Associate label sets with wall profiles

[nsavoire]

Based on #98

Adds ability to associate custom label sets to wall samples. This can then be used by dd-trace-js to associate span ids with said samples.

Implementation notes

The implementation adds JS API to set and clear labels. In practice, the WallProfiler JS class will gain getters and setters for properties labels.

Internally, the implementation uses a PPROF signal handler executing before V8's signal handler used for profiling. It will associate the current label set with a timestamp, and put it in a preallocated buffer (so allocations are avoided in the signal handler.) This happens once every 10ms while profiling. The buffer is sized generously, for 2x the number of samples for a single profile.

When the profile is post-processed once every minute, the labels are matched to samples based on the timestamp, and emitted into the serialized profile.

Timestamp matching

Two timestamps are collected in profiler signal handler and are associated with the current label: one before v8 handler is invoked and one after.
These two timestamps allow easily matching v8 profiling samples to labels, each sample is matched with the label whose timestamp interval contains the sample timestamp.
v8 does not expose publicly its clock API (v8::base::TimeTicks::Now()), matching samples / labels without a common clock source proved to be difficult, hence profiler uses private v8 API by redeclaring it.

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-06-28 11:18:02

Comparing candidate commit 516a9d6 in PR branch nsavoire/use_timestamps with baseline commit 19d5e87 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 114 metrics, 66 unstable metrics.

[github-actions]

Overall package size

Self size: 2.08 MB
Deduped: 2.96 MB
No deduping: 2.96 MB

Dependency sizes

name	version	self size	total size
pprof-format	2.0.7	588.12 kB	588.12 kB
source-map	0.7.4	226 kB	226 kB
split	1.0.1	12.29 kB	24.82 kB
p-limit	3.1.0	7.75 kB	13.78 kB
delay	5.0.0	11.17 kB	11.17 kB
pify	5.0.0	8.87 kB	8.87 kB
node-gyp-build	3.9.0	8.81 kB	8.81 kB

_{🤖 This report was automatically generated by heaviest-objects-in-the-universe}

[r1viollet]

great comment, should we add a ticket to v8 for this ?

[r1viollet]

LGTM
A very well thought approach

[r1viollet]

LGTM

[theanarkh]

@nsavoire Hi, I want to know why this happened and why do you need to deal with this situation ? I read the V8 source code and found that the timestamp of samples is incremental (test/cctest/test-cpu-profiler.cc).

  uint64_t end_time = profile->GetEndTime();
  uint64_t current_time = profile->GetStartTime();
  CHECK_LE(current_time, end_time);
  for (int i = 0; i < profile->GetSamplesCount(); i++) {
    CHECK(profile->GetSample(i));
    uint64_t timestamp = profile->GetSampleTimestamp(i);
    // <=
    CHECK_LE(current_time, timestamp);
    CHECK_LE(timestamp, end_time);
    current_time = timestamp;
  }