为什么G1在达到初始抱负率时不开始标记周期？

根据文件，XX:InitiatingHeapOccupancyPercent

设置触发标记周期的 Java 堆占用阈值。默认占用率为整个 Java 堆的 45%。

在我目前的环境中，这种情况不会发生。

我的 G1 垃圾回收配置如下

-Xms25000m
-Xmx25000m
-XX:+UseG1GC
-XX:MaxGCPauseMillis=1000
-XX:GCTimeRatio=99
-XX:InitiatingHeapOccupancyPercent=70
-XX:MaxTenuringThreshold=8
-XX:+UnlockExperimentalVMOptions
-XX:G1MixedGCCountTarget=16
-XX:G1OldCSetRegionThresholdPercent=3
-XX:G1NewSizePercent=30
-XX:G1RSetUpdatingPauseTimePercent=5

对于25g堆和70%的堆，您会期望在18g被占用时开始标记周期。我正在跟踪垃圾回收日志，但这并没有发生。XX:InitiatingHeapOccupancyPercent

这是一个摘录：

{Heap before GC invocations=592 (full 0):
 garbage-first heap   total 25600000K, used 22802164K [0x00000001a5800000, 0x00000001a60061a8, 0x00000007c0000000)
  region size 8192K, 1526 young (12500992K), 25 survivors (204800K)
 Metaspace       used 37386K, capacity 37948K, committed 38144K, reserved 1083392K
  class space    used 3948K, capacity 4080K, committed 4096K, reserved 1048576K
2016-04-20T22:06:38.272+0000: 4213.406: [GC pause (GCLocker Initiated GC) (young)
Desired survivor size 801112064 bytes, new threshold 8 (max 8)
- age   1:   98537800 bytes,   98537800 total
- age   2:    7053912 bytes,  105591712 total
- age   3:    6556320 bytes,  112148032 total
- age   4:    8836064 bytes,  120984096 total
- age   5:    5725448 bytes,  126709544 total
- age   6:    6702728 bytes,  133412272 total
- age   7:    3831920 bytes,  137244192 total
- age   8:    4166336 bytes,  141410528 total
 4213.406: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 184844, predicted base time: 44.67 ms, remaining time: 955.33 ms, target pause time: 1000.00 ms]
 4213.406: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 1501 regions, survivors: 25 regions, predicted young region time: 21.21 ms]
 4213.406: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 1501 regions, survivors: 25 regions, old: 0 regions, predicted pause time: 65.88 ms, target pause time: 1000.00 ms]
 4213.475: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason: recent GC overhead higher than threshold after GC, recent GC overhead: 1.40 %, threshold: 1.00 %, uncommitted: 0 bytes, calculated expansion amount: 0 bytes (20.00 %)]
, 0.0687163 secs]
   [Parallel Time: 61.7 ms, GC Workers: 28]
      [GC Worker Start (ms): Min: 4213406.9, Avg: 4213407.1, Max: 4213407.3, Diff: 0.4]
      [Ext Root Scanning (ms): Min: 6.0, Avg: 6.2, Max: 6.4, Diff: 0.4, Sum: 173.1]
      [Update RS (ms): Min: 33.5, Avg: 34.0, Max: 34.6, Diff: 1.1, Sum: 951.9]
         [Processed Buffers: Min: 27, Avg: 36.6, Max: 48, Diff: 21, Sum: 1024]
      [Scan RS (ms): Min: 0.1, Avg: 0.2, Max: 0.5, Diff: 0.4, Sum: 6.3]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
      [Object Copy (ms): Min: 20.1, Avg: 20.6, Max: 20.8, Diff: 0.7, Sum: 577.5]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.7]
         [Termination Attempts: Min: 1, Avg: 13.2, Max: 19, Diff: 18, Sum: 371]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.3, Sum: 4.7]
      [GC Worker Total (ms): Min: 60.9, Avg: 61.2, Max: 61.6, Diff: 0.6, Sum: 1714.2]
      [GC Worker End (ms): Min: 4213468.2, Avg: 4213468.3, Max: 4213468.5, Diff: 0.3]
   [Code Root Fixup: 0.4 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 1.2 ms]
   [Other: 5.4 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.5 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.8 ms]
      [Humongous Register: 0.2 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 2.4 ms]
   [Eden: 11.7G(11.7G)->0.0B(11.7G) Survivors: 200.0M->200.0M Heap: 21.7G(24.4G)->10.0G(24.4G)]
Heap after GC invocations=593 (full 0):
 garbage-first heap   total 25600000K, used 10516798K [0x00000001a5800000, 0x00000001a60061a8, 0x00000007c0000000)
  region size 8192K, 25 young (204800K), 25 survivors (204800K)
 Metaspace       used 37386K, capacity 37948K, committed 38144K, reserved 1083392K
  class space    used 3948K, capacity 4080K, committed 4096K, reserved 1048576K
}
 [Times: user=1.70 sys=0.01, real=0.07 secs] 
2016-04-20T22:06:38.342+0000: 4213.475: Total time for which application threads were stopped: 0.0701353 seconds, Stopping threads took: 0.0001600 seconds

我会提请你注意

{Heap before GC invocations=592 (full 0):
   garbage-first heap   total 25600000K, used 22802164K [0x00000001a5800000, 0x00000001a60061a8, 0x00000007c0000000)
[...]
[Eden: 11.7G(11.7G)->0.0B(11.7G) Survivors: 200.0M->200.0M Heap: 21.7G(24.4G)->10.0G(24.4G)]

在此集合之前，超过 70% 的堆已被占用。为什么没有触发标记周期？

该应用程序继续进行年轻一代的收集，填满了旧区域，最终导致分配失败和冗长的全GC。

减少到55没有明显的效果。InitiatingHeapOccupancyPercent

在20岁时，它确实开始进行混合收集，但只有当大约80%的堆被占用时。