用于测量 GC 活动的 V8 垃圾收集器回调答案

【问题标题】：V8 garbage collector callbacks for measuring GC activity用于测量 GC 活动的 V8 垃圾收集器回调
【发布时间】：2019-11-28 17:11:18
【问题描述】：

我有一个关于 V8 6.7.240 GC 行为和 AddGCPrologueCallback/AddGCEpilogueCallback 回调的小问题。

问题背后的小故事：我们使用 V8 引擎启动自定义 JS 代码，为了按时间限制执行，我们有一个 watchdog（带有 isolate->TerminateExecution() 调用的单独线程）监控代码并在以防万一时将其杀死长时间运行，但与 GC 活动有关。

所以，如果代码执行超时等于200ms，GC 活动占用300ms，代码占用199ms，我们会没事的（199ms 200ms，300ms 不包括在内）。

另一方面，如果代码执行超时等于200ms，则GC活动占用300ms，代码占用201ms，这将是执行超时（201ms>200ms，300ms不包括在内)。

如您所见，精确的 GC 测量非常重要，因为如果 GC 活动花费的时间超过回调指示的时间，这可能会导致watchdog 会注意到代码运行时间过长并终止它，但是实际上，GC 活动（以及“停止世界”方法）“进食”时间没有任何迹象。

似乎这正是我们在测试和调试过程中注意到的。让我们看看：

V8 方面：

// init step
api,v8::Context::New
[api,v8::FunctionTemplate::New]

// a little bit fancy way of converting string to object
// like JSON:Parse call, but doing that using global JSON object
// no problem with that, just part of the log
api,v8::String::NewFromUtf8
api,v8::Object::Get
api,v8::String::NewFromUtf8
api,v8::Object::Get
api,v8::String::NewFromUtf8
timer-event-start,V8.GCIncrementalMarking,6406056058
timer-event-end,V8.GCIncrementalMarking,6406056581
api,v8::Function::Call

// actually function run
// script->Run(context);
timer-event-start,V8.Execute,6406057062
timer-event-start,V8.GCIncrementalMarking,6406057179
timer-event-end,V8.GCIncrementalMarking,6406059180
timer-event-start,V8.GCIncrementalMarking,6406060424
timer-event-end,V8.GCIncrementalMarking,6406062569
timer-event-start,V8.GCIncrementalMarking,6406063674
timer-event-end,V8.GCIncrementalMarking,6406065864
timer-event-start,V8.GCIncrementalMarking,6406066891
timer-event-end,V8.GCIncrementalMarking,6406068970
timer-event-start,V8.GCIncrementalMarking,6406069912
timer-event-end,V8.GCIncrementalMarking,6406070711
timer-event-start,V8.GCIncrementalMarking,6406071368
timer-event-end,V8.GCIncrementalMarking,6406073392
timer-event-start,V8.GCIncrementalMarking,6406074204
timer-event-end,V8.GCIncrementalMarking,6406076411
timer-event-start,V8.GCIncrementalMarking,6406077223
timer-event-end,V8.GCIncrementalMarking,6406079326
timer-event-start,V8.GCIncrementalMarking,6406080096
timer-event-end,V8.GCIncrementalMarking,6406082253
timer-event-start,V8.GCIncrementalMarking,6406083041
timer-event-end,V8.GCIncrementalMarking,6406085169
timer-event-start,V8.GCIncrementalMarking,6406085754
timer-event-end,V8.GCIncrementalMarking,6406087852
timer-event-start,V8.GCIncrementalMarking,6406088753
timer-event-end,V8.GCIncrementalMarking,6406090888
timer-event-start,V8.GCIncrementalMarking,6406091704
timer-event-end,V8.GCIncrementalMarking,6406093860
timer-event-start,V8.GCIncrementalMarking,6406094638
timer-event-end,V8.GCIncrementalMarking,6406096819
timer-event-start,V8.GCIncrementalMarking,6406097737
timer-event-end,V8.GCIncrementalMarking,6406099851
timer-event-start,V8.GCIncrementalMarking,6406100651
timer-event-end,V8.GCIncrementalMarking,6406102158
timer-event-start,V8.GCIncrementalMarking,6406102830
timer-event-end,V8.GCIncrementalMarking,6406102949
timer-event-start,V8.GCIncrementalMarkingFinalize,6406103476
timer-event-end,V8.GCIncrementalMarkingFinalize,6406103720
timer-event-start,V8.GCIncrementalMarking,6406103781
timer-event-end,V8.GCIncrementalMarking,6406103805
timer-event-start,V8.GCFinalizeMC,6406106106
markcompact,begin,37,733899,1573929817539
sfi-move,0x26c8c6b42258,0x39727ab446f0
sfi-move,0x26c8c6b423d0,0x39727ab44858
code-move,0x26c8c6b424a8,0x39727ab44920
code-move,0x26c8c6b42990,0x39727ab44e08
[delete,MemoryChunk,0x20803a500000]
markcompact,end,37,750515,1573929817560
timer-event-end,V8.GCFinalizeMC,6406126613
[delete,MemoryChunk,0x2fcf14300000]
timer-event-start,V8.GCIncrementalMarkingStart,
timer-event-end,V8.GCIncrementalMarkingStart,6406148920
timer-event-start,V8.GCIncrementalMarking,6406148975
timer-event-end,V8.GCIncrementalMarking,6406150059
timer-event-start,V8.GCIncrementalMarking,6406151014
timer-event-end,V8.GCIncrementalMarking,6406152657
timer-event-start,V8.GCIncrementalMarking,6406153356
timer-event-end,V8.GCIncrementalMarking,6406154995
timer-event-start,V8.GCIncrementalMarking,6406155703
timer-event-end,V8.GCIncrementalMarking,6406157341
new,MemoryChunk,0x24eee1900000,524288
timer-event-start,V8.GCIncrementalMarking,6406158486
timer-event-end,V8.GCIncrementalMarking,6406160149
new,MemoryChunk,0x7f310200000,524288
timer-event-start,V8.GCIncrementalMarking,6406161218
timer-event-end,V8.GCIncrementalMarking,6406162914
new,MemoryChunk,0x178aad500000,524288
timer-event-start,V8.GCIncrementalMarking,6406163990
timer-event-end,V8.GCIncrementalMarking,6406165681
new,MemoryChunk,0x34d7b2580000,524288
timer-event-start,V8.GCIncrementalMarking,6406166748
timer-event-end,V8.GCIncrementalMarking,6406168439
new,MemoryChunk,0x225fec080000,524288
timer-event-start,V8.GCIncrementalMarking,6406169481
timer-event-end,V8.GCIncrementalMarking,6406171229
new,MemoryChunk,0x502e7380000,524288
timer-event-start,V8.GCIncrementalMarking,6406172280
timer-event-end,V8.GCIncrementalMarking,6406174003
new,MemoryChunk,0x208b2af00000,524288
timer-event-start,V8.GCIncrementalMarking,6406175047
timer-event-end,V8.GCIncrementalMarking,6406176787
new,MemoryChunk,0x39ffd8400000,524288
timer-event-start,V8.GCIncrementalMarking,6406177851
timer-event-end,V8.GCIncrementalMarking,6406179600
new,MemoryChunk,0x10408d480000,524288
timer-event-start,V8.GCIncrementalMarking,6406180631
timer-event-end,V8.GCIncrementalMarking,6406182384
new,MemoryChunk,0x25c069e80000,524288
timer-event-start,V8.GCIncrementalMarking,6406183415
timer-event-end,V8.GCIncrementalMarking,6406185165
new,MemoryChunk,0x20cb51c80000,524288
timer-event-start,V8.GCIncrementalMarking,6406186210
timer-event-end,V8.GCIncrementalMarking,6406186919
new,MemoryChunk,0xe774a300000,524288
timer-event-start,V8.GCIncrementalMarking,6406187940
timer-event-end,V8.GCIncrementalMarking,6406188387
new,MemoryChunk,0x23a0a1180000,524288
timer-event-start,V8.GCIncrementalMarking,6406189172
timer-event-end,V8.GCIncrementalMarking,6406189924
new,MemoryChunk,0x16bb70600000,524288
timer-event-start,V8.GCIncrementalMarking,6406190724
timer-event-end,V8.GCIncrementalMarking,6406192029
new,MemoryChunk,0x18b8a0200000,524288
timer-event-start,V8.GCIncrementalMarking,6406192894
timer-event-end,V8.GCIncrementalMarking,6406194253
new,MemoryChunk,0x11c87c180000,524288
timer-event-start,V8.GCIncrementalMarking,6406195137
timer-event-end,V8.GCIncrementalMarking,6406196462
new,MemoryChunk,0x353b80280000,524288
timer-event-start,V8.GCIncrementalMarking,6406197381
timer-event-end,V8.GCIncrementalMarking,6406198513
new,MemoryChunk,0x991e5a80000,524288
timer-event-start,V8.GCIncrementalMarking,6406199347
timer-event-end,V8.GCIncrementalMarking,6406200659
new,MemoryChunk,0x77559500000,524288
timer-event-start,V8.GCIncrementalMarking,6406201554
timer-event-end,V8.GCIncrementalMarking,6406202952
new,MemoryChunk,0x51fcc580000,524288
timer-event-start,V8.GCIncrementalMarking,6406203842
timer-event-end,V8.GCIncrementalMarking,6406205229
new,MemoryChunk,0x11134ca80000,524288
timer-event-start,V8.GCIncrementalMarking,6406206124
timer-event-end,V8.GCIncrementalMarking,6406207561
new,MemoryChunk,0x17b32eb00000,524288
timer-event-start,V8.GCIncrementalMarking,6406208473
timer-event-end,V8.GCIncrementalMarking,6406209913
new,MemoryChunk,0xa7929400000,524288
timer-event-start,V8.GCIncrementalMarking,6406210795
timer-event-end,V8.GCIncrementalMarking,6406212249
new,MemoryChunk,0x1f76e7200000,524288
timer-event-start,V8.GCIncrementalMarking,6406213115
timer-event-end,V8.GCIncrementalMarking,6406214629
new,MemoryChunk,0x4bb9cd00000,524288
timer-event-start,V8.GCIncrementalMarking,6406215548
timer-event-end,V8.GCIncrementalMarking,6406216079
new,MemoryChunk,0x26fd04080000,524288
timer-event-start,V8.GCIncrementalMarking,6406216911
timer-event-end,V8.GCIncrementalMarking,6406217106
timer-event-start,V8.GCIncrementalMarkingFinalize,6406217606
timer-event-end,V8.GCIncrementalMarkingFinalize,6406217921
new,MemoryChunk,0x3e8b51000000,524288
timer-event-start,V8.GCIncrementalMarking,6406218332
timer-event-end,V8.GCIncrementalMarking,6406218348
timer-event-start,V8.GCFinalizeMC,6406218897
markcompact,begin,38,36462,1573929817653
markcompact,end,38,54306,1573929817670
timer-event-end,V8.GCFinalizeMC,6406237199
new,MemoryChunk,0x3e70a1a00000,524288
new,MemoryChunk,0x14c9e4180000,524288
new,MemoryChunk,0x335947c80000,524288
timer-event-start,V8.GCIncrementalMarkingStart,6406263779
timer-event-end,V8.GCIncrementalMarkingStart,6406264145
new,MemoryChunk,0x1bf6fd00000,524288
timer-event-start,V8.GCIncrementalMarking,6406264575
timer-event-end,V8.GCIncrementalMarking,6406266175
new,MemoryChunk,0x109d6f780000,524288
timer-event-start,V8.GCIncrementalMarking,6406267498
timer-event-end,V8.GCIncrementalMarking,6406269125
new,MemoryChunk,0xcf3b9200000,524288
timer-event-start,V8.GCIncrementalMarking,6406270226
timer-event-end,V8.GCIncrementalMarking,6406271868
new,MemoryChunk,0x33eb2c880000,524288
timer-event-start,V8.GCIncrementalMarking,6406272919
timer-event-end,V8.GCIncrementalMarking,6406274608
new,MemoryChunk,0x10c056a80000,524288
timer-event-start,V8.GCIncrementalMarking,6406275660
timer-event-end,V8.GCIncrementalMarking,6406277318
new,MemoryChunk,0x1846c1880000,524288
timer-event-start,V8.GCIncrementalMarking,6406278406
timer-event-end,V8.GCIncrementalMarking,6406280119
new,MemoryChunk,0x21eed5900000,524288
timer-event-start,V8.GCIncrementalMarking,6406281176
timer-event-end,V8.GCIncrementalMarking,6406282888
new,MemoryChunk,0x323ac6b80000,524288
timer-event-start,V8.GCIncrementalMarking,6406283954
timer-event-end,V8.GCIncrementalMarking,6406285682
new,MemoryChunk,0x3c2009d00000,524288
timer-event-start,V8.GCIncrementalMarking,6406286739
timer-event-end,V8.GCIncrementalMarking,6406288466
new,MemoryChunk,0x3d504cd00000,524288
timer-event-start,V8.GCIncrementalMarking,6406289535
timer-event-end,V8.GCIncrementalMarking,6406291291
new,MemoryChunk,0x235494980000,524288
timer-event-start,V8.GCIncrementalMarking,6406292373
timer-event-end,V8.GCIncrementalMarking,6406294127
new,MemoryChunk,0xecfc3600000,524288
timer-event-start,V8.GCIncrementalMarking,6406295191
timer-event-end,V8.GCIncrementalMarking,6406296997
new,MemoryChunk,0x1a2eae600000,524288
timer-event-start,V8.GCIncrementalMarking,6406298083
timer-event-end,V8.GCIncrementalMarking,6406299894
timer-event-end,V8.Execute,6406300603

应用端：

(1) // kGCTypeIncrementalMarking
[18:43:37.536(1573929817536752)][DEBUG]: GCPrologueCallback type: 4
[18:43:37.536(1573929817536955)][DEBUG]: GCEpilogCallback type: 4
Total: 203 micro


(2) // kGCTypeMarkSweepCompact
[18:43:37.539(1573929817539368)][DEBUG]: GCPrologueCallback type: 2
[18:43:37.559(1573929817559840)][DEBUG]: GCEpilogCallback type: 2
Total: 20472 micro


(3) // kGCTypeIncrementalMarking
[18:43:37.650(1573929817650874)][DEBUG]: GCPrologueCallback type: 4
[18:43:37.651(1573929817651154)][DEBUG]: GCEpilogCallback type: 4
Total: 280 micro


(4) // kGCTypeMarkSweepCompact
[18:43:37.652(1573929817652160)][DEBUG]: GCPrologueCallback type: 2
[18:43:37.670(1573929817670422)][DEBUG]: GCEpilogCallback type: 2
Total: 18262 micro

所以，总共 39217 微秒或 40 毫秒。

这样读取V8日志太复杂了。所以我通过测量timer-event-start/timer-event-end 事件之间的时间对其进行了一些后期处理。

(V8.GCIncrementalMarking, 523)
(V8.GCIncrementalMarking, 2001)
(V8.GCIncrementalMarking, 2145)
(V8.GCIncrementalMarking, 2190)
(V8.GCIncrementalMarking, 2079)
(V8.GCIncrementalMarking, 799)
(V8.GCIncrementalMarking, 2024)
(V8.GCIncrementalMarking, 2207)
(V8.GCIncrementalMarking, 2103)
(V8.GCIncrementalMarking, 2157)
(V8.GCIncrementalMarking, 2128)
(V8.GCIncrementalMarking, 2098)
(V8.GCIncrementalMarking, 2135)
(V8.GCIncrementalMarking, 2156)
(V8.GCIncrementalMarking, 2181)
(V8.GCIncrementalMarking, 2114)
(V8.GCIncrementalMarking, 1507)
(V8.GCIncrementalMarking, 119)
(V8.GCIncrementalMarkingFinalize, 244)        (1)
(V8.GCIncrementalMarking, 24)
(V8.GCFinalizeMC, 20507)                      (2)
(V8.GCIncrementalMarkingStart, 428)
(V8.GCIncrementalMarking, 1084)
(V8.GCIncrementalMarking, 1643)
(V8.GCIncrementalMarking, 1639)
(V8.GCIncrementalMarking, 1638)
(V8.GCIncrementalMarking, 1663)
(V8.GCIncrementalMarking, 1696)
(V8.GCIncrementalMarking, 1691)
(V8.GCIncrementalMarking, 1691)
(V8.GCIncrementalMarking, 1748)
(V8.GCIncrementalMarking, 1723)
(V8.GCIncrementalMarking, 1740)
(V8.GCIncrementalMarking, 1749)
(V8.GCIncrementalMarking, 1753)
(V8.GCIncrementalMarking, 1750)
(V8.GCIncrementalMarking, 709)
(V8.GCIncrementalMarking, 447)
(V8.GCIncrementalMarking, 752)
(V8.GCIncrementalMarking, 1305)
(V8.GCIncrementalMarking, 1359)
(V8.GCIncrementalMarking, 1325)
(V8.GCIncrementalMarking, 1132)
(V8.GCIncrementalMarking, 1312)
(V8.GCIncrementalMarking, 1398)
(V8.GCIncrementalMarking, 1387)
(V8.GCIncrementalMarking, 1437)
(V8.GCIncrementalMarking, 1440)
(V8.GCIncrementalMarking, 1454)
(V8.GCIncrementalMarking, 1514)
(V8.GCIncrementalMarking, 531)
(V8.GCIncrementalMarking, 195)
(V8.GCIncrementalMarkingFinalize, 315)        (3)
(V8.GCIncrementalMarking, 16)
(V8.GCFinalizeMC, 18302)                      (4)
(V8.GCIncrementalMarkingStart, 366)
(V8.GCIncrementalMarking, 1600)
(V8.GCIncrementalMarking, 1627)
(V8.GCIncrementalMarking, 1642)
(V8.GCIncrementalMarking, 1689)
(V8.GCIncrementalMarking, 1658)
(V8.GCIncrementalMarking, 1713)
(V8.GCIncrementalMarking, 1712)
(V8.GCIncrementalMarking, 1728)
(V8.GCIncrementalMarking, 1727)
(V8.GCIncrementalMarking, 1756)
(V8.GCIncrementalMarking, 1754)
(V8.GCIncrementalMarking, 1806)
(V8.GCIncrementalMarking, 1811)
(Total, 135996)

如您所见，我们只能匹配 4 个案例，但很多 GCIncrementalMarking 活动未被注意到。真正的总时间等于136ms。远远超过 40 毫秒！，我们能够在应用程序端进行测量..

我知道 Orinoco 和并行方法，但我不知道对 GCIncrementalMarking 的调用是否会阻止代码执行。

我们已经多次注意到相同的情况，并且在所有情况下，代码终止的原因都是与 V8 日志文件相比，应用程序端的 GC 测量不正确。

这是故意的，我们不能这么信任 GC 回调吗？或者我错过了什么？作为一个临时解决方案，我们已将 V8 降级为 5.6.316 并且它可以正常工作..

6.7.240 有新的 GC 方法，它会是问题的根源吗？

附言。 --single-threaded-gc for 6.7.240 也无济于事

PPS。一些与V8和GC逻辑相关的代码：

void cnode::V8Runner::init() {    
    v8::V8::InitializeICU();

    v8::Platform *platform = v8::platform::CreateDefaultPlatform();
    v8::V8::InitializePlatform(platform);
    v8::V8::Initialize();

    auto flags = "--log --log-all --logfile=/tmp/v8.log --nolazy";
    auto isolate = cnode::V8Runner::getIsolate();

    isolate->AddGCPrologueCallback(cnode::V8Runner::_GCPrologue);
    isolate->AddGCEpilogueCallback(cnode::V8Runner::_GCEpilog);
}

void cnode::V8Runner::_GCPrologue(v8::Isolate *isolate, v8::GCType type,
                                  v8::GCCallbackFlags flags) {    
    cnode::V8Runner::_GCPrologueTimePoint = std::chrono::high_resolution_clock::now();
    cnode::V8Runner::_GCEpilogTimePoint = std::chrono::high_resolution_clock::now();

    LOG_DEBUG("GCPrologueCallback type: %d", type);
}

void cnode::V8Runner::_GCEpilog(v8::Isolate *isolate, v8::GCType type,
                                v8::GCCallbackFlags flags) {    
    cnode::V8Runner::_GCEpilogTimePoint = std::chrono::high_resolution_clock::now();

    std::chrono::milliseconds _GCPrologueTimePointMs = std::chrono::duration_cast<std::chrono::milliseconds>(
            cnode::V8Runner::_GCPrologueTimePoint.time_since_epoch());
    std::chrono::milliseconds _GCEpilogTimePointMs = std::chrono::duration_cast<std::chrono::milliseconds>(
            cnode::V8Runner::_GCEpilogTimePoint.time_since_epoch());

    const int diff = _GCEpilogTimePointMs.count() - _GCPrologueTimePointMs.count();

    cnode::V8Runner::_currentGCActionMs += diff;

    cnode::V8Runner::_GCPrologueTimePoint = std::chrono::high_resolution_clock::now();
    cnode::V8Runner::_GCEpilogTimePoint = std::chrono::high_resolution_clock::now();

    LOG_DEBUG("GCEpilogCallback type: %d", type);
}

购买力平价。在这篇文章之后，我决定在每次代码执行后手动调用Isolate::LowMemoryNotification() 重新检查。

原始 V8 日志：

api,v8::Context::New
api,v8::FunctionTemplate::New
api,v8::ObjectTemplate::New
[api,v8::FunctionTemplate::New]
api,v8::String::NewFromUtf8
api,v8::JSON::Parse
[new,MemoryChunk,0x1940b7d00000,524288]
timer-event-start,V8.GCIncrementalMarkingStart,4351782723
timer-event-end,V8.GCIncrementalMarkingStart,4351783066
new,MemoryChunk,0x106dca200000,524288
timer-event-start,V8.GCIncrementalMarking,4351783463
timer-event-end,V8.GCIncrementalMarking,4351785131
new,MemoryChunk,0x225019c00000,524288
timer-event-start,V8.GCIncrementalMarking,4351786485
timer-event-end,V8.GCIncrementalMarking,4351788194
new,MemoryChunk,0xdfc50c80000,524288
timer-event-start,V8.GCIncrementalMarking,4351789202
timer-event-end,V8.GCIncrementalMarking,4351790879
new,MemoryChunk,0x26861a780000,524288
timer-event-start,V8.GCIncrementalMarking,4351791905
timer-event-end,V8.GCIncrementalMarking,4351793611
new,MemoryChunk,0x44086a00000,524288
timer-event-start,V8.GCIncrementalMarking,4351794615
timer-event-end,V8.GCIncrementalMarking,4351796319
new,MemoryChunk,0x4debd600000,524288
timer-event-start,V8.GCIncrementalMarking,4351797315
timer-event-end,V8.GCIncrementalMarking,4351799066
new,MemoryChunk,0x2db69c500000,524288
timer-event-start,V8.GCIncrementalMarking,4351800060
timer-event-end,V8.GCIncrementalMarking,4351801814
new,MemoryChunk,0x1f1825000000,524288
timer-event-start,V8.GCIncrementalMarking,4351802804
timer-event-end,V8.GCIncrementalMarking,4351804535
new,MemoryChunk,0x86246080000,524288
timer-event-start,V8.GCIncrementalMarking,4351805573
timer-event-end,V8.GCIncrementalMarking,4351807383
new,MemoryChunk,0xfc3bb580000,524288
timer-event-start,V8.GCIncrementalMarking,4351808377
timer-event-end,V8.GCIncrementalMarking,4351810192
new,MemoryChunk,0x10451e500000,524288
timer-event-start,V8.GCIncrementalMarking,4351811223
timer-event-end,V8.GCIncrementalMarking,4351813019
new,MemoryChunk,0x16e643c00000,524288
timer-event-start,V8.GCIncrementalMarking,4351814010
timer-event-end,V8.GCIncrementalMarking,4351815842
new,MemoryChunk,0x2a4379600000,524288
timer-event-start,V8.GCIncrementalMarking,4351816837
timer-event-end,V8.GCIncrementalMarking,4351818621
new,MemoryChunk,0x18436f400000,524288
timer-event-start,V8.GCIncrementalMarking,4351819650
timer-event-end,V8.GCIncrementalMarking,4351821491
new,MemoryChunk,0x3b377700000,524288
timer-event-start,V8.GCIncrementalMarking,4351822503
timer-event-end,V8.GCIncrementalMarking,4351824007
new,MemoryChunk,0x1f2f22600000,524288
timer-event-start,V8.GCIncrementalMarking,4351825011
timer-event-end,V8.GCIncrementalMarking,4351825827
new,MemoryChunk,0x1cd733a80000,524288
timer-event-start,V8.GCIncrementalMarking,4351826871
timer-event-end,V8.GCIncrementalMarking,4351827291
new,MemoryChunk,0x3cf039a80000,524288
timer-event-start,V8.GCIncrementalMarking,4351828058
timer-event-end,V8.GCIncrementalMarking,4351828500
new,MemoryChunk,0x31ae7100000,524288
timer-event-start,V8.GCIncrementalMarking,4351829286
timer-event-end,V8.GCIncrementalMarking,4351830586    
new,MemoryChunk,0x2265e4c00000,524288
timer-event-start,V8.GCIncrementalMarking,4351831462
timer-event-end,V8.GCIncrementalMarking,4351832806
new,MemoryChunk,0x34586ae00000,524288
timer-event-start,V8.GCIncrementalMarking,4351833700
timer-event-end,V8.GCIncrementalMarking,4351835122
new,MemoryChunk,0x21c0aa580000,524288    
timer-event-start,V8.GCIncrementalMarking,4351836000
timer-event-end,V8.GCIncrementalMarking,4351837409
new,MemoryChunk,0x2d29cda00000,524288
timer-event-start,V8.GCIncrementalMarking,4351838280
timer-event-end,V8.GCIncrementalMarking,4351839701
new,MemoryChunk,0x216cbbb80000,524288
timer-event-start,V8.GCIncrementalMarking,4351840569
timer-event-end,V8.GCIncrementalMarking,4351842015
new,MemoryChunk,0x1fb82e480000,524288
timer-event-start,V8.GCIncrementalMarking,4351842910
timer-event-end,V8.GCIncrementalMarking,4351844380
new,MemoryChunk,0x329529f00000,524288
timer-event-start,V8.GCIncrementalMarking,4351845255
timer-event-end,V8.GCIncrementalMarking,4351846654    
new,MemoryChunk,0x957ccf80000,524288
timer-event-start,V8.GCIncrementalMarking,4351847553
timer-event-end,V8.GCIncrementalMarking,4351848991
new,MemoryChunk,0x2df008e00000,524288
timer-event-start,V8.GCIncrementalMarking,4351849864
timer-event-end,V8.GCIncrementalMarking,4351851358
new,MemoryChunk,0x8be2be80000,524288
timer-event-start,V8.GCIncrementalMarking,4351852237
timer-event-end,V8.GCIncrementalMarking,4351853720
new,MemoryChunk,0x146846980000,524288
timer-event-start,V8.GCIncrementalMarking,4351854588
timer-event-end,V8.GCIncrementalMarking,4351856080
new,MemoryChunk,0x367cfe280000,524288
timer-event-start,V8.GCIncrementalMarking,4351856969
timer-event-end,V8.GCIncrementalMarking,4351858491
new,MemoryChunk,0x6ed38180000,524288
timer-event-start,V8.GCIncrementalMarking,4351859382
timer-event-end,V8.GCIncrementalMarking,4351860900
new,MemoryChunk,0xfbd5f600000,524288
timer-event-start,V8.GCIncrementalMarking,4351861805
timer-event-end,V8.GCIncrementalMarking,4351863367
new,MemoryChunk,0x1a5e01680000,524288
timer-event-start,V8.GCIncrementalMarking,4351864277
timer-event-end,V8.GCIncrementalMarking,4351865092
new,MemoryChunk,0x24120d700000,524288
timer-event-start,V8.GCIncrementalMarking,4351865984
timer-event-end,V8.GCIncrementalMarking,4351866048
new,MemoryChunk,0x125d39480000,524288
timer-event-start,V8.GCIncrementalMarking,4351866808
timer-event-end,V8.GCIncrementalMarking,4351866824
timer-event-start,V8.GCIncrementalMarkingFinalize,4351867246
timer-event-end,V8.GCIncrementalMarkingFinalize,4351867419
new,MemoryChunk,0x2f31d9400000,524288
timer-event-start,V8.GCIncrementalMarking,4351867767
timer-event-end,V8.GCIncrementalMarking,4351867774
timer-event-start,V8.GCFinalizeMC,4351869855
markcompact,begin,7143,825650,1575032707066
markcompact,end,7143,846265,1575032707086
timer-event-end,V8.GCFinalizeMC,4351890363
[new,MemoryChunk,0x19b52bb00000,524288]
timer-event-start,V8.GCLowMemoryNotification,4351984850
timer-event-start,V8.GCCompactor,4351984910
markcompact,begin,7143,949018,1575032707181
[delete,MemoryChunk,0xd202f700000]
markcompact,end,7144,188469,1575032707268
timer-event-end,V8.GCCompactor,4352073048
[delete,MemoryChunk,0x1c887fa80000]    
timer-event-start,V8.GCCompactor,4352073343
markcompact,begin,7144,192691,1575032707270
new,MemoryChunk,0x33c543500000,524288
new,MemoryChunk,0xd4241800000,524288
[sfi-move,0x8b6f5421390,0xd424181d470]
[delete,MemoryChunk,0x1455b1780000]
markcompact,end,7144,453893,1575032707371
timer-event-end,V8.GCCompactor,4352175676
delete,MemoryChunk,0x25fae9b80000
delete,MemoryChunk,0x2590e9700000
timer-event-end,V8.GCLowMemoryNotification,4352175844
api,v8::ObjectTemplate::New
api,v8::FunctionTemplate::New
api,v8::String::NewFromUtf8

应用日志：

(1) // kGCTypeIncrementalMarking
[13:05:07.063(1575032707063270)][DEBUG]: GCPrologueCallback type: 4
[13:05:07.063(1575032707063418)][DEBUG]: GCEpilogCallback type: 4
Total: 148micro

(2) // kGCTypeMarkSweepCompact
[13:05:07.065(1575032707065876)][DEBUG]: GCPrologueCallback type: 2
[13:05:07.086(1575032707086356)][DEBUG]: GCEpilogCallback type: 2
Total: 20480 micro

(3) // kGCTypeMarkSweepCompact
[13:05:07.180(1575032707180925)][DEBUG]: GCPrologueCallback type: 2
[13:05:07.269(1575032707269036)][DEBUG]: GCEpilogCallback type: 2

(4) // kGCTypeMarkSweepCompact
[13:05:07.269(1575032707269366)][DEBUG]: GCPrologueCallback type: 2
[13:05:07.371(1575032707371667)][DEBUG]: GCEpilogCallback type: 2

后处理后的V8 Log：

(V8.GCIncrementalMarkingStart, 343)          (1)?
(V8.GCIncrementalMarking, 1668)
(V8.GCIncrementalMarking, 1709)
(V8.GCIncrementalMarking, 1677)
(V8.GCIncrementalMarking, 1706)
(V8.GCIncrementalMarking, 1704)
(V8.GCIncrementalMarking, 1751)
(V8.GCIncrementalMarking, 1754)
(V8.GCIncrementalMarking, 1731)
(V8.GCIncrementalMarking, 1810)
(V8.GCIncrementalMarking, 1815)
(V8.GCIncrementalMarking, 1796)
(V8.GCIncrementalMarking, 1832)
(V8.GCIncrementalMarking, 1784)
(V8.GCIncrementalMarking, 1841)
(V8.GCIncrementalMarking, 1504)
(V8.GCIncrementalMarking, 816)
(V8.GCIncrementalMarking, 420)
(V8.GCIncrementalMarking, 442)
(V8.GCIncrementalMarking, 1300)
(V8.GCIncrementalMarking, 1344)
(V8.GCIncrementalMarking, 1422)
(V8.GCIncrementalMarking, 1409)
(V8.GCIncrementalMarking, 1421)
(V8.GCIncrementalMarking, 1446)
(V8.GCIncrementalMarking, 1470)
(V8.GCIncrementalMarking, 1399)
(V8.GCIncrementalMarking, 1438)
(V8.GCIncrementalMarking, 1494)
(V8.GCIncrementalMarking, 1483)
(V8.GCIncrementalMarking, 1492)
(V8.GCIncrementalMarking, 1522)
(V8.GCIncrementalMarking, 1518)
(V8.GCIncrementalMarking, 1562)
(V8.GCIncrementalMarking, 815)
(V8.GCIncrementalMarking, 64)
(V8.GCIncrementalMarking, 16)
(V8.GCIncrementalMarkingFinalize, 173)
(V8.GCIncrementalMarking, 7)
(V8.GCFinalizeMC, 20508)                  (2)
// call to LowMemoryNotification
(V8.GCCompactor, 88138)                   (3)
(V8.GCCompactor, 102333)                  (4)
(V8.GCLowMemoryNotification, 190994)
(Total, 452871)

看到这个，我的第一个想法是——V8.GCLowMemoryNotification(190994)包含V8.GCCompactor(88138)和V8.GCCompactor(102333)..毕竟(88138 + 102333)

所以，V8.GCIncrementalMarking 可能也包含在V8.GCFinalizeMC 中。但事实证明并非如此 - 20508 远小于 GCIncrementalMarkings 值的数量 (50555)。

这个故事的最终结果是一样的——执行超时。代码时间限制等于200ms。

调用LowMemoryNotification 不包括在watchdog 监控中。所以452871 - 190994 = 261877。

请记住，我们尊重 GC 活动，所以 261877 - 20508 = 241369。仍然大于 200 毫秒。

但是，如果我们开始计算所有 V8.GCIncrementalMarking 活动 (50555microseconds)，最终结果将是 241369 - 50555 = 190814。 190ms - 不是超时！

我仍然认为 V8 (6.7.240) 在 GC 活动方面没有提供足够的准确性。

【问题讨论】：

标签： c++ garbage-collection v8 embedded-v8

【解决方案1】：

V8 开发人员在这里。如果我对这个问题的理解正确，那么答案是：GC prologue/epilogue 回调在一个主要 GC 周期的开始/结束时被调用，它由（可能数百个）增量标记步骤和一个最后的最终确定阶段稍长一些，所有这些都与常规程序执行混合在一起。回调不会在每个增量标记步骤中调用，也不会在次要 GC 周期（“scavenges”，奇怪的是，上面的各种日志中完全缺失）调用，并且不打算提供精确的时间信息。对不起。

理想情况下，您不必执行任何这些操作。对于大多数程序来说，GC 所花费的时间占总执行时间的 1% 到 10% 之间，所以应该没什么大不了的。

如果您确实坚持跟踪主要 GC 上花费的主线程时间，那么您最好的选择可能是关闭增量标记（标志 --noincremental-marking）。请注意，这将使用户体验更加不稳定（而不是数百次低于 1 毫秒的暂停，您将获得几秒钟长的暂停），但也许在您的场景中您并不关心。我不建议涵盖次要 GC。

也就是说，如果您允许发表更笼统的评论，我对整体方法表示怀疑。 GC 活动由分配触发，可以看作是分配成本的一部分。如果您的目标是“公平地”对待程序（甚至在竞争意义上？），那么我绝对会将 GC 时间包括在每个程序的时间预算中。 GC 并不是“突然随机地攻击行为良好的代码”——不得不花时间在 GC 活动上是分配大量代码的情况；如果代码想要保持在一定的时间预算内，那么最好尽量少分配，这只是快速/高效的一个方面。

也许潜在的心理模型是一种非常简单的内存管理策略，例如许多 sn-ps 代码可以在没有任何 GC 活动的情况下运行，并且每个都分配几兆字节，然后其中一个是压倒骆驼的倒霉稻草，越过某个阈值，并在清理垃圾的主要减速中运行其他人都留下了吗？在这样的 VM 中，您的方法完全有意义，但这与 V8 的工作方式相去甚远。

此外，GC 不是主线程的唯一中断，还有（前台部分）函数优化，它周围没有任何嵌入器控制的回调。如果你想排除所有干扰，你必须破解 V8 来创建一种方法来排除这些干扰。另一方面，我上面的论点也适用于这里：优化不会随机发生，它发生在长时间运行的代码上，并且通常有利于整体性能（否则我们不会这样做），所以我认为它会是公平/适当地将其包括在预算中，就像 GC 时间一样。

【讨论】：

嗨，你能帮我处理一下stackoverflow.com/questions/59544711/… 吗？
考虑到“如果代码想要保持在一定的时间预算内，那么最好尽量少分配”，GC 回调对 JavaScript 作者来说是一个好的信号吗？使用回调运行的次数或 GC 启动/停止之间花费的时间来指示脚本分配过多是否有意义？
我不这么认为。原因之一：“太多”和“尽可能少”是（必然！）非常模糊的定义；它在很大程度上取决于用例，某些程序/算法只需要比其他程序/算法更多的内存/分配。第二个原因：因为它取决于用例，V8 的堆/GC 会尽力适应程序正在做的事情，所以像“启动了多少个主要 GC 周期”这样的简单指标会给出高度误导/无法比较的结果。第三个原因：短期对象比长期对象清理得更快，因此并非所有分配在性能方面都是相同的。