前言:在 Node.js 中,我们有时候需要抓取进程堆快照来判断是否有内存泄漏,本文介绍Node.js 中抓取堆快照的实现。
首先来看一下 Node.js 中如何抓取堆快照。
- const { Session } = require('inspector');
-
- const session = new Session();
-
- let chunk = '';
-
- const cb = (result) => {
-
- chunk += result.params.chunk;
-
- };
-
-
- session.on('HeapProfiler.addHeapSnapshotChunk', cb);
- session.post('HeapProfiler.takeHeapSnapshot', (err, r) => {
- session.off('HeapProfiler.addHeapSnapshotChunk', cb);
- console.log(err || chunk);
-
- });
下面看一下 HeapProfiler.addHeapSnapshotChunk 命令的实现。
- {
- v8_crdtp::SpanFrom("takeHeapSnapshot"),
- &DomainDispatcherImpl::takeHeapSnapshot
- }
对应 DomainDispatcherImpl::takeHeapSnapshot 函数。
- void DomainDispatcherImpl::takeHeapSnapshot(const v8_crdtp::Dispatchable& dispatchable){
- std::unique_ptr
weak = weakPtr(); - // 抓取快照
- DispatchResponse response = m_backend->takeHeapSnapshot(std::move(params.reportProgress), std::move(params.treatGlobalObjectsAsRoots), std::move(params.captureNumericValue));
- // 抓取完毕,响应
- if (weak->get())
- weak->get()->sendResponse(dispatchable.CallId(), response);
- return;
-
- }
上面代码中 m_backend 是 V8HeapProfilerAgentImpl 对象。
- Response V8HeapProfilerAgentImpl::takeHeapSnapshot(
- Maybe
reportProgress, Maybe treatGlobalObjectsAsRoots, - Maybe
captureNumericValue) { - v8::HeapProfiler* profiler = m_isolate->GetHeapProfiler();
- // 抓取快照
- const v8::HeapSnapshot* snapshot = profiler->TakeHeapSnapshot(
- progress.get(), &resolver, treatGlobalObjectsAsRoots.fromMaybe(true),
- captureNumericValue.fromMaybe(false));
- // 抓取完毕后通知调用方
- HeapSnapshotOutputStream stream(&m_frontend);
- snapshot->Serialize(&stream);
- const_cast
(snapshot)->Delete(); - // HeapProfiler.takeHeapSnapshot 命令结束,回调调用方
- return Response::Success();
-
- }
我们重点看一下 profiler->TakeHeapSnapshot。
- const HeapSnapshot* HeapProfiler::TakeHeapSnapshot(
- ActivityControl* control, ObjectNameResolver* resolver,
- bool treat_global_objects_as_roots, bool capture_numeric_value) {
- return reinterpret_cast
( - reinterpret_cast
(this)->TakeSnapshot( - control, resolver, treat_global_objects_as_roots,
- capture_numeric_value));
-
- }
继续看真正的 TakeSnapshot。
- HeapSnapshot* HeapProfiler::TakeSnapshot(
- v8::ActivityControl* control,
- v8::HeapProfiler::ObjectNameResolver* resolver,
- bool treat_global_objects_as_roots, bool capture_numeric_value) {
- is_taking_snapshot_ = true;
- HeapSnapshot* result = new HeapSnapshot(this, treat_global_objects_as_roots,
- capture_numeric_value);
- {
- HeapSnapshotGenerator generator(result, control, resolver, heap());
- if (!generator.GenerateSnapshot()) {
- delete result;
- result = nullptr;
- } else {
- snapshots_.emplace_back(result);
- }
- }
- return result;
-
- }
我们看到新建了一个 HeapSnapshot 对象,然后通过 HeapSnapshotGenerator 对象的 GenerateSnapshot 抓取快照。看一下 GenerateSnapshot。
- bool HeapSnapshotGenerator::GenerateSnapshot() {
- Isolate* isolate = Isolate::FromHeap(heap_);
- base::Optional
handle_scope(base::in_place, isolate); - v8_heap_explorer_.CollectGlobalObjectsTags();
- // 抓取前先回收不用内存,保证看到的是存活的对象,否则影响内存泄漏的分析
- heap_->CollectAllAvailableGarbage(GarbageCollectionReason::kHeapProfiler);
- // 收集内存信息
- snapshot_->AddSyntheticRootEntries();
- FillReferences();
- snapshot_->FillChildren();
- return true;
-
- }
GenerateSnapshot 的逻辑是首先进行GC 回收不用的内存,然后收集 GC 后的内存信息到 HeapSnapshot 对象。接着看收集完后的逻辑。
- HeapSnapshotOutputStream stream(&m_frontend);
- snapshot->Serialize(&stream);
HeapSnapshotOutputStream 是用于通知调用方收集的数据(通过 m_frontend)。
- explicit HeapSnapshotOutputStream(protocol::HeapProfiler::Frontend* frontend)
- : m_frontend(frontend) {}
- void EndOfStream() override {}
- int GetChunkSize() override { return 102400; }
- WriteResult WriteAsciiChunk(char* data, int size) override {
- m_frontend->addHeapSnapshotChunk(String16(data, size));
- m_frontend->flush();
- return kContinue;
- }
HeapSnapshotOutputStream 通过 WriteAsciiChunk 告诉调用方收集的数据,但是目前我们还没有数据源,下面看看数据源怎么来的。
- snapshot->Serialize(&stream);
看一下 Serialize。
- void HeapSnapshot::Serialize(OutputStream* stream,
- HeapSnapshot::SerializationFormat format) const {
- i::HeapSnapshotJSONSerializer serializer(ToInternal(this));
- serializer.Serialize(stream);
-
- }
最终调了 HeapSnapshotJSONSerializer 的 Serialize。
- void HeapSnapshotJSONSerializer::Serialize(v8::OutputStream* stream) {
- // 写者
- writer_ = new OutputStreamWriter(stream);
- // 开始写
- SerializeImpl();
-
- }
我们看一下 SerializeImpl。
- void HeapSnapshotJSONSerializer::SerializeImpl() {
- DCHECK_EQ(0, snapshot_->root()->index());
- writer_->AddCharacter('{');
- writer_->AddString("\"snapshot\":{");
- SerializeSnapshot();
- if (writer_->aborted()) return;
- writer_->AddString("},\n");
- writer_->AddString("\"nodes\":[");
- SerializeNodes();
- if (writer_->aborted()) return;
- writer_->AddString("],\n");
- writer_->AddString("\"edges\":[");
- SerializeEdges();
- if (writer_->aborted()) return;
- writer_->AddString("],\n");
-
- writer_->AddString("\"trace_function_infos\":[");
- SerializeTraceNodeInfos();
- if (writer_->aborted()) return;
- writer_->AddString("],\n");
- writer_->AddString("\"trace_tree\":[");
- SerializeTraceTree();
- if (writer_->aborted()) return;
- writer_->AddString("],\n");
-
- writer_->AddString("\"samples\":[");
- SerializeSamples();
- if (writer_->aborted()) return;
- writer_->AddString("],\n");
-
- writer_->AddString("\"locations\":[");
- SerializeLocations();
- if (writer_->aborted()) return;
- writer_->AddString("],\n");
-
- writer_->AddString("\"strings\":[");
- SerializeStrings();
- if (writer_->aborted()) return;
- writer_->AddCharacter(']');
- writer_->AddCharacter('}');
- writer_->Finalize();
-
- }
SerializeImpl 函数的逻辑就是把快照数据通过 OutputStreamWriter 对象 writer_ 写到 writer_ 持有的 stream 中。写的数据有很多种类型,这里以 AddCharacter 为例。
- void AddCharacter(char c) {
- chunk_[chunk_pos_++] = c;
- MaybeWriteChunk();
-
- }
每次写的时候都会判断是不达到阈值,是的话则先推给调用方。看一下 MaybeWriteChunk。
- void MaybeWriteChunk() {
- if (chunk_pos_ == chunk_size_) {
- WriteChunk();
- }
-
- }
-
-
-
- void WriteChunk() {
-
- // stream 控制是否还需要写入,通过 kAbort 和 kContinue
- if (stream_->WriteAsciiChunk(chunk_.begin(), chunk_pos_) ==
- v8::OutputStream::kAbort)
- aborted_ = true;
- chunk_pos_ = 0;
-
- }
我们看到最终通过 stream 的 WriteAsciiChunk 写到 stream 中。
- WriteResult WriteAsciiChunk(char* data, int size) override {
- m_frontend->addHeapSnapshotChunk(String16(data, size));
- m_frontend->flush();
- return kContinue;
-
- }
WriteAsciiChunk 调用 addHeapSnapshotChunk 通知调用方。
- void Frontend::addHeapSnapshotChunk(const String& chunk){
- v8_crdtp::ObjectSerializer serializer;
- serializer.AddField(v8_crdtp::MakeSpan("chunk"), chunk);
- frontend_channel_->SendProtocolNotification(v8_crdtp::CreateNotification("HeapProfiler.addHeapSnapshotChunk", serializer.Finish()));
-
- }
触发 HeapProfiler.addHeapSnapshotChunk 事件,并传入快照的数据,最终触发 JS 层的事件。再看一下文章开头的代码。
- let chunk = '';
-
- const cb = (result) => {
-
- chunk += result.params.chunk;
-
- };
-
-
-
- session.on('HeapProfiler.addHeapSnapshotChunk', cb);
- session.post('HeapProfiler.takeHeapSnapshot', (err, r) => {
- session.off('HeapProfiler.addHeapSnapshotChunk', cb);
- console.log(err || chunk);
-
- });
这个过程是否清晰了很多。从过程中也看到,抓取快照虽然传入了回调,但是其实是以同步的方式执行的,因为提交 HeapProfiler.takeHeapSnapshot 命令后,V8 就开始收集内存,然后不断触发
HeapProfiler.addHeapSnapshotChunk 事件,直到堆数据写完,然后执行 JS 回调。
总结:整个过程不算复杂,因为我们没有涉及到堆内存管理那部分,V8 Inspector 提供了很多命令,有时间的话后续再分析其他的命令。