为什么似乎不必要地触发了 G1 Full GC？答案

【问题标题】：Why is G1 Full GC being triggered seeminly unnecessarily?为什么似乎不必要地触发了 G1 Full GC？
【发布时间】：2013-01-15 02:40:24
【问题描述】：

我正在使用

java version "1.7.0_09"
Java(TM) SE Runtime Environment (build 1.7.0_09-b05)
Java HotSpot(TM) 64-Bit Server VM (build 23.5-b02, mixed mode)

使用 G1 垃圾收集器。 JVM 参数是

-server -Xss4096k -XX:MaxPermSize=512m -XX:PermSize=512m -Xms30G 
-Xmx30G -Xnoclassgc -XX:+UseNUMA -XX:+UseFastAccessorMethods 
-XX:ReservedCodeCacheSize=48m -XX:+UseStringCache 
-XX:+UseGCOverheadLimit -Duser.timezone=EET 
-Xmaxf1 -XX:+UseCompressedOops -XX:+DisableExplicitGC 
-XX:+AggressiveOpts -XX:CMSInitiatingOccupancyFraction=70 
-XX:+ParallelRefProcEnabled -XX:+UseAdaptiveSizePolicy 
-XX:MaxGCPauseMillis=500 -XX:+UseG1GC 
-XX:InitiatingHeapOccupancyPercent=0 
-XX:GCPauseIntervalMillis=10000 -XX:+PrintGCDetails 
-XX:+PrintHeapAtGC -XX:+PrintGCDateStamps -Xloggc:gc.log

但是，我在没有任何明显原因的情况下经历了完整的 GC 扫描，如何摆脱它们？

GC 日志，带有来自先前事件的一些尾部：

{Heap before GC invocations=206 (full 8):
 garbage-first heap   total 31457280K, used 18323299K [0x00007fc6eb800000, 0x00007fce6b800000, 0x00007fce6b800000)
  region size 8192K, 768 young (6291456K), 31 survivors (253952K)
 compacting perm gen  total 524288K, used 145771K [0x00007fce6b800000, 0x00007fce8b800000, 0x00007fce8b800000)
   the space 524288K,  27% used [0x00007fce6b800000, 0x00007fce7465af18, 0x00007fce7465b000, 0x00007fce8b800000)
No shared spaces configured.
2013-01-16T11:45:31.290+0200: 20430.451: [GC pause (young), 0.54738500 secs]
   [Parallel Time: 510.7 ms]
      [GC Worker Start (ms):  20430451.7  20430451.8  20430451.8  20430451.8
       Avg: 20430451.8, Min: 20430451.7, Max: 20430451.8, Diff:   0.1]
      [Ext Root Scanning (ms):  22.3  68.3  21.8  20.8
       Avg:  33.3, Min:  20.8, Max:  68.3, Diff:  47.5]
      [Update RS (ms):  21.6  0.0  23.6  23.7
       Avg:  17.3, Min:   0.0, Max:  23.7, Diff:  23.7]
         [Processed Buffers : 77 0 18 55
          Sum: 150, Avg: 37, Min: 0, Max: 77, Diff: 77]
      [Scan RS (ms):  19.6  0.1  19.3  19.6
       Avg:  14.7, Min:   0.1, Max:  19.6, Diff:  19.5]
      [Object Copy (ms):  439.1  434.2  437.8  438.4
       Avg: 437.4, Min: 434.2, Max: 439.1, Diff:   4.9]
      [Termination (ms):  0.0  0.0  0.0  0.0
       Avg:   0.0, Min:   0.0, Max:   0.0, Diff:   0.0]
         [Termination Attempts : 1 1 1 1
          Sum: 4, Avg: 1, Min: 1, Max: 1, Diff: 0]
      [GC Worker End (ms):  20430954.5  20430954.6  20430954.5  20430954.4
       Avg: 20430954.5, Min: 20430954.4, Max: 20430954.6, Diff:   0.1]
      [GC Worker (ms):  502.7  502.8  502.7  502.6
       Avg: 502.7, Min: 502.6, Max: 502.8, Diff:   0.2]
      [GC Worker Other (ms):  8.0  8.0  8.0  8.1
       Avg:   8.0, Min:   8.0, Max:   8.1, Diff:   0.1]
   [Clear CT:   2.6 ms]
   [Other:  34.1 ms]
      [Choose CSet:   0.1 ms]
      [Ref Proc:  27.4 ms]
      [Ref Enq:   0.2 ms]
      [Free CSet:   5.4 ms]
   [Eden: 5896M(5896M)->0B(5688M) Survivors: 248M->456M Heap: 17893M(30720M)->12208M(30720M)]
 [Times: user=2.12 sys=0.02, real=0.55 secs]
Heap after GC invocations=207 (full 8):
 garbage-first heap   total 31457280K, used 12501684K [0x00007fc6eb800000, 0x00007fce6b800000, 0x00007fce6b800000)
  region size 8192K, 57 young (466944K), 57 survivors (466944K)
 compacting perm gen  total 524288K, used 145771K [0x00007fce6b800000, 0x00007fce8b800000, 0x00007fce8b800000)
   the space 524288K,  27% used [0x00007fce6b800000, 0x00007fce7465af18, 0x00007fce7465b000, 0x00007fce8b800000)
No shared spaces configured.
}
{Heap before GC invocations=207 (full 8):
 garbage-first heap   total 31457280K, used 19058524K [0x00007fc6eb800000, 0x00007fce6b800000, 0x00007fce6b800000)
  region size 8192K, 727 young (5955584K), 57 survivors (466944K)
 compacting perm gen  total 524288K, used 145775K [0x00007fce6b800000, 0x00007fce8b800000, 0x00007fce8b800000)
   the space 524288K,  27% used [0x00007fce6b800000, 0x00007fce7465bdd8, 0x00007fce7465be00, 0x00007fce8b800000)
No shared spaces configured.
2013-01-16T11:46:26.619+0200: 20485.781: [GC pause (mixed), 1.42356500 secs]
   [Parallel Time: 1385.9 ms]
      [GC Worker Start (ms):  20485786.6  20485786.6  20485786.6  20485786.7
       Avg: 20485786.6, Min: 20485786.6, Max: 20485786.7, Diff:   0.1]
      [Ext Root Scanning (ms):  22.1  70.0  22.2  21.3
       Avg:  33.9, Min:  21.3, Max:  70.0, Diff:  48.7]
      [Update RS (ms):  9.9  0.0  10.9  11.2
       Avg:   8.0, Min:   0.0, Max:  11.2, Diff:  11.2]
         [Processed Buffers : 13 0 25 17
          Sum: 55, Avg: 13, Min: 0, Max: 25, Diff: 25]
      [Scan RS (ms):  324.6  276.4  324.6  324.6
       Avg: 312.5, Min: 276.4, Max: 324.6, Diff:  48.2]
      [Object Copy (ms):  1021.1  1031.2  1019.8  1020.5
       Avg: 1023.1, Min: 1019.8, Max: 1031.2, Diff:  11.4]
      [Termination (ms):  0.0  0.0  0.0  0.0
       Avg:   0.0, Min:   0.0, Max:   0.0, Diff:   0.0]
         [Termination Attempts : 6 5 5 1
          Sum: 17, Avg: 4, Min: 1, Max: 6, Diff: 5]
      [GC Worker End (ms):  20487164.3  20487164.3  20487164.3  20487164.3
       Avg: 20487164.3, Min: 20487164.3, Max: 20487164.3, Diff:   0.0]
      [GC Worker (ms):  1377.7  1377.6  1377.6  1377.6
       Avg: 1377.7, Min: 1377.6, Max: 1377.7, Diff:   0.1]
      [GC Worker Other (ms):  8.2  8.3  8.3  8.3
       Avg:   8.3, Min:   8.2, Max:   8.3, Diff:   0.1]
   [Clear CT:   4.4 ms]
   [Other:  33.3 ms]
      [Choose CSet:   5.5 ms]
      [Ref Proc:   5.0 ms]
      [Ref Enq:   0.2 ms]
      [Free CSet:   9.1 ms]
   [Eden: 5360M(5688M)->0B(5784M) Survivors: 456M->360M Heap: 18616M(30720M)->11359M(30720M)]
 [Times: user=5.56 sys=0.01, real=1.42 secs]
Heap after GC invocations=208 (full 8):
 garbage-first heap   total 31457280K, used 11632594K [0x00007fc6eb800000, 0x00007fce6b800000, 0x00007fce6b800000)
  region size 8192K, 45 young (368640K), 45 survivors (368640K)
 compacting perm gen  total 524288K, used 145775K [0x00007fce6b800000, 0x00007fce8b800000, 0x00007fce8b800000)
   the space 524288K,  27% used [0x00007fce6b800000, 0x00007fce7465bdd8, 0x00007fce7465be00, 0x00007fce8b800000)
No shared spaces configured.
}
{Heap before GC invocations=208 (full 8):
 garbage-first heap   total 31457280K, used 11632594K [0x00007fc6eb800000, 0x00007fce6b800000, 0x00007fce6b800000)
  region size 8192K, 45 young (368640K), 45 survivors (368640K)
 compacting perm gen  total 524288K, used 145775K [0x00007fce6b800000, 0x00007fce8b800000, 0x00007fce8b800000)
   the space 524288K,  27% used [0x00007fce6b800000, 0x00007fce7465bdd8, 0x00007fce7465be00, 0x00007fce8b800000)
No shared spaces configured.
2013-01-16T11:46:28.060+0200: 20487.222: [Full GC 11359M->8807M(30720M), 32.4879420 secs]
 [Times: user=45.41 sys=0.21, real=32.48 secs]
Heap after GC invocations=209 (full 9):
 garbage-first heap   total 31457280K, used 9019206K [0x00007fc6eb800000, 0x00007fce6b800000, 0x00007fce6b800000)
  region size 8192K, 0 young (0K), 0 survivors (0K)
 compacting perm gen  total 524288K, used 145775K [0x00007fce6b800000, 0x00007fce8b800000, 0x00007fce8b800000)
   the space 524288K,  27% used [0x00007fce6b800000, 0x00007fce7465bdd8, 0x00007fce7465be00, 0x00007fce8b800000)
No shared spaces configured.
}

其他类似的：

{Heap before GC invocations=105 (full 4):
 garbage-first heap   total 31457280K, used 12638024K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 56 young (458752K), 56 survivors (458752K)
 compacting perm gen  total 524288K, used 147352K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  28% used [0x00007fd2a2800000, 0x00007fd2ab7e61f8, 0x00007fd2ab7e6200, 0x00007fd2c2800000)
No shared spaces configured.
2013-01-14T10:31:31.240+0200: 14185.812: [Full GC 12341M->7092M(30720M), 25.3417840 secs]
 [Times: user=35.12 sys=0.21, real=25.33 secs]
Heap after GC invocations=106 (full 5):
 garbage-first heap   total 31457280K, used 7262578K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 0 young (0K), 0 survivors (0K)
 compacting perm gen  total 524288K, used 147352K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  28% used [0x00007fd2a2800000, 0x00007fd2ab7e61f8, 0x00007fd2ab7e6200, 0x00007fd2c2800000)
No shared spaces configured.
}

{Heap before GC invocations=248 (full 11):
 garbage-first heap   total 31457280K, used 12262418K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 37 young (303104K), 37 survivors (303104K)
 compacting perm gen  total 524288K, used 152608K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  29% used [0x00007fd2a2800000, 0x00007fd2abd082e8, 0x00007fd2abd08400, 0x00007fd2c2800000)
No shared spaces configured.
2013-01-14T13:01:39.869+0200: 23194.441: [Full GC 11975M->7331M(30720M), 28.6147390 secs]
 [Times: user=40.31 sys=0.03, real=28.62 secs]
Heap after GC invocations=249 (full 12):
 garbage-first heap   total 31457280K, used 7507920K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 0 young (0K), 0 survivors (0K)
 compacting perm gen  total 524288K, used 152608K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  29% used [0x00007fd2a2800000, 0x00007fd2abd082e8, 0x00007fd2abd08400, 0x00007fd2c2800000)
No shared spaces configured.
}

{Heap before GC invocations=399 (full 19):
 garbage-first heap   total 31457280K, used 13016093K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 54 young (442368K), 54 survivors (442368K)
 compacting perm gen  total 524288K, used 155475K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  29% used [0x00007fd2a2800000, 0x00007fd2abfd4c70, 0x00007fd2abfd4e00, 0x00007fd2c2800000)
No shared spaces configured.
2013-01-14T15:31:36.484+0200: 32191.056: [Full GC 12711M->7543M(30720M), 29.5013210 secs]
 [Times: user=41.39 sys=0.06, real=29.50 secs]
Heap after GC invocations=400 (full 20):
 garbage-first heap   total 31457280K, used 7724994K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 0 young (0K), 0 survivors (0K)
 compacting perm gen  total 524288K, used 155475K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  29% used [0x00007fd2a2800000, 0x00007fd2abfd4c70, 0x00007fd2abfd4e00, 0x00007fd2c2800000)
No shared spaces configured.
}

{Heap before GC invocations=476 (full 26):
 garbage-first heap   total 31457280K, used 14622246K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 27 young (221184K), 27 survivors (221184K)
 compacting perm gen  total 524288K, used 156716K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  29% used [0x00007fd2a2800000, 0x00007fd2ac10b2b0, 0x00007fd2ac10b400, 0x00007fd2c2800000)
No shared spaces configured.
2013-01-14T18:46:41.545+0200: 43896.117: [Full GC 14279M->8107M(30720M), 31.6936770 secs]
 [Times: user=44.40 sys=0.18, real=31.69 secs]
Heap after GC invocations=477 (full 27):
 garbage-first heap   total 31457280K, used 8302054K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 0 young (0K), 0 survivors (0K)
 compacting perm gen  total 524288K, used 156551K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  29% used [0x00007fd2a2800000, 0x00007fd2ac0e1fa0, 0x00007fd2ac0e2000, 0x00007fd2c2800000)
No shared spaces configured.
}

{Heap before GC invocations=522 (full 34):
 garbage-first heap   total 31457280K, used 16096219K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 30 young (245760K), 30 survivors (245760K)
 compacting perm gen  total 524288K, used 156869K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  29% used [0x00007fd2a2800000, 0x00007fd2ac1315d8, 0x00007fd2ac131600, 0x00007fd2c2800000)
No shared spaces configured.
2013-01-14T22:31:40.711+0200: 57395.283: [Full GC 15718M->11933M(30720M), 42.0857250 secs]
 [Times: user=60.13 sys=0.07, real=42.08 secs]
Heap after GC invocations=523 (full 35):
 garbage-first heap   total 31457280K, used 12219927K [0x00007fcb22800000, 0x00007fd2a2800000, 0x00007fd2a2800000)
  region size 8192K, 0 young (0K), 0 survivors (0K)
 compacting perm gen  total 524288K, used 156869K [0x00007fd2a2800000, 0x00007fd2c2800000, 0x00007fd2c2800000)
   the space 524288K,  29% used [0x00007fd2a2800000, 0x00007fd2ac1315d8, 0x00007fd2ac131600, 0x00007fd2c2800000)
No shared spaces configured.
}

其他类似问题报告：

http://grokbase.com/t/openjdk/hotspot-gc-use/1192sy84j5/g1c-strange-full-gc-behavior http://grokbase.com/p/openjdk/hotspot-gc-use/123ydf9c92/puzzling-why-is-a-full-gc-triggered-here http://mail.openjdk.java.net/pipermail/hotspot-gc-use/2013-February/001484.html

我一直在使用 appdynamics profiler 分析问题，我发现每次发生 Full GC 时，代码缓存（配置为最大值）都已满。这似乎是 GC 中的一个错误。

另请参阅分析器图像，两个不必要的 Full GC：在 24/5 和 25/5 之间。更重要的是，它们扼杀了服务器的可用性，因为它们每个持续 60 秒：

Profiler log image http://eisler.vps.kotisivut.com/logs/g1gc-code-cache-full-gc-bug-illustration.png

另见关于 Azul 的 pauseless GC 的讨论，他们似乎已经解决了这类问题http://www.artima.com/lejava/articles/azul_pauseless_gc.html

【问题讨论】：

这是整个 GS 日志，还是您留下/编辑了内容？
整个 GS 日志超过 24 小时，所以是的，它是简单多汁部分的编辑版本。
关于为什么 JVM 需要执行完整 GC 的线索不在这里。
好吧，如果您仔细阅读，这就是整个问题的重点：日志中没有任何线索说明为什么需要 Full GC -> 是 JVM 中的错误吗？我添加了一些前面的事件，请查看是否有任何迹象表明有任何可能导致 Full GC。
好吧，如果你仔细考虑的话，我的评论的重点是，虽然你在你遗漏的日志部分中看不到任何线索 other人们可能！

标签： java garbage-collection g1gc

【解决方案1】：

请看我的回复 G1GC long pause with initial-mark

你的每一个设置都应该有充分的理由来这里......不幸的是其中一些没有例如 -XX:+UseBiasedLocking (used for combination of tenured and young generation GCs, but G1GC is capable to handle both) -XX:+DisableExplicitGC (its unpredictable, in my experience its never restrict explicit gc calls)

请相应地使用/调整下面提到的设置以获得最佳结果，我正在为您提供前进的基准，希望这些设置对您有用： Java G1 garbage collection in production

【讨论】：

谢谢。我们有一些“shot弹枪”参数的原因是因为它们没有很好地记录什么去哪里以及哪些影响什么......为什么你认为+UseAdaptiveSizePolicy不是一个好主意？ -XX:+DoEscapeAnalysis 在JDK7 中是否应该默认开启？ GCPauseIntervalMillis 的实际值重要还是 MaxGCPauseMillis/GCPauseIntervalMillis 的比率重要？
这个答案还没有帮助解决问题。
@emkays G1 当您的堆大小大于 6GB 时推荐使用。

【解决方案2】：

我们在 1.7._u06 上记录了这个 bug，并在几天前升级到 1.7.0_21-b11，升级后我们还没有看到任何 Full GC，所以这个 bug 似乎在 JVM 中得到了修复。代码缓存内存配置文件现在在分析器中看起来也更好了。过去，这个问题曾经是每天一个，每天一到多次。

如果情况发生变化，我会回来报告。在那之前，我认为升级解决了这个问题。

【讨论】：

我们遇到了更多负载较高的问题，而热点 jvm 无法解决这些问题。最后，我们找到了使用 Azul 的 Zing jvm 的长期解决方案。我的经验是非常积极的，几乎所有问题都消失了，我花零时间调整它。考虑到我以前每天会花费几个小时来调整和分析 g1gc，这非常重要，这可以节省每月 20 天几个小时的工作成本。最显着的好处是：1）无需调整 2）不再需要 Full GC:s，最长延迟以毫秒为单位 3）azulsystems.com/products/zing/technical-case