T22:14:19.006+0800: 5433.841: [GC (Allocation Failure) [PSYoungGen: 606785K->3041K(656896K)] 746943K->143356K(2055168K), 0.0157837 secs] [Times: user=0.03 sys=0.01, real=0.02 secs]
    UseAdaptiveSizePolicy actions to meet  *** reduced footprint ***
                       GC overhead (%)
    Young generation:        0.02         (attempted to shrink)
    Tenured generation:      0.00         (attempted to shrink)
    Tenuring threshold:    (attempted to decrease to balance GC costs) = 1
2018-12-27T22:21:36.581+0800: 5871.417: [GC (Allocation Failure) [PSYoungGen: 615905K->3089K(654848K)] 756220K->143504K(2053120K), 0.0157796 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]
    UseAdaptiveSizePolicy actions to meet  *** reduced footprint ***
                       GC overhead (%)
    Young generation:        0.01         (attempted to shrink)
    Tenured generation:      0.00         (attempted to shrink)
    Tenuring threshold:    (attempted to decrease to balance GC costs) = 1
2018-12-27T22:28:51.669+0800: 6306.505: [GC (Allocation Failure) [PSYoungGen: 615953K->3089K(660992K)] 756368K->143664K(2059264K), 0.0178418 secs] [Times: user=0.03 sys=0.01, real=0.02 secs]
    UseAdaptiveSizePolicy actions to meet  *** reduced footprint ***
                       GC overhead (%)
    Young generation:        0.01         (attempted to shrink)
    Tenured generation:      0.00         (attempted to shrink)
    Tenuring threshold:    (attempted to decrease to balance GC costs) = 1
2018-12-27T22:36:17.738+0800: 6752.573: [GC (Allocation Failure) [PSYoungGen: 624145K->2896K(658944K)] 764720K->143576K(2057216K), 0.0144179 secs] [Times: user=0.02 sys=0.01, real=0.01 secs]
    UseAdaptiveSizePolicy actions to meet  *** reduced footprint ***
                       GC overhead (%)
    Young generation:        0.01         (attempted to shrink)
    Tenured generation:      0.00         (attempted to shrink)
    Tenuring threshold:    (attempted to decrease to balance GC costs) = 1
2018-12-27T22:43:40.208+0800: 7195.043: [GC (Allocation Failure) [PSYoungGen: 623952K->2976K(665088K)] 764632K->143720K(2063360K), 0.0135656 secs] [Times: user=0.03 sys=0.01, real=0.02 secs]
    UseAdaptiveSizePolicy actions to meet  *** reduced footprint ***
                       GC overhead (%)
    Young generation:        0.01         (attempted to shrink)
    Tenured generation:      0.00         (attempted to shrink)
    Tenuring threshold:    (attempted to decrease to balance GC costs) = 1
2018-12-27T22:48:59.110+0800: 7513.945: [GC (Allocation Failure) [PSYoungGen: 632224K->5393K(663040K)] 772968K->146241K(2061312K), 0.0230613 secs] [Times: user=0.05 sys=0.01, real=0.02 secs]
    UseAdaptiveSizePolicy actions to meet  *** reduced footprint ***
                       GC overhead (%)
    Young generation:        0.01         (attempted to shrink)
    Tenured generation:      0.00         (attempted to shrink)
    Tenuring threshold:    (attempted to decrease to balance GC costs) = 1
2018-12-27T22:54:05.871+0800: 7820.706: [GC (Allocation Failure) [PSYoungGen: 634641K->4785K(669696K)] 775489K->147601K(2067968K), 0.0173448 secs] [Times: user=0.04 sys=0.01, real=0.02 secs]
    UseAdaptiveSizePolicy actions to meet  *** reduced footprint ***
                       GC overhead (%)
    Young generation:        0.01         (attempted to shrink)
    Tenured generation:      0.00         (attempted to shrink)
    Tenuring threshold:    (attempted to decrease to balance GC costs) = 1

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-XX:CMSInitiatingOccupancyFraction=60
-XX:+UseCMSInitiatingOccupancyOnly

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-XX:+PrintGCDetails
-XX:+PrintPromotionFailure
-XX:PrintFLSStatistics=1

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592.079: [ParNew (0: promotion failure size = 2698)  (promotion failed): 135865K->134943K(138240K), 0.1433555 secs]
Statistics for BinaryTreeDictionary:
------------------------------------
Total Free Space: 40115394
Max   Chunk Size: 38808526
Number of Blocks: 1360
Av.  Block  Size: 29496
Tree      Height: 22

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  // concurrentMarkSweepGeneration.cpp#collect_in_background
  while (_collectorState != Idling) {
     ....
    switch (_collectorState) {
      case InitialMarking:
        {
          ReleaseForegroundGC x(this);
          stats().record_cms_begin();
          VM_CMS_Initial_Mark initial_mark_op(this);
          VMThread::execute(&initial_mark_op);
        }
        // The collector state may be any legal state at this point
        // since the background collector may have yielded to the
        // foreground collector.
        break;
      case Marking:
        // initial marking in checkpointRootsInitialWork has been completed
        if (markFromRoots(true)) { // we were successful
          assert(_collectorState == Precleaning, "Collector state should "
            "have changed");
        } else {
          assert(_foregroundGCIsActive, "Internal state inconsistency");
        }
        break;
      case Precleaning:
        if (UseAdaptiveSizePolicy) {
          size_policy()->concurrent_precleaning_begin();
        }
        // marking from roots in markFromRoots has been completed
        preclean();
        if (UseAdaptiveSizePolicy) {
          size_policy()->concurrent_precleaning_end();
        }
        assert(_collectorState == AbortablePreclean ||
               _collectorState == FinalMarking,
               "Collector state should have changed");
        break;
      case AbortablePreclean:
        if (UseAdaptiveSizePolicy) {
        size_policy()->concurrent_phases_resume();
        }
        abortable_preclean();
        if (UseAdaptiveSizePolicy) {
          size_policy()->concurrent_precleaning_end();
        }
        assert(_collectorState == FinalMarking, "Collector state should "
          "have changed");
        break;
      case FinalMarking:
        {
          ReleaseForegroundGC x(this);

          VM_CMS_Final_Remark final_remark_op(this);
          VMThread::execute(&final_remark_op);
        }
        assert(_foregroundGCShouldWait, "block post-condition");
        break;
      case Sweeping:
        if (UseAdaptiveSizePolicy) {
          size_policy()->concurrent_sweeping_begin();
        }
        // final marking in checkpointRootsFinal has been completed
        sweep(true);
        assert(_collectorState == Resizing, "Collector state change "
          "to Resizing must be done under the free_list_lock");
        _full_gcs_since_conc_gc = 0;

        // Stop the timers for adaptive size policy for the concurrent phases
        if (UseAdaptiveSizePolicy) {
          size_policy()->concurrent_sweeping_end();
          size_policy()->concurrent_phases_end(gch->gc_cause(),
                                             gch->prev_gen(_cmsGen)->capacity(),
                                             _cmsGen->free());
        }

      case Resizing: {
        ....
        break;
      }
      case Resetting:
        ......
        break;
      case Idling:
      default:
        ShouldNotReachHere();
        break;
    }
    .......
  }

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  // concurrentMarkSweepGeneration.cpp#abortable_preclean()
  
  if (get_eden_used() > CMSScheduleRemarkEdenSizeThreshold) {
    size_t loops = 0, workdone = 0, cumworkdone = 0, waited = 0;
    while (!(should_abort_preclean() ||
             ConcurrentMarkSweepThread::should_terminate())) {
      workdone = preclean_work(CMSPrecleanRefLists2, CMSPrecleanSurvivors2);
      cumworkdone += workdone;
      loops++;
      // Voluntarily terminate abortable preclean phase if we have
      // been at it for too long.
      if ((CMSMaxAbortablePrecleanLoops != 0) &&
          loops >= CMSMaxAbortablePrecleanLoops) {
        if (PrintGCDetails) {
          gclog_or_tty->print(" CMS: abort preclean due to loops ");
        }
        break;
      }
      if (pa.wallclock_millis() > CMSMaxAbortablePrecleanTime) {
        if (PrintGCDetails) {
          gclog_or_tty->print(" CMS: abort preclean due to time ");
        }
        break;
      }
      // If we are doing little work each iteration, we should
      // take a short break.
      if (workdone < CMSAbortablePrecleanMinWorkPerIteration) {
        // Sleep for some time, waiting for work to accumulate
        stopTimer();
        cmsThread()->wait_on_cms_lock(CMSAbortablePrecleanWaitMillis);
        startTimer();
        waited++;
      }
    }
    if (PrintCMSStatistics > 0) {
      gclog_or_tty->print(" [%d iterations, %d waits, %d cards)] ",
                          loops, waited, cumworkdone);
    }
  }

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// concurrentMarkSweepGeneration.cpp 
// preclean_work 会调用 sample_eden，但是这里的 !CMSEdenChunksRecordAlways 默认为 false
// 所以这里不会进行抽样
void CMSCollector::sample_eden() {
  if (_eden_chunk_array != NULL && !CMSEdenChunksRecordAlways) {
    ...... do sample
  }

}
// defNewGeneration.cpp#allocate() 
  HeapWord* result = eden()->par_allocate(word_size);
  if (result != NULL) {
    if (CMSEdenChunksRecordAlways && _next_gen != NULL) {
      // 这里会调用 concurrentMarkSweepGeneration 里的 sample_eden_chunk
      _next_gen->sample_eden_chunk();
    }
    return result;
  }
// concurrentMarkSweepGeneration.cpp 
void CMSCollector::sample_eden_chunk() {
  // 默认会在这里进行抽样
  if (CMSEdenChunksRecordAlways && _eden_chunk_array != NULL) {
     ..... do sample
  }
}

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the promotion rate*the maximum Old Gen collection time*(1 + a little bit)

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docker run -d -p 8080:80 gcplot/gcplot