如何使用Java长时间制作Full GC [英] How to make a long time Full GC in Java manually

问题描述

如何编写一些代码来将对象分配到Old Gen中，这将导致完整的GC时间超过三到五秒?

How to write some code to allocate the objects into Old Gen that will cause the full GC time longer than three or five seconds?

推荐答案

将对象分配给Old Gen"和长时间GC暂停"很难合并，因为您对垃圾收集器最糟糕的事情是创建许多小的，链接的活动对象，形成垃圾收集器必须遍历的图形.

"Allocate the objects into Old Gen" and "long GC pauses" are hard to combine, as the worst thing you could do to the garbage collector, is to create lots of small, linked, live objects forming a graph that the garbage collector must traverse.

但是小对象没有分配到旧一代中，只有大对象，例如数组直接分配给Old Gen，但是这些不会给GC带来太大的压力.尤其是原始类型数组，因为您通常不会包含对象引用，因此您在第一次尝试中使用的原始类型数组就没有什么挑战.

But small objects are not allocated into the Old Gen. Only large objects, e.g. arrays, are allocated directly into the Old Gen, but these do not stress the GC that much. Especially primitive type arrays, which you stated to have been used in your first attempt, are no challenge, as they can not contain object references in general.

因此，要强调GC，您必须创建许多小的链接对象，足以使它们最终升级为Old Gen，然后修改旧对象以强制执行Full GC.记忆受影响的记忆卡的更新.

So, to stress the GC, you have to create lots of small linked objects, enough to get them eventually promoted to the Old Gen, followed by modifying the old objects to enforce Full GC, resp. updates of the remembered sets of the affected cards.

如果这听起来像是熟悉的模式，那您是对的.我们在这里谈论LinkedList.

If that’s sounds like familiar pattern, you’re right. We’re talking about LinkedList here.

LinkedList<Object> list = new LinkedList<>();
for(ListIterator<Object> it = list.listIterator();;) {
    for(;;) {
        it.add(new Object());
        if(!it.hasNext()) break;
        it.next();
    }
    while(it.hasPrevious()) {
        it.previous();
        it.add(new Object());
        it.previous();
    }
}

这在列表上来回循环，在旧对象之间插入新对象，这意味着修改

This iterates over the list back and forth, inserting new objects between the old ones, which implies modifying the next pointer of the preceding and the prev pointer of the succeeding list node. We have to do this via ListIterator, as trying to use add(int index, E element) would become too slow for a growing linked list, not creating enough objects to stress the GC.

使用-Xmx7G -Xlog:gc在Java 9.0.4上运行给了我

Running on Java 9.0.4 with -Xmx7G -Xlog:gc gave me

[0.023s][info][gc] Using G1
[0.230s][info][gc] GC(0) Pause Young (G1 Evacuation Pause) 24M->22M(256M) 60.748ms
[0.263s][info][gc] GC(1) Pause Young (G1 Evacuation Pause) 34M->36M(256M) 21.686ms
[0.299s][info][gc] GC(2) Pause Young (G1 Evacuation Pause) 46M->47M(256M) 18.848ms
[0.336s][info][gc] GC(3) Pause Young (G1 Evacuation Pause) 57M->58M(256M) 20.904ms
[0.391s][info][gc] GC(4) Pause Young (G1 Evacuation Pause) 68M->68M(768M) 31.420ms
[0.521s][info][gc] GC(5) Pause Young (G1 Evacuation Pause) 104M->105M(768M) 56.681ms
[0.620s][info][gc] GC(6) Pause Young (G1 Evacuation Pause) 138M->140M(768M) 46.616ms
[0.737s][info][gc] GC(7) Pause Young (G1 Evacuation Pause) 173M->174M(768M) 58.947ms
[0.906s][info][gc] GC(8) Pause Young (G1 Evacuation Pause) 207M->209M(2304M) 100.361ms
[1.272s][info][gc] GC(9) Pause Young (G1 Evacuation Pause) 319M->321M(2304M) 142.440ms
[1.656s][info][gc] GC(10) Pause Young (G1 Evacuation Pause) 421M->423M(2304M) 162.770ms
[1.992s][info][gc] GC(11) Pause Young (G1 Evacuation Pause) 523M->525M(2304M) 145.804ms
[2.404s][info][gc] GC(12) Pause Young (G1 Evacuation Pause) 625M->627M(4250M) 221.090ms
[3.088s][info][gc] GC(13) Pause Young (G1 Evacuation Pause) 824M->826M(4250M) 302.307ms
[3.721s][info][gc] GC(14) Pause Young (G1 Evacuation Pause) 1011M->1013M(4250M) 250.395ms
[4.421s][info][gc] GC(15) Pause Young (G1 Evacuation Pause) 1198M->1199M(4250M) 304.642ms
[5.179s][info][gc] GC(16) Pause Young (G1 Evacuation Pause) 1384M->1386M(5418M) 334.825ms
[6.134s][info][gc] GC(17) Pause Young (G1 Evacuation Pause) 1629M->1631M(5418M) 381.155ms
[6.897s][info][gc] GC(18) Pause Young (G1 Evacuation Pause) 1867M->1868M(5418M) 309.688ms
[7.670s][info][gc] GC(19) Pause Young (G1 Evacuation Pause) 2104M->2106M(5418M) 399.689ms
[8.498s][info][gc] GC(20) Pause Young (G1 Evacuation Pause) 2342M->2344M(6118M) 388.430ms
[9.515s][info][gc] GC(21) Pause Young (G1 Evacuation Pause) 2615M->2617M(6118M) 439.557ms
[10.477s][info][gc] GC(22) Pause Young (G1 Evacuation Pause) 2883M->2884M(6118M) 451.488ms
[11.489s][info][gc] GC(23) Pause Initial Mark (G1 Evacuation Pause) 3150M->3152M(6118M) 443.329ms
[11.489s][info][gc] GC(24) Concurrent Cycle
[12.660s][info][gc] GC(25) Pause Young (G1 Evacuation Pause) 3418M->3419M(6538M) 467.964ms
[14.124s][info][gc] GC(26) Pause Young (G1 Evacuation Pause) 3706M->3708M(6538M) 454.921ms
[15.538s][info][gc] GC(27) Pause Young (G1 Evacuation Pause) 3993M->3995M(6538M) 465.929ms
[16.945s][info][gc] GC(28) Pause Young (G1 Evacuation Pause) 4280M->4281M(6538M) 460.913ms
[18.490s][info][gc] GC(29) Pause Young (G1 Evacuation Pause) 4566M->4568M(6790M) 493.803ms
[20.083s][info][gc] GC(30) Pause Young (G1 Evacuation Pause) 4866M->4868M(6790M) 491.910ms
[21.516s][info][gc] GC(31) Pause Young (G1 Evacuation Pause) 5164M->5166M(6790M) 486.847ms
[22.895s][info][gc] GC(32) Pause Young (G1 Evacuation Pause) 5462M->5464M(6790M) 443.031ms
[24.342s][info][gc] GC(33) Pause Young (G1 Evacuation Pause) 5760M->5762M(6942M) 475.235ms
[25.258s][info][gc] GC(24) Pause Remark 5988M->5988M(6942M) 0.794ms

[25.827s][info][gc] GC(34) Pause Young (G1 Evacuation Pause) 6066M->6067M(6942M) 458.463ms
[27.021s][info][gc] GC(35) Pause Young (G1 Evacuation Pause) 6370M->6372M(6942M) 448.794ms
[27.485s][info][gc] GC(24) Pause Cleanup 6484M->6484M(6942M) 28.631ms
[27.490s][info][gc] GC(24) Concurrent Cycle 16000.856ms
[28.250s][info][gc] GC(36) Pause Young (G1 Evacuation Pause) 6675M->6676M(7024M) 481.748ms
[29.651s][info][gc] GC(37) To-space exhausted
[29.651s][info][gc] GC(37) Pause Initial Mark (G1 Evacuation Pause) 6983M->7127M(7168M) 751.288ms
[29.651s][info][gc] GC(38) Concurrent Cycle
[30.135s][info][gc] GC(39) To-space exhausted
[30.135s][info][gc] GC(39) Pause Young (G1 Evacuation Pause) 7168M->7168M(7168M) 203.578ms
[30.205s][info][gc] GC(40) Pause Young (G1 Evacuation Pause) 7168M->7168M(7168M) 69.831ms
[53.924s][info][gc] GC(41) Pause Full (Allocation Failure) 7168M->6955M(7168M) 23719.072ms
[53.924s][info][gc] GC(38) Concurrent Cycle 24273.544ms
[54.654s][info][gc] GC(42) To-space exhausted
[54.654s][info][gc] GC(42) Pause Young (G1 Evacuation Pause) 7167M->7167M(7168M) 484.771ms
[54.811s][info][gc] GC(43) Pause Initial Mark (G1 Evacuation Pause) 7167M->7167M(7168M) 156.984ms
[54.811s][info][gc] GC(44) Concurrent Cycle
[75.187s][info][gc] GC(45) Pause Full (Allocation Failure) 7167M->7166M(7168M) 20375.807ms
[75.187s][info][gc] GC(44) Concurrent Cycle 20375.993ms
[75.197s][info][gc] GC(46) To-space exhausted
[75.197s][info][gc] GC(46) Pause Young (G1 Evacuation Pause) 7167M->7167M(7168M) 8.585ms
[75.199s][info][gc] GC(47) Pause Initial Mark (G1 Evacuation Pause) 7167M->7167M(7168M) 1.773ms
[75.199s][info][gc] GC(48) Concurrent Cycle
[95.382s][info][gc] GC(49) Pause Full (Allocation Failure) 7167M->7167M(7168M) 20183.050ms
[115.575s][info][gc] GC(50) Pause Full (Allocation Failure) 7167M->7167M(7168M) 20192.989ms
[115.575s][info][gc] GC(48) Concurrent Cycle 40376.227ms
[115.581s][info][gc] GC(51) Pause Young (G1 Evacuation Pause) 7167M->7167M(7168M) 2.649ms
[115.613s][info][gc] GC(52) Pause Initial Mark (G1 Evacuation Pause) 7167M->7167M(7168M) 0.629ms
[115.613s][info][gc] GC(53) Concurrent Cycle
[119.694s][info][gc] GC(54) Pause Full (Allocation Failure) 7167M->0M(8M) 4080.414ms
[119.694s][info][gc] GC(53) Concurrent Cycle 4080.622ms
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.base/java.util.LinkedList.linkBefore(LinkedList.java:162)
    at java.base/java.util.LinkedList$ListItr.add(LinkedList.java:952)
    at test.Tmp.main(Tmp.java:32)

请注意，报告的在[53.924s]，[75.187s]，[95.382s]和[115.575s]处的暂停分别超过20秒…

Note the reported pauses at [53.924s], [75.187s], [95.382s], and [115.575s] of over 20 seconds each…

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