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src: enable detailed source positions in V8 #24274

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dc102e8
deps: backport 073073b4f1 from upstream V8
hashseed Nov 9, 2018
aa32194
src: enable detailed source positions in V8
hashseed Nov 9, 2018
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2 changes: 1 addition & 1 deletion common.gypi
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Original file line number Diff line number Diff line change
Expand Up @@ -32,7 +32,7 @@

# Reset this number to 0 on major V8 upgrades.
# Increment by one for each non-official patch applied to deps/v8.
'v8_embedder_string': '-node.10',
'v8_embedder_string': '-node.11',

# Enable disassembler for `--print-code` v8 options
'v8_enable_disassembler': 1,
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6 changes: 6 additions & 0 deletions deps/v8/include/v8-profiler.h
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Expand Up @@ -341,6 +341,12 @@ class V8_EXPORT CpuProfiler {
V8_DEPRECATED("Use Isolate::SetIdle(bool) instead.",
void SetIdle(bool is_idle));

/**
* Generate more detailed source positions to code objects. This results in
* better results when mapping profiling samples to script source.
*/
static void UseDetailedSourcePositionsForProfiling(Isolate* isolate);

private:
CpuProfiler();
~CpuProfiler();
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5 changes: 5 additions & 0 deletions deps/v8/src/api.cc
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Expand Up @@ -10132,6 +10132,11 @@ void CpuProfiler::SetIdle(bool is_idle) {
isolate->SetIdle(is_idle);
}

void CpuProfiler::UseDetailedSourcePositionsForProfiling(Isolate* isolate) {
reinterpret_cast<i::Isolate*>(isolate)
->set_detailed_source_positions_for_profiling(true);
}

uintptr_t CodeEvent::GetCodeStartAddress() {
return reinterpret_cast<i::CodeEvent*>(this)->code_start_address;
}
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3 changes: 2 additions & 1 deletion deps/v8/src/isolate.cc
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Expand Up @@ -3257,7 +3257,8 @@ bool Isolate::use_optimizer() {
}

bool Isolate::NeedsDetailedOptimizedCodeLineInfo() const {
return NeedsSourcePositionsForProfiling() || FLAG_detailed_line_info;
return NeedsSourcePositionsForProfiling() ||
detailed_source_positions_for_profiling();
}

bool Isolate::NeedsSourcePositionsForProfiling() const {
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3 changes: 2 additions & 1 deletion deps/v8/src/isolate.h
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Expand Up @@ -553,7 +553,8 @@ typedef std::vector<HeapObject*> DebugObjectCache;
V(int, last_console_context_id, 0) \
V(v8_inspector::V8Inspector*, inspector, nullptr) \
V(bool, next_v8_call_is_safe_for_termination, false) \
V(bool, only_terminate_in_safe_scope, false)
V(bool, only_terminate_in_safe_scope, false) \
V(bool, detailed_source_positions_for_profiling, FLAG_detailed_line_info)

#define THREAD_LOCAL_TOP_ACCESSOR(type, name) \
inline void set_##name(type v) { thread_local_top_.name##_ = v; } \
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41 changes: 41 additions & 0 deletions deps/v8/test/cctest/test-cpu-profiler.cc
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Expand Up @@ -40,6 +40,7 @@
#include "src/objects-inl.h"
#include "src/profiler/cpu-profiler-inl.h"
#include "src/profiler/profiler-listener.h"
#include "src/source-position-table.h"
#include "src/utils.h"
#include "test/cctest/cctest.h"
#include "test/cctest/profiler-extension.h"
Expand Down Expand Up @@ -2544,6 +2545,46 @@ TEST(MultipleProfilers) {
profiler2->StopProfiling("2");
}

UNINITIALIZED_TEST(DetailedSourcePositionAPI) {
i::FLAG_detailed_line_info = false;
i::FLAG_allow_natives_syntax = true;
v8::Isolate::CreateParams create_params;
create_params.array_buffer_allocator = CcTest::array_buffer_allocator();
v8::Isolate* isolate = v8::Isolate::New(create_params);

const char* source =
"function fib(i) {"
" if (i <= 1) return 1; "
" return fib(i - 1) +"
" fib(i - 2);"
"}"
"fib(5);"
"%OptimizeFunctionOnNextCall(fib);"
"fib(5);"
"fib";
{
v8::Isolate::Scope isolate_scope(isolate);
v8::HandleScope handle_scope(isolate);
v8::Local<v8::Context> context = v8::Context::New(isolate);
v8::Context::Scope context_scope(context);
i::Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);

CHECK(!i_isolate->NeedsDetailedOptimizedCodeLineInfo());

int non_detailed_positions = GetSourcePositionEntryCount(i_isolate, source);

v8::CpuProfiler::UseDetailedSourcePositionsForProfiling(isolate);
CHECK(i_isolate->NeedsDetailedOptimizedCodeLineInfo());

int detailed_positions = GetSourcePositionEntryCount(i_isolate, source);

CHECK((non_detailed_positions == -1 && detailed_positions == -1) ||
non_detailed_positions < detailed_positions);
}

isolate->Dispose();
}

} // namespace test_cpu_profiler
} // namespace internal
} // namespace v8
1 change: 1 addition & 0 deletions src/node.cc
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Expand Up @@ -2548,6 +2548,7 @@ Isolate* NewIsolate(ArrayBufferAllocator* allocator, uv_loop_t* event_loop) {
isolate->SetMicrotasksPolicy(MicrotasksPolicy::kExplicit);
isolate->SetFatalErrorHandler(OnFatalError);
isolate->SetAllowWasmCodeGenerationCallback(AllowWasmCodeGenerationCallback);
v8::CpuProfiler::UseDetailedSourcePositionsForProfiling(isolate);
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@refack refack Nov 9, 2018 •
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Question: does this have a penalty? And even if not could this be conditional on inspector being attached or profiler run requested (I mean is it possible)?

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The problem with only enabling this once the profiler run is requested is that functions that get optimized before profiling starts don't have the detailed line information. In the browser you can work around this by reloading the page with profiling enabled (devtools has a button for this) but for a running Node instance this is not really feasible, particularly if you want to debug some rarely-occurring thing. Unfortunately we see this in the wild quite a bit so having it on by default makes sense IMO.

The overhead is very low - on average it makes no difference, it the worst case a few percent increase in optimized compile time, but this is currently being moved off-thread anyway (plus the general performance boost of upgrading the V8 version is going to easily outweigh this).

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Thanks for the explanation.

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@refack refack Nov 12, 2018 •
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In the browser you can work around this by reloading the page with profiling enabled

I'm thinking about this some more...
For node we have one main way to enable inspetor, via CLI args. There are two other ways which I don't have usage information about SIGUSR and inspector.open().

It might be a better balance to turn this on only for processes started with inspector listening, and save the performance cost for regular runs. IIUC the down side is less accurate code position information?

I'm ambivalent...

/CC @nodejs/performance

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CpuProfiler can be also used without inspector, see e.g. NPM modules like v8-profiler. There are tools out in the wild which start profiling automatically for some time if they notice performance issues.

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It's also possible to use the CPU profiler directly through the V8 API without the inspector and there are some native modules that do this - that approach wouldn't work for them as far as I understand

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The thoughts leading to this were:

  • It's hard to foresee whether the user is going to profile, both for Chrome and Node.
  • For Chrome, you can at least reload, with DevTools open.
  • Node.js does not have the reload option.
  • Adding more accurate data has a small one-time cost for the script. Chrome suffers a bit more for this as you pay more for optimizing compiles, possibly while starting up a page, and this occurs on every page navigation. Node.js does not encounter new scripts to compile and optimize as often.

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I agree that it's a tough call:
slightly slower compile time but for everyone ⚖️ improvement not available but only for profiling started out-of-band


return isolate;
}
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