G1GC

G1GC - Tenured如何在没有混合GC的情况下进行清洁(G1GC - How can Tenured get cleaned without a mixed GC)

我已经创建了一个测试Java程序来帮助理解G1GC的工作原理。

我的程序有两个线程 - 一个加载一个大型的ArrayList，大约200MB的对象（只有1000个字符串），然后它删除一个元素，并在一个循环中添加一个元素，其中有一个sleep。 这里的想法是让它慢慢老化并最终落入Tenured。

另一个线程只是在while循环中创建一个大字符串来创建大量垃圾以导致GC运行。

这一切似乎都运作正常，但我感到困惑的是，终身的浪费似乎得到清理，没有混合的GC运行。

[Eden: 581.0M(581.0M)->0.0B(573.0M) Survivors: 9216.0K->9216.0K Heap: 904.3M(1024.0M)->330.9M(1024.0M)] [Times: user=0.03 sys=0.00, real=0.00 secs] 2017-09-28T16:22:14.889+0000: 2.415: 2017-09-28T16:22:14.889+0000[GC concurrent-root-region-scan-start] : 2.415: Total time for which application threads were stopped: 0.0081917 seconds, Stopping threads took: 0.0000852 seconds 2017-09-28T16:22:14.889+0000: 2.415: [GC concurrent-root-region-scan-end, 0.0002779 secs] 2017-09-28T16:22:14.889+0000: 2.415: [GC concurrent-mark-start] 2017-09-28T16:22:14.904+0000: 2.430: [GC concurrent-mark-end, 0.0143310 secs] 2017-09-28T16:22:14.905+0000: 2.431: [GC remark 2017-09-28T16:22:14.905+0000: 2.431: [Finalize Marking, 0.0003653 secs] 2017-09-28T16:22:14.905+0000: 2.431: [GC ref-proc2017-09-28T16:22:14.905+0000: 2.431: [SoftReference, 0 refs, 0.0008964 secs]2017-09-28T16:22:14.906+0000: 2.432: [WeakReference, 0 refs, 0.0005380 secs]2017-09-28T16:22:14.907+0000: 2.433: [FinalReference, 2 refs, 0.0005460 secs]2017-09-28T16:22:14.907+0000: 2.434: [PhantomReference, 0 refs, 0 refs, 0.0010243 secs]2017-09-28T16:22:14.908+0000: 2.435: [JNI Weak Reference, 0.0000090 secs], 0.0034419 secs] 2017-09-28T16:22:14.909+0000: 2.435: [Unloading, 0.0008396 secs], 0.0057362 secs] [Times: user=0.03 sys=0.00, real=0.01 secs] 2017-09-28T16:22:14.911+0000: 2.437: Total time for which application threads were stopped: 0.0069810 seconds, Stopping threads took: 0.0011536 seconds 2017-09-28T16:22:14.911+0000: 2.437: [GC cleanup 418M->302M(1024M), 0.0014100 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 2017-09-28T16:22:14.912+0000: 2.438: Total time for which application threads were stopped: 0.0015190 seconds, Stopping threads took: 0.0000697 seconds 2017-09-28T16:22:14.912+0000: 2.438: [GC concurrent-cleanup-start] 2017-09-28T16:22:14.912+0000: 2.439: [GC concurrent-cleanup-end, 0.0000987 secs] 2017-09-28T16:22:14.987+0000: 2.513: [GC pause (G1 Evacuation Pause) (young) 2.513: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 4816, predicted base time: 5.38 ms, remaining time: 44.62 ms, target pause time: 50.00 ms] 2.513: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 573 regions, survivors: 9 regions, predicted young region time: 3.54 ms] 2.513: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 573 regions, survivors: 9 regions, old: 0 regions, predicted pause time: 8.92 ms, target pause time: 50.00 ms] 2017-09-28T16:22:14.991+0000: 2.517: [SoftReference, 0 refs, 0.0010742 secs]2017-09-28T16:22:14.992+0000: 2.518: [WeakReference, 0 refs, 0.0006150 secs]2017-09-28T16:22:14.992+0000: 2.519: [FinalReference, 0 refs, 0.0004760 secs]2017-09-28T16:22:14.993+0000: 2.519: [PhantomReference, 0 refs, 0 refs, 0.0008707 secs]2017-09-28T16:22:14.994+0000: 2.520: [JNI Weak Reference, 0.0000094 secs] 2.521: [G1Ergonomics (Mixed GCs) do not start mixed GCs, reason: reclaimable percentage not over threshold, candidate old regions: 14 regions, reclaimable: 13071400 bytes (1.22 %), threshold: 5.00 %] , 0.0077749 secs] [Parallel Time: 3.6 ms, GC Workers: 16] [GC Worker Start (ms): Min: 2513.1, Avg: 2513.6, Max: 2515.2, Diff: 2.1] [Ext Root Scanning (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.1, Sum: 0.9] [Update RS (ms): Min: 0.0, Avg: 0.4, Max: 1.1, Diff: 1.1, Sum: 6.3] [Processed Buffers: Min: 0, Avg: 1.2, Max: 4, Diff: 4, Sum: 20] [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.8] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1] [Object Copy (ms): Min: 1.2, Avg: 2.3, Max: 2.9, Diff: 1.7, Sum: 36.9] [Termination (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 2.1] [Termination Attempts: Min: 1, Avg: 1.1, Max: 2, Diff: 1, Sum: 18] [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.0, Sum: 0.3] [GC Worker Total (ms): Min: 1.4, Avg: 3.0, Max: 3.5, Diff: 2.1, Sum: 47.3] [GC Worker End (ms): Min: 2516.5, Avg: 2516.6, Max: 2516.6, Diff: 0.1] [Code Root Fixup: 0.0 ms] [Code Root Purge: 0.0 ms] [Clear CT: 0.2 ms] [Other: 4.0 ms] [Choose CSet: 0.0 ms] [Ref Proc: 3.4 ms] [Ref Enq: 0.2 ms] [Redirty Cards: 0.1 ms] [Humongous Register: 0.0 ms] [Humongous Reclaim: 0.0 ms] [Free CSet: 0.2 ms] [Eden: 573.0M(573.0M)->0.0B(607.0M) Survivors: 9216.0K->7168.0K Heap: 787.9M(1024.0M)->221.4M(1024.0M)]

这是GC日志的片段。 注意这次运行之间：

[Eden: 581.0M(581.0M)->0.0B(573.0M) Survivors: 9216.0K->9216.0K Heap: 904.3M(1024.0M)->330.9M(1024.0M)] ... [Eden: 573.0M(573.0M)->0.0B(607.0M) Survivors: 9216.0K->7168.0K Heap: 787.9M(1024.0M)->221.4M(1024.0M)]

因此Tenured的用法似乎在这次运行中缩减了。 但是，没有提到混合GC，事实上它给出的是：

[G1Ergonomics (Mixed GCs) do not start mixed GCs, reason: reclaimable percentage not over threshold, candidate old regions: 14 regions, reclaimable: 13071400 bytes (1.22 %), threshold: 5.00 %]

所以我的问题是 - 如果没有混合（或完全）GC发生，终身空间如何得到清理？

I have created a test Java program to help understand how the G1GC works.

My program has two threads - one loads a large ArrayList with about 200MB of objects (just 1000 character strings) and then it removes an element and adds an element in a loop with a sleep in it. The idea here it to have something that ages slowly and will end up in Tenured.

The other thread just creates a large string in a while loop to create lots of garbage to cause GC runs.

This all seems to work OK, but what I am confused about is that the waste in tenured seems to get cleaned with no mixed GCs running.

[Eden: 581.0M(581.0M)->0.0B(573.0M) Survivors: 9216.0K->9216.0K Heap: 904.3M(1024.0M)->330.9M(1024.0M)] [Times: user=0.03 sys=0.00, real=0.00 secs] 2017-09-28T16:22:14.889+0000: 2.415: 2017-09-28T16:22:14.889+0000[GC concurrent-root-region-scan-start] : 2.415: Total time for which application threads were stopped: 0.0081917 seconds, Stopping threads took: 0.0000852 seconds 2017-09-28T16:22:14.889+0000: 2.415: [GC concurrent-root-region-scan-end, 0.0002779 secs] 2017-09-28T16:22:14.889+0000: 2.415: [GC concurrent-mark-start] 2017-09-28T16:22:14.904+0000: 2.430: [GC concurrent-mark-end, 0.0143310 secs] 2017-09-28T16:22:14.905+0000: 2.431: [GC remark 2017-09-28T16:22:14.905+0000: 2.431: [Finalize Marking, 0.0003653 secs] 2017-09-28T16:22:14.905+0000: 2.431: [GC ref-proc2017-09-28T16:22:14.905+0000: 2.431: [SoftReference, 0 refs, 0.0008964 secs]2017-09-28T16:22:14.906+0000: 2.432: [WeakReference, 0 refs, 0.0005380 secs]2017-09-28T16:22:14.907+0000: 2.433: [FinalReference, 2 refs, 0.0005460 secs]2017-09-28T16:22:14.907+0000: 2.434: [PhantomReference, 0 refs, 0 refs, 0.0010243 secs]2017-09-28T16:22:14.908+0000: 2.435: [JNI Weak Reference, 0.0000090 secs], 0.0034419 secs] 2017-09-28T16:22:14.909+0000: 2.435: [Unloading, 0.0008396 secs], 0.0057362 secs] [Times: user=0.03 sys=0.00, real=0.01 secs] 2017-09-28T16:22:14.911+0000: 2.437: Total time for which application threads were stopped: 0.0069810 seconds, Stopping threads took: 0.0011536 seconds 2017-09-28T16:22:14.911+0000: 2.437: [GC cleanup 418M->302M(1024M), 0.0014100 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 2017-09-28T16:22:14.912+0000: 2.438: Total time for which application threads were stopped: 0.0015190 seconds, Stopping threads took: 0.0000697 seconds 2017-09-28T16:22:14.912+0000: 2.438: [GC concurrent-cleanup-start] 2017-09-28T16:22:14.912+0000: 2.439: [GC concurrent-cleanup-end, 0.0000987 secs] 2017-09-28T16:22:14.987+0000: 2.513: [GC pause (G1 Evacuation Pause) (young) 2.513: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 4816, predicted base time: 5.38 ms, remaining time: 44.62 ms, target pause time: 50.00 ms] 2.513: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 573 regions, survivors: 9 regions, predicted young region time: 3.54 ms] 2.513: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 573 regions, survivors: 9 regions, old: 0 regions, predicted pause time: 8.92 ms, target pause time: 50.00 ms] 2017-09-28T16:22:14.991+0000: 2.517: [SoftReference, 0 refs, 0.0010742 secs]2017-09-28T16:22:14.992+0000: 2.518: [WeakReference, 0 refs, 0.0006150 secs]2017-09-28T16:22:14.992+0000: 2.519: [FinalReference, 0 refs, 0.0004760 secs]2017-09-28T16:22:14.993+0000: 2.519: [PhantomReference, 0 refs, 0 refs, 0.0008707 secs]2017-09-28T16:22:14.994+0000: 2.520: [JNI Weak Reference, 0.0000094 secs] 2.521: [G1Ergonomics (Mixed GCs) do not start mixed GCs, reason: reclaimable percentage not over threshold, candidate old regions: 14 regions, reclaimable: 13071400 bytes (1.22 %), threshold: 5.00 %] , 0.0077749 secs] [Parallel Time: 3.6 ms, GC Workers: 16] [GC Worker Start (ms): Min: 2513.1, Avg: 2513.6, Max: 2515.2, Diff: 2.1] [Ext Root Scanning (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.1, Sum: 0.9] [Update RS (ms): Min: 0.0, Avg: 0.4, Max: 1.1, Diff: 1.1, Sum: 6.3] [Processed Buffers: Min: 0, Avg: 1.2, Max: 4, Diff: 4, Sum: 20] [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.8] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1] [Object Copy (ms): Min: 1.2, Avg: 2.3, Max: 2.9, Diff: 1.7, Sum: 36.9] [Termination (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 2.1] [Termination Attempts: Min: 1, Avg: 1.1, Max: 2, Diff: 1, Sum: 18] [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.0, Sum: 0.3] [GC Worker Total (ms): Min: 1.4, Avg: 3.0, Max: 3.5, Diff: 2.1, Sum: 47.3] [GC Worker End (ms): Min: 2516.5, Avg: 2516.6, Max: 2516.6, Diff: 0.1] [Code Root Fixup: 0.0 ms] [Code Root Purge: 0.0 ms] [Clear CT: 0.2 ms] [Other: 4.0 ms] [Choose CSet: 0.0 ms] [Ref Proc: 3.4 ms] [Ref Enq: 0.2 ms] [Redirty Cards: 0.1 ms] [Humongous Register: 0.0 ms] [Humongous Reclaim: 0.0 ms] [Free CSet: 0.2 ms] [Eden: 573.0M(573.0M)->0.0B(607.0M) Survivors: 9216.0K->7168.0K Heap: 787.9M(1024.0M)->221.4M(1024.0M)]

This is a snippet of the GC log. Note that between this run:

[Eden: 581.0M(581.0M)->0.0B(573.0M) Survivors: 9216.0K->9216.0K Heap: 904.3M(1024.0M)->330.9M(1024.0M)] ... [Eden: 573.0M(573.0M)->0.0B(607.0M) Survivors: 9216.0K->7168.0K Heap: 787.9M(1024.0M)->221.4M(1024.0M)]

So the usage in Tenured seems to have shrank on this run. However, there is no mention of a mixed GC, infact all it gives is:

[G1Ergonomics (Mixed GCs) do not start mixed GCs, reason: reclaimable percentage not over threshold, candidate old regions: 14 regions, reclaimable: 13071400 bytes (1.22 %), threshold: 5.00 %]

So my question is - how is the tenured space getting cleaned without a mixed (or a full) GC happening?

最满意答案

不需要混合收集来撤离已被发现完全没有被并发周期引起的旧区域。

2017-09-28T16：22：14.911 + 0000：2.437：[GC清理418M-> 302M（1024M），0.0014100秒]

还有一些额外的日志记录标志，例如-XX:+G1PrintHeapRegions ，可用于跟踪此类清理。 有关详细说明，请参阅有关G1GC日志的oracle博客 。

No mixed collections are needed to evacuate old regions that have been found to be totally unreferenced by the concurrent cycle.

2017-09-28T16:22:14.911+0000: 2.437: [GC cleanup 418M->302M(1024M), 0.0014100 secs]

There are some additional logging flags, e.g. -XX:+G1PrintHeapRegions, that can be used to track such cleanup. See the oracle blog on G1GC logs for in-depth explanation.