java g1 gc

java g1 gc"/>

java g1 gc

我不是Java的新手，但是我对垃圾收集只了解一点点。现在，我想通过一些实践经验来改变这种状况。我的目标是在0.3秒以下的延迟，或者在极端情况下0.5也可以。

我有一个带有-Xmx50gb(-Xms50gb)的应用程序，并设置了以下其他GC选项：

-XX:+UseG1GC -Xloggc:somewhere.gc.log -XX:+PrintGCDateStamps

但是现在由于垃圾回收，尽管偶尔有足够的可用内存，但我偶尔会在5秒内停顿很长时间。我发现的原因之一：

[GC pause (G1 Evacuation Pause) (young) 42G->40G(48G), 5.9409662 secs]

为什么GCG1仍为此做一个“停止世界”？(或者至少我看到它恰好在此时停止了我的应用程序)以及为什么由于不需要的可用内存(如果可用RAM超过12％)而没有必要进行此类负面清理。我还认为默认值-XX:MaxGCPauseMillis是200毫秒，为什么这个值违反了29甚至50倍(请参阅下文)？

延迟的另一个原因是：

[GC pause (Metadata GC Threshold) (young) (initial-mark) 40G->39G(48G), 10.4667233 secs]

这可能会通过此答案解决，例如仅增加元数据空间-XX:MetaspaceSize=100M

顺便说一句：使用JSE 1.8.0_91-b14

更新： 此类事件的详细GC日志

2016-08-12T09:20:31.589+0200: 1178.312: [GC pause (G1 Evacuation Pause) (young) 1178.312: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 3159, predicted base time: 1.52 ms, remaining time: 198.48 ms, target pause time: 200.00 ms]

1178.312: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 136 regions, survivors: 20 regions, predicted young region time: 1924.75 ms]

1178.312: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 136 regions, survivors: 20 regions, old: 0 regions, predicted pause time: 1926.27 ms, target pause time: 200.00 ms]

1185.330: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason: recent GC overhead higher than threshold after GC, recent GC overhead: 21.83 %, threshold: 10.00 %, uncommitted: 0 bytes, calculated expansion amount: 0 bytes (20.00 %)]

1185.330: [G1Ergonomics (Concurrent Cycles) do not request concurrent cycle initiation, reason: still doing mixed collections, occupancy: 42580574208 bytes, allocation request: 0 bytes, threshold: 23592960000 bytes (45.00 %), source: end of GC]

1185.330: [G1Ergonomics (Mixed GCs) do not start mixed GCs, reason: reclaimable percentage not over threshold, candidate old regions: 1 regions, reclaimable: 3381416 bytes (0.01 %), threshold: 5.00 %]

, 7.0181903 secs]

[Parallel Time: 6991.8 ms, GC Workers: 10]

[GC Worker Start (ms): Min: 1178312.6, Avg: 1178312.8, Max: 1178312.9, Diff: 0.2]

[Ext Root Scanning (ms): Min: 1.1, Avg: 1.5, Max: 2.3, Diff: 1.2, Sum: 15.0]

[Update RS (ms): Min: 0.0, Avg: 0.3, Max: 1.3, Diff: 1.3, Sum: 3.4]

[Processed Buffers: Min: 0, Avg: 2.1, Max: 5, Diff: 5, Sum: 21]

[Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.4]

[Code Root Scanning (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 1.7]

[Object Copy (ms): Min: 6964.1, Avg: 6973.0, Max: 6989.5, Diff: 25.3, Sum: 69730.4]

[Termination (ms): Min: 0.0, Avg: 16.4, Max: 25.3, Diff: 25.3, Sum: 164.4]

[Termination Attempts: Min: 1, Avg: 3.2, Max: 13, Diff: 12, Sum: 32]

[GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.2]

[GC Worker Total (ms): Min: 6991.5, Avg: 6991.6, Max: 6991.7, Diff: 0.2, Sum: 69915.5]

[GC Worker End (ms): Min: 1185304.3, Avg: 1185304.3, Max: 1185304.3, Diff: 0.0]

[Code Root Fixup: 0.1 ms]

[Code Root Purge: 0.0 ms]

[Clear CT: 0.3 ms]

[Other: 26.0 ms]

[Choose CSet: 0.0 ms]

[Ref Proc: 25.3 ms]

[Ref Enq: 0.1 ms]

[Redirty Cards: 0.1 ms]

[Humongous Register: 0.2 ms]

[Humongous Reclaim: 0.0 ms]

[Free CSet: 0.2 ms]

[Eden: 2176.0M(2176.0M)->0.0B(2176.0M) Survivors: 320.0M->320.0M Heap: 40.6G(48.8G)->40.0G(48.8G)]

[Times: user=0.55 sys=46.58, real=7.02 secs]

在此处阅读有关此内容的信息：复制(停止世界活动)-这是世界暂停撤离活动物体或将活动物体复制到新的未使用区域的停止点。这可以用记录为[GC暂停(年轻)]的年轻一代区域来完成。或记录为[GC暂停(混合)]的年轻一代区域和老一代区域。