为什么G1GC在开始混合系列之前缩小了年轻一代？
2022-09-03 15:12:41
当G1决定需要开始混合收集时，它会积极地将我们的伊甸园空间从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
这是在它用10-11克伊甸园运行60个或更多系列之后。
以下是我们运行的相应 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"
根据本演示的第 55 页，它需要调整 Eden 的大小，以便最大暂停目标适用于整个堆，而不仅仅是新一代。为什么收藏家如此咄咄逼人？
对于10g的堆大小，年轻一代的平均暂停时间在50-150ms之间。如果演示是正确的（我没有找到其他任何东西来支持这个说法），我预计会缩小一半（20克堆），而不是10倍。
