我不是Java的初学者,但我只知道一点垃圾回收相关的知识。现在我想通过一些实践经验来改变这种情况。我的目标是实现低于0.3秒的延迟,极端情况下0.5秒也可以接受。
我有一个应用程序,使用了-Xmx50gb(-Xms50gb)以及以下其他GC选项:
-XX:+UseG1GC -Xloggc:somewhere.gc.log -XX:+PrintGCDateStamps
但现在由于垃圾回收问题,偶尔会出现超过5秒的长时间暂停,尽管似乎有足够的可用内存。我发现其中一个原因是:
[GC pause (G1 Evacuation Pause) (young) 42G->40G(48G), 5.9409662 secs]
为什么GCG1对此仍然要执行"停止整个世界(stop the world)"操作呢?(或者至少我看到它在这个时候停止了我的应用程序)。如果可用RAM空间超过12%,那么为什么需要进行如此负面的清理呢?另外,我认为-XX:MaxGCPauseMillis
的默认值是200毫秒,为什么该值会被违反29倍甚至50倍(请参见下文)?
另一个延迟的原因是:
[GC pause (Metadata GC Threshold) (young) (initial-mark) 40G->39G(48G), 10.4667233 secs]
这可能通过这个答案得到解决,例如只需要增加元数据空间-XX:MetaspaceSize=100M
顺便提一下: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]
点击这里了解更多信息:复制(暂停全局事件) - 这些是停止全局暂停,以便将活动对象转移到新的未使用区域。 可以使用年轻一代区域执行此操作,这些区域会被标记为[GC pause (young)]。 或同时使用年轻和老一代区域进行操作,这些区域会被标记为[GC Pause (mixed)]。
System.gc()
方法? - Mick Mnemonic