src: add event loop based metrics into node by pabloerhard · Pull Request #62935 · nodejs/node

pabloerhard · 2026-04-24T18:31:31Z

Adds an opt in into monitorEventLoopDelay(), this option adds support for per iteration sampling using libuv hooks, while preserving the existing timer based approach.

When the new samplePerIteration option is set to true, monitorEventLoopDelay() samples directly from event loop iterations instead of using the interval timer. When samplePerIteration is omitted or false, the existing interval based implementation remains unchanged, including resolution handling and the older interval based trace counters.

Motivation
The interval based implementation has two limitations that the new option addresses:

Inaccurate measurements. Delay was approximated from a timer dispatch jitter (RecordDelta() between timer ticks) rather than measured from the loop itself. Samples had an implicit floor of resolution ms and could miss delays that didn't happen to coincide with a timer tick.
Wasteful wake ups. The timer forced the event loop to iterate every resolution ms just to record a sample, even when the application was otherwise idle and no one was reading the histogram.

This is an alternative implementation to the one in #62934

This PR directly addresses the following issue #56064

codecov · 2026-04-30T22:28:02Z

Codecov Report

❌ Patch coverage is 78.51852% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.64%. Comparing base (80e0f14) to head (27ead1e).
⚠️ Report is 109 commits behind head on main.

Files with missing lines	Patch %	Lines
src/histogram.cc	79.50%	11 Missing and 14 partials ⚠️
src/histogram.h	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62935      +/-   ##
==========================================
- Coverage   89.65%   89.64%   -0.02%     
==========================================
  Files         708      708              
  Lines      220413   220546     +133     
  Branches    42275    42290      +15     
==========================================
+ Hits       197607   197702      +95     
- Misses      14658    14675      +17     
- Partials     8148     8169      +21

Files with missing lines	Coverage Δ
lib/internal/perf/event_loop_delay.js	`95.95% <100.00%> (+0.12%)`	⬆️
src/node_perf.cc	`86.61% <100.00%> (+0.22%)`	⬆️
src/histogram.h	`50.00% <0.00%> (-10.00%)`	⬇️
src/histogram.cc	`79.08% <79.50%> (+0.12%)`	⬆️

... and 33 files with indirect coverage changes

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add a samplePerIteration option to monitorEventLoopDelay that records event loop delay from libuv event loop iterations instead of the existing timer interval sampler. The default remains the interval based, uses of monitorEventLoopDelay() will still behave the same unless the samplePerIteration options is passed through. Signed-off-by: Pablo Erhard Hernandez <pablo.erhardhernandez@datadoghq.com>

Add test coverage for the per iteration event loop delay histogram start and stop callbacks Use disitinct debug tracking keys for the event loop delay histogram callbacks.

Use the super Close method instead of duplicating code

Added extra test to add coverage of the overall functionality of the new ELD histogram.

Modified docs to be on par with the changes and modifications made.

BridgeAR

Pretty much LGTM, just left two nits. I would feel more comfortable with someone else reviewing it as well.

@addaleax @jasnell maybe?

nodejs-github-bot · 2026-06-17T15:20:57Z

CI: https://ci.nodejs.org/job/node-test-pull-request/74208/

The C++ class `ELDHistogram` is the per-iteration sampling implementation that sits next to `IntervalHistogram` (the timer-based one). Its name collided with the JS exposed `ELDHistogram` class that `monitorEventLoopDelay()` returns regardless of sampling mode, making the layering confusing: "ELDHistogram" was both the generic JS concept and the name of one specific C++ backing.

align perf_hooks docs after renaming internal ELDHistogram to IterationHistogram

Added shared templates for duplicated start/stop logic in IntevalHistogram and IterationHistogram.

BridgeAR

LGTM, while I would still like to get another LG who works on CPP frequently

nodejs-github-bot · 2026-06-18T15:55:44Z

CI: https://ci.nodejs.org/job/node-test-pull-request/74238/

nodejs-github-bot added c++ Issues and PRs that require attention from people who are familiar with C++. needs-ci PRs that need a full CI run. labels Apr 24, 2026

rochdev reviewed Apr 24, 2026

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Comment thread src/histogram.cc Outdated

addaleax reviewed Apr 24, 2026

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Comment thread src/histogram.cc Outdated

rochdev mentioned this pull request Apr 24, 2026

src: add a native mode to monitorEventLoopDelay #62934

Closed

pabloerhard force-pushed the pabloerhard/add-event-loop-metrics branch from edb66ea to e7093c8 Compare April 28, 2026 20:12

pabloerhard marked this pull request as ready for review April 29, 2026 18:38

pabloerhard force-pushed the pabloerhard/add-event-loop-metrics branch from 6f5cdca to bc0a59c Compare April 30, 2026 17:55

bengl approved these changes Apr 30, 2026

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pabloerhard force-pushed the pabloerhard/add-event-loop-metrics branch from bc0a59c to 27ead1e Compare April 30, 2026 23:33

BridgeAR added the request-ci Add this label to start a Jenkins CI on a PR. label May 11, 2026

pabloerhard requested review from addaleax and bengl May 12, 2026 19:51

pabloerhard added 4 commits May 19, 2026 12:59

perf_hooks: test event loop delay fast api callbacks

2660e2e

Add test coverage for the per iteration event loop delay histogram start and stop callbacks Use disitinct debug tracking keys for the event loop delay histogram callbacks.

perf_hooks: format cpp

89a5c17

refactor: use super Close method

3b21617

Use the super Close method instead of duplicating code

pabloerhard force-pushed the pabloerhard/add-event-loop-metrics branch from 27ead1e to 3b21617 Compare May 19, 2026 21:21

BridgeAR reviewed May 27, 2026

View reviewed changes

Comment thread doc/api/perf_hooks.md

BridgeAR reviewed May 27, 2026

View reviewed changes

Comment thread doc/api/perf_hooks.md

BridgeAR reviewed May 27, 2026

View reviewed changes

Comment thread src/histogram.cc Outdated

BridgeAR reviewed May 27, 2026

View reviewed changes

Comment thread test/sequential/test-performance-eventloopdelay.js

pabloerhard added 2 commits May 27, 2026 15:38

test: improve event loop delay sample-per-iteration coverage

d243595

Added extra test to add coverage of the overall functionality of the new ELD histogram.

docs: improve docs

c52710d

Modified docs to be on par with the changes and modifications made.

pabloerhard requested a review from BridgeAR June 11, 2026 09:05

BridgeAR approved these changes Jun 17, 2026

View reviewed changes

Comment thread doc/api/perf_hooks.md Outdated

Comment thread src/histogram.cc Outdated

github-actions Bot removed the request-ci Add this label to start a Jenkins CI on a PR. label Jun 17, 2026

doc: align perf_hooks histogram class name with implementation

88fce8d

align perf_hooks docs after renaming internal ELDHistogram to IterationHistogram

pabloerhard force-pushed the pabloerhard/add-event-loop-metrics branch from 71f121f to 88fce8d Compare June 17, 2026 16:56

pabloerhard requested a review from BridgeAR June 17, 2026 20:18

perf_hooks: deduplicate histogram Start/Stop via shared templates

29b708d

Added shared templates for duplicated start/stop logic in IntevalHistogram and IterationHistogram.

pabloerhard force-pushed the pabloerhard/add-event-loop-metrics branch from 50f8eab to 29b708d Compare June 18, 2026 13:32

BridgeAR approved these changes Jun 18, 2026

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BridgeAR requested review from Qard, jasnell, joyeecheung and legendecas June 18, 2026 15:24

BridgeAR added the request-ci Add this label to start a Jenkins CI on a PR. label Jun 18, 2026

github-actions Bot removed the request-ci Add this label to start a Jenkins CI on a PR. label Jun 18, 2026

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src: add event loop based metrics into node#62935

src: add event loop based metrics into node#62935
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