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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bindings/translate-time-profile.cc

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

ts/src/time-profiler.ts

+  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

bindings/profilers/wall.cc

+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!

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 0d480d5 Compare 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 0e5f1dc Compare March 3, 2026 19:10

IlyasShabi and 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 b617efb Compare March 4, 2026 15:02

compute totalHintCount on c++ side

c4d4158

IlyasShabi force-pushed the ishabi/lazy-time-profiler branch from b617efb to c4d4158 Compare 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-connector bot reviewed Mar 5, 2026

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bindings/translate-time-profile.cc Outdated Show resolved Hide resolved

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 main Mar 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

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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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