Java 垃圾回收器 G1GC 需要很长时间才能进行“对象复制”（疏散暂停）
2022-09-04 22:18:53
我不是Java新手，但我对垃圾回收只知道一点点。现在我想用一些实践经验来改变这种状况。我的目标是延迟低于0.3秒，或者在极端情况下，0.5也是可以的。
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，我偶尔会在5秒内长时间停顿，尽管似乎有足够的可用内存。我发现的一个原因：
[GC pause (G1 Evacuation Pause) (young) 42G->40G(48G), 5.9409662 secs]
为什么GCG1还在为此做“停止世界”？（或者至少我看到它恰好在这个时候停止了我的应用程序）如果不是真的有必要，为什么它会进行这样的负面清理，因为有超过12%的可用RAM可用。另外，我认为默认值为200毫秒，为什么这个值被29甚至50的系数（见下文）所违反？-XX:MaxGCPauseMillis
延迟的另一个原因是：
[GC pause (Metadata GC Threshold) (young) (initial-mark) 40G->39G(48G), 10.4667233 secs]
这可能通过这个答案来解决，例如，只是增加元数据空间-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]
在这里阅读有关它的信息：复制（停止世界事件） - 这些是世界暂停以疏散或复制活动对象到新的未使用区域的停止。这可以通过记录为[GC暂停（年轻）]的年轻一代区域来完成。或者记录为[GC暂停（混合）]的年轻一代和老一代区域。
