当G1决定需要开始进行混合收集时，它会积极地将我们的Eden空间从10g缩小到大约1g。
{Heap before GC invocations=294 (full 0):
garbage-first heap total 20480000K, used 18304860K [0x00000002de000000, 0x00000002de804e20, 0x00000007c0000000)
region size 8192K, 1363 young (11165696K), 11 survivors (90112K)
Metaspace used 37327K, capacity 37826K, committed 38096K, reserved 1083392K
class space used 3935K, capacity 4081K, committed 4096K, reserved 1048576K
2016-03-31T20:57:31.002+0000: 7196.427: [GC pause (G1 Evacuation Pause) (young)
Desired survivor size 717225984 bytes, new threshold 1 (max 1)
- age 1: 41346816 bytes, 41346816 total
7196.427: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 144693, predicted base time: 48.88 ms, remaining time: 951.12 ms, target pause time: 1000.00 ms]
7196.427: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 1352 regions, survivors: 11 regions, predicted young region time: 20.72 ms]
7196.427: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 1352 regions, survivors: 11 regions, old: 0 regions, predicted pause time: 69.60 ms, target pause time: 1000.00 ms]
7196.494: [G1Ergonomics (Mixed GCs) start mixed GCs, reason: candidate old regions available, candidate old regions: 789 regions, reclaimable: 4703761904 bytes (22.43 %), threshold: 5.00 %]
, 0.0673540 secs]
[Parallel Time: 60.1 ms, GC Workers: 18]
[GC Worker Start (ms): Min: 7196427.8, Avg: 7196428.1, Max: 7196428.2, Diff: 0.4]
[Ext Root Scanning (ms): Min: 7.3, Avg: 7.5, Max: 7.7, Diff: 0.4, Sum: 134.2]
[Update RS (ms): Min: 28.2, Avg: 28.8, Max: 29.9, Diff: 1.7, Sum: 518.4]
[Processed Buffers: Min: 41, Avg: 57.7, Max: 80, Diff: 39, Sum: 1039]
[Scan RS (ms): Min: 0.1, Avg: 0.2, Max: 0.5, Diff: 0.4, Sum: 3.7]
[Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
[Object Copy (ms): Min: 22.1, Avg: 23.1, Max: 23.4, Diff: 1.3, Sum: 416.2]
[Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
[Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 18]
[GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 2.5]
[GC Worker Total (ms): Min: 59.5, Avg: 59.7, Max: 60.0, Diff: 0.5, Sum: 1075.1]
[GC Worker End (ms): Min: 7196487.7, Avg: 7196487.8, Max: 7196487.9, Diff: 0.2]
[Code Root Fixup: 0.2 ms]
[Code Root Purge: 0.0 ms]
[Clear CT: 1.9 ms]
[Other: 5.2 ms]
[Choose CSet: 0.0 ms]
[Ref Proc: 0.5 ms]
[Ref Enq: 0.0 ms]
[Redirty Cards: 0.5 ms]
[Humongous Register: 0.2 ms]
[Humongous Reclaim: 0.1 ms]
[Free CSet: 2.3 ms]
[Eden: 10.6G(10.6G)->0.0B(848.0M) Survivors: 88.0M->152.0M Heap: 17.5G(19.5G)->7128.3M(19.5G)]
Heap after GC invocations=295 (full 0):
garbage-first heap total 20480000K, used 7299344K [0x00000002de000000, 0x00000002de804e20, 0x00000007c0000000)
region size 8192K, 19 young (155648K), 19 survivors (155648K)
Metaspace used 37327K, capacity 37826K, committed 38096K, reserved 1083392K
class space used 3935K, capacity 4081K, committed 4096K, reserved 1048576K
}
[Times: user=1.09 sys=0.00, real=0.07 secs]
2016-03-31T20:57:31.070+0000: 7196.495: Total time for which application threads were stopped: 0.0699324 seconds, Stopping threads took: 0.0003462 seconds
这是因为它已经与10-11克伊甸园一起运行了60个或更多的 Collection 。
这是我们正在运行的适当的JVM GC参数
-Xms20000m -Xmx20000m
-XX:+UseG1GC
-XX:G1RSetUpdatingPauseTimePercent=5
-XX:MaxGCPauseMillis=1000
-XX:GCTimeRatio=99
-XX:InitiatingHeapOccupancyPercent=35
-XX:MaxTenuringThreshold=1
-XX:G1ConcRefinementThreads=6
-XX:ConcGCThreads=18
-XX:ParallelGCThreads=18
-XX:+PrintGCDetails"
-XX:+PrintGCDateStamps"
-XX:+PrintHeapAtGC"
-XX:+PrintTenuringDistribution"
-XX:+PrintGCApplicationStoppedTime"
-XX:+PrintPromotionFailure"
-XX:+PrintAdaptiveSizePolicy"
根据page 55 of this presentation的说法，它需要调整Eden的大小，以便最大暂停目标占整个堆的空间，而不仅仅是新一代的。为什么 Collection 家如此激进？
对于10g的堆，平均年轻一代的暂停时间在50-150ms之间。如果演示文稿是正确的(我没有找到其他支持该声明的内容)，我希望收缩一半(20g堆)，而不是10x。
最佳答案
您可以在幻灯片编号56中找到查询的答案。
因此，将工厂缩小10倍并不奇怪。
摘自Monica Beckwith的infoQ文章，介绍了G1GC的调优技巧:
由于您尚未明确设置年轻一代的大小，因此默认值为
-XX:G1NewSizePercent=5
设置要用作年轻代大小的最小值的堆百分比。
因此要尊重您的暂停时间目标
-XX:MaxGCPauseMillis=1000
年轻的一代最多可以缩小总堆的5％。
我在以下位置找到了一篇有关G1GC的不错的Google组文章:
https://groups.google.com/a/jclarity.com/forum/#!msg/friends/hsZiz6HTm9M/klrRjBclCwAJ
如果G1预测的暂停时间目标大于目标暂停时间目标，则收缩年轻代，但不超过当前Java堆大小(而不是最大大小)的 G1NewSizePercent 。同样，基于计算的GC时间比率与 GCTimeRatio 的值，整个Java堆将增长(或收缩)。
注意:不要将G1NewSizePercent和G1MaxNewSizePercent与NewSize或MaxNewSize混淆。
G1NewSizePercent和G1MaxNewSizePercent分别为G1可以确定多大的年轻一代设定了下限和上限。
另一方面，您已配置了许多参数，如果大多数默认参数已设置为默认值，则可能不必要的正弦G1GC可以正常工作。有关更多详细信息，请引用此SE问题。
Java 7 (JDK 7) garbage collection and documentation on G1
总结:根据暂停时间目标，年轻一代会缩小。如果您真的担心年轻一代缩小到低值，请配置
-XX:G1NewSizePercent。但只要满足-XX:MaxGCPauseMillis，我就不建议您这样做编辑:
在G1GC ergonomics页面中，