Reduce time profiler memory usage with lazy profile tree by IlyasShabi · Pull Request #287 · DataDog/pprof-nodejs

IlyasShabi · 2026-03-02T20:48:32Z

What does this PR do?:
Introduces a new stopAndCollect function for the time profiler that reduces memory usage during profile serialization. Instead of materializing the entire V8 CPU profile tree as JS objects upfront, the new API resolves children on demand using getters.

This also moves totalHitCount() computation to C++ instead of TS to avoid getting children twice from C++

Motivation:
The current stop() API recursively translates the entire profile tree into JS objects, creating a full copy in heap memory which can be significant for large applications.
The new stopAndCollect() API uses a callback to keep the V8 profile alive while the TS serializer traverses the tree lazily.
Scalar properties are set at creation time, but children are resolved on first access, reducing peak memory usage.

Additional Notes:

Similar to heap profiling
Benchmark on CI shows:

* ~99% on initial load which normal since we are not materializing the profiler to JS objects.
* ~52% after traversing the profiler tree to explicitly call getters.
* ~58% after calling afterHitCount() which is moved to c++.

How to test the change?:
Unit tests

github-actions · 2026-03-02T20:49:14Z

Overall package size

Self size: 1.98 MB
Deduped: 2.34 MB
No deduping: 2.34 MB

Dependency sizes

| name | version | self size | total size | |------|---------|-----------|------------| | source-map | 0.7.6 | 185.63 kB | 185.63 kB | | pprof-format | 2.2.1 | 163.06 kB | 163.06 kB | | node-gyp-build | 3.9.0 | 8.81 kB | 8.81 kB |

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

pr-commenter · 2026-03-03T14:49:49Z

Benchmarks

Benchmark execution time: 2026-03-10 13:04:35

Comparing candidate commit 5434977 in PR branch ishabi/lazy-time-profiler with baseline commit 2db9741 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 91 metrics, 29 unstable metrics.

IlyasShabi · 2026-03-05T08:11:11Z

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szegedi

Overall, great work. I'm sorry you had to go through all that trouble for supporting line-info. I have two refactoring suggestions to reduce code duplication between two modes of stopping.

szegedi · 2026-03-09T14:33:01Z

+ if (restart) {
+ gV8ProfilerStuckEventLoopDetected =
+ gProfiler.v8ProfilerStuckEventLoopDetected();
+ if (gV8ProfilerStuckEventLoopDetected > 0) {
+ gProfiler.stop(false);
+ gProfiler.start();
+ }
+ } else {
+ gV8ProfilerStuckEventLoopDetected = 0;
+ gProfiler.dispose();
+ gProfiler = undefined;
+ gSourceMapper = undefined;
+ if (gStore !== undefined) {
+ gStore.disable();
+ gStore = undefined;
+ }
+ }


You could extract these into two module-private functions (the way they are in the original stop method) and call these functions instead, to reduce some code duplication.

szegedi · 2026-03-09T14:38:15Z

+Result WallProfiler::StopAndCollectImpl(bool restart,
+ v8::Local<v8::Function> callback,
+ v8::Local<v8::Value>& result) {


This method is very similar to ::Stop. I had Claude extract the common code from the two to see if it can be done; I put it up as a draft PR here. Feel free to incorporate the commit here, I just put the PR up for the ease of review.

IlyasShabi · 2026-03-10T13:08:53Z

@szegedi Thanks for your suggestions! I actually preferred not to use shared helper functions for the duplicated code because the upcoming PR will remove the old stop function, but I’m also fine with this approach.

szegedi

Looks good, 🚢 it!

Reduce time profiler memory usage with lazy profile tree

IlyasShabi added the semver-patch Bug or security fixes, mainly label Mar 2, 2026

Reduce time profiler memory usage with lazy profile tree
0d480d5

IlyasShabi force-pushed the ishabi/lazy-time-profiler branch from 216d01f to 0d480d5Compare March 3, 2026 14:45

Merge branch 'main' into ishabi/lazy-time-profiler
0e5f1dc

IlyasShabi force-pushed the ishabi/lazy-time-profiler branch from 9881a82 to 0e5f1dcCompare March 3, 2026 19:10

IlyasShabiand others added 2 commits March 4, 2026 12:12

Merge branch 'main' into ishabi/lazy-time-profiler
d5848ac

asan 25 issue
94ff9f0

IlyasShabi force-pushed the ishabi/lazy-time-profiler branch 6 times, most recently from 7ee93d7 to b617efbCompare March 4, 2026 15:02

compute totalHintCount on c++ side
c4d4158

IlyasShabi force-pushed the ishabi/lazy-time-profiler branch from b617efb to c4d4158Compare March 4, 2026 19:58

add profilev2 tests
3a567e0

IlyasShabi marked this pull request as ready for review March 4, 2026 21:05

IlyasShabi requested review from nsavoire, r1viollet and szegedi as code owners March 4, 2026 21:05

chatgpt-codex-connectorBot reviewed Mar 5, 2026
View reviewed changes

Comment threadbindings/translate-time-profile.cc Outdated

IlyasShabi added 2 commits March 5, 2026 11:23

Preserve line-number root children in lazy view
f8d317c

fix linter
3ed4376

szegedi requested changes Mar 9, 2026
View reviewed changes

use common functions for duplicate code
5434977

szegedi approved these changes Mar 10, 2026
View reviewed changes

IlyasShabi merged commit bd5ca5d into mainMar 10, 2026
67 checks passed

IlyasShabi deleted the ishabi/lazy-time-profiler branch March 10, 2026 17:31

szegedi mentioned this pull request Mar 13, 2026
v5.14.0 #295
Merged

szegedi pushed a commit that referenced this pull request Mar 18, 2026
Reduce time profiler memory usage with lazy profile tree (#287)
5db3031
Reduce time profiler memory usage with lazy profile tree

szegedi pushed a commit that referenced this pull request Mar 18, 2026
Reduce time profiler memory usage with lazy profile tree (#287)
825dcc1
Reduce time profiler memory usage with lazy profile tree

IlyasShabi mentioned this pull request Mar 19, 2026
Switch time profiling to use lazy load stop and collect method by default #305
Merged

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Reduce time profiler memory usage with lazy profile tree - #287

Reduce time profiler memory usage with lazy profile tree#287
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ishabi/lazy-time-profiler

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