为什么G1不释放内存？

Question

我有一个java 11应用程序运行在一个带有以下标志的docker容器中：

-XX:CICompilerCount=12
-XX:ConcGCThreads=3
-XX:+FlightRecorder
-XX:G1ConcRefinementThreads=13
-XX:G1HeapRegionSize=4194304
-XX:GCDrainStackTargetSize=64
-XX:InitialHeapSize=1052770304
-XX:MarkStackSize=4194304
-XX:MaxHeapSize=16823353344
-XX:MaxNewSize=10091495424
-XX:MinHeapDeltaBytes=4194304
-XX:NonNMethodCodeHeapSize=7594288
-XX:NonProfiledCodeHeapSize=122031976
-XX:ProfiledCodeHeapSize=122031976
-XX:ReservedCodeCacheSize=251658240
-XX:+SegmentedCodeCache
-XX:+UseCompressedClassPointers
-XX:+UseCompressedOops
-XX:+UseFastUnorderedTimeStamps
-XX:+UseG1GC

所发生的情况是，在加载下，docker stats显示的内存使用量非常高(7Gb)。这已经很奇怪了，因为尽管我创建了许多对象，但它们应该是动态收集的，因为对象彼此之间没有链接，而且我没有保留对这些对象的任何引用(我只是在迭代器上迭代)。无论如何，当加载停止时，所有这些对象都应该被垃圾收集，但我看到的是，报告的内存使用情况保持不变(7Gb)。只有在运行jcmd 1 GC.run时，才会看到内存使用量下降到大约1Gb。

我怀疑GC可能出于某种原因无法运行，因此我使用-Xlog:gc打开了GC日志，但事实证明它正在按预期运行：

[236.335s][info][gc] GC(14) Concurrent Cycle
[236.352s][info][gc] GC(15) Pause Young (Normal) (G1 Evacuation Pause) 84M->81M(124M) 4.344ms
[236.361s][info][gc] GC(16) Pause Young (Normal) (G1 Evacuation Pause) 85M->84M(124M) 2.911ms
[236.370s][info][gc] GC(17) Pause Young (Normal) (G1 Evacuation Pause) 88M->87M(124M) 2.207ms
[236.380s][info][gc] GC(18) Pause Young (Normal) (G1 Evacuation Pause) 91M->89M(124M) 2.192ms
[236.390s][info][gc] GC(19) Pause Young (Normal) (G1 Evacuation Pause) 93M->92M(124M) 2.139ms
[236.400s][info][gc] GC(20) Pause Young (Normal) (G1 Evacuation Pause) 96M->94M(124M) 2.191ms
[236.409s][info][gc] GC(21) Pause Young (Normal) (G1 Evacuation Pause) 98M->96M(124M) 2.132ms
[236.418s][info][gc] GC(22) Pause Young (Normal) (G1 Evacuation Pause) 100M->99M(124M) 2.139ms
[236.426s][info][gc] GC(23) Pause Young (Normal) (G1 Evacuation Pause) 103M->101M(124M) 2.121ms
[236.435s][info][gc] GC(24) Pause Young (Normal) (G1 Evacuation Pause) 105M->104M(124M) 2.075ms
[236.441s][info][gc] GC(14) Pause Remark 106M->106M(124M) 3.191ms
[236.447s][info][gc] GC(25) Pause Young (Normal) (G1 Evacuation Pause) 108M->106M(124M) 2.184ms
[236.456s][info][gc] GC(26) Pause Young (Normal) (G1 Evacuation Pause) 110M->108M(568M) 3.698ms
[236.491s][info][gc] GC(14) Pause Cleanup 122M->122M(568M) 0.141ms
[236.493s][info][gc] GC(14) Concurrent Cycle 157.955ms
[236.827s][info][gc] GC(27) Pause Young (Normal) (G1 Evacuation Pause) 260M->141M(568M) 19.249ms
[237.196s][info][gc] GC(28) Pause Young (Normal) (G1 Evacuation Pause) 289M->142M(568M) 19.387ms
[237.441s][info][gc] GC(29) Pause Young (Normal) (G1 Evacuation Pause) 302M->144M(568M) 12.322ms
[237.531s][info][gc] GC(30) Pause Young (Normal) (G1 Evacuation Pause) 192M->143M(1704M) 14.226ms
[238.441s][info][gc] GC(31) Pause Young (Normal) (G1 Evacuation Pause) 699M->157M(1704M) 23.429ms
[238.763s][info][gc] GC(32) Pause Young (Normal) (G1 Evacuation Pause) 241M->197M(1704M) 44.961ms
[239.907s][info][gc] GC(33) Pause Young (Normal) (G1 Evacuation Pause) 1201M->274M(1704M) 57.209ms
[240.671s][info][gc] GC(34) Pause Young (Normal) (G1 Evacuation Pause) 1198M->344M(3596M) 88.736ms
[error] ErrorHandler - An error occurred (404): 
[241.836s][info][gc] GC(35) Pause Young (Normal) (G1 Evacuation Pause) 1360M->434M(3596M) 91.769ms
[242.729s][info][gc] GC(36) Pause Young (Normal) (G1 Evacuation Pause) 1518M->490M(3596M) 95.333ms
[243.834s][info][gc] GC(37) Pause Young (Normal) (G1 Evacuation Pause) 1594M->481M(3596M) 51.726ms
[245.636s][info][gc] GC(38) Pause Young (Normal) (G1 Evacuation Pause) 2545M->490M(6108M) 46.050ms
[249.053s][info][gc] GC(39) Pause Young (Normal) (G1 Evacuation Pause) 4054M->500M(6108M) 46.525ms
[251.903s][info][gc] GC(40) Pause Young (Normal) (G1 Evacuation Pause) 4052M->498M(6108M) 41.389ms
[254.797s][info][gc] GC(41) Pause Young (Normal) (G1 Evacuation Pause) 4054M->495M(6108M) 44.757ms
[257.506s][info][gc] GC(42) Pause Young (Normal) (G1 Evacuation Pause) 4051M->484M(6108M) 35.766ms
[260.329s][info][gc] GC(43) Pause Young (Normal) (G1 Evacuation Pause) 4052M->487M(6108M) 34.923ms
[263.337s][info][gc] GC(44) Pause Young (Normal) (G1 Evacuation Pause) 4051M->472M(6108M) 30.865ms
[266.368s][info][gc] GC(45) Pause Young (Normal) (G1 Evacuation Pause) 4052M->481M(6108M) 35.517ms
[269.631s][info][gc] GC(46) Pause Young (Normal) (G1 Evacuation Pause) 4053M->491M(6108M) 39.524ms
[272.931s][info][gc] GC(47) Pause Young (Normal) (G1 Evacuation Pause) 4051M->484M(6108M) 37.010ms
[info] c.HomeController - Redirecting to swagger generated API
[276.067s][info][gc] GC(48) Pause Young (Normal) (G1 Evacuation Pause) 4052M->485M(6108M) 38.312ms
[279.409s][info][gc] GC(49) Pause Young (Normal) (G1 Evacuation Pause) 4053M->479M(6108M) 33.003ms
[308.226s][info][gc] GC(50) Pause Full (Diagnostic Command) 1220M->51M(256M) 652.015ms

所以问题是为什么内存使用量没有下降，因为它看起来像G1看起来像预期的那样释放内存。为什么jcmd 1 GC.run能有效地释放内存？？

编辑

我试过用CMS GC代替(-XX:+UseConcMarkSweepGC)和.完全没有记忆问题！内存按预期动态释放(最大使用量1.5Gb)。所以我现在假设问题是G1不是我的应用程序使用7Gb，而是G1本身是罪魁祸首(使用6Gb)。但为什么？

Answer 1

正如霍格所解释的:由于我将堆大小限制在16 G1，所以G1可以在这个限制范围内自由地使用它想要的内存。将Xmx设置为4Gb，我可以看到G1正在设法保持在4GB以下，这意味着没有内存问题！