Question

如何编写一些代码来将对象分配到Old Gen中，这会导致GC的完整时间超过三或五秒？

Answer 1

“将对象分配到Old Gen”并且“长GC暂停”难以组合，因为您可以对垃圾收集器做的最糟糕的事情是创建大量小的，链接的，活动对象，形成一个图形，垃圾收集器必须遍历。

但是小对象没有分配到Old Gen.只有大型对象，例如数组，直接分配给Old Gen，但这些并没有给GC带来太多压力。特别是原始类型的数组，你声称它是你第一次尝试使用的，不是挑战，因为它们通常不能包含对象引用。

因此，为了强调GC，你必须创建许多小的链接对象，足以使它们最终被提升为Old Gen，然后修改旧对象以强制执行Full GC，resp。更新了受影响卡片的记忆。

如果这听起来像熟悉的模式，那你就是对的。我们在这里谈论LinkedList。

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();
    }
}

这来回迭代列表，在旧的列表之间插入新对象，这意味着修改前面的next指针和后续列表节点的prev指针。我们必须通过ListIterator执行此操作，因为尝试使用add(int index, E element)对于不断增长的链接列表来说会变得太慢，而不会创建足够的对象来强调GC。

使用-Xmx7G -Xlog:gc在Java 9.0.4上运行给了我

[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[53.924s]，[75.187s]，[95.382s]和[115.575s]报告的暂停时间超过20秒......

如何手动在Java中长时间使用Full GC

1 个答案: