perf: reduce EventSourceStream parser allocations#5032
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trivikr merged 4 commits intonodejs:mainfrom Apr 30, 2026
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Assisted-by: OpenAI:gpt-5.4 Signed-off-by: Kamat, Trivikram <[email protected]>
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BenchmarksBefore$ node benchmarks/eventsource/eventsource-stream.mjs
clk: ~3.18 GHz
cpu: Apple M1 Pro
runtime: node 24.14.0 (arm64-darwin)
benchmark avg (min … max) p75 / p99 (min … top 1%)
------------------------------------------- -------------------------------
• EventSourceStream parsing (250 events)
------------------------------------------- -------------------------------
single chunk 523.13 µs/iter 517.21 µs █
(493.29 µs … 853.88 µs) 731.96 µs █
(167.25 kb … 1.18 mb) 659.19 kb █▆▂▁▁▂▃▃▂▂▁▁▁▁▁▁▁▁▁▁▁
256-byte chunks 544.53 µs/iter 538.29 µs █
(519.42 µs … 1.71 ms) 749.08 µs █▆
(110.61 kb … 2.11 mb) 711.91 kb ██▆▂▂▂▂▂▂▁▁▂▁▂▁▁▁▁▁▁▁
64-byte chunks 597.89 µs/iter 600.50 µs █
(580.54 µs … 844.83 µs) 726.08 µs ▆█
(148.83 kb … 1.41 mb) 868.50 kb ██▇▇▄▂▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁
8-byte chunks 1.14 ms/iter 1.14 ms █ ▃
(1.11 ms … 1.30 ms) 1.27 ms █▇█▅
(659.22 kb … 2.29 mb) 2.29 mb ▆████▆▃▂▂▂▂▁▁▁▁▂▂▃▂▁▂
1-byte chunks 5.49 ms/iter 5.56 ms ▃█▆▇
(5.20 ms … 6.90 ms) 6.63 ms █████▃▂
( 13.86 mb … 13.86 mb) 13.86 mb ███████▆▂█▁▁▃▁▂▁▂▁▁▂▂
• EventSourceStream parsing with BOM
------------------------------------------- -------------------------------
BOM + 8-byte chunks 1.16 ms/iter 1.17 ms █
(1.13 ms … 1.39 ms) 1.34 ms █ ▄
( 2.29 mb … 2.30 mb) 2.29 mb ▆███▅▅▂▂▂▁▁▁▁▁▁▁▁▁▁▂▁
BOM + 1-byte chunks 5.42 ms/iter 5.49 ms █▂
(5.32 ms … 5.74 ms) 5.70 ms ██
( 13.86 mb … 13.87 mb) 13.86 mb ▃██▆▂▆▄▂▂██▂▃▅▁▁▂▁▂▂▂After$ node benchmarks/eventsource/eventsource-stream.mjs
clk: ~3.14 GHz
cpu: Apple M1 Pro
runtime: node 24.14.0 (arm64-darwin)
benchmark avg (min … max) p75 / p99 (min … top 1%)
------------------------------------------- -------------------------------
• EventSourceStream parsing (250 events)
------------------------------------------- -------------------------------
single chunk 356.42 µs/iter 333.46 µs █▇
(320.71 µs … 6.80 ms) 661.83 µs ██
( 10.12 kb … 1.12 mb) 210.89 kb ██▃▄▂▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁
256-byte chunks 364.56 µs/iter 363.38 µs █
(352.96 µs … 517.63 µs) 443.25 µs ███
( 62.13 kb … 1.30 mb) 301.20 kb ███▆▂▂▂▂▂▂▂▁▁▁▁▁▁▁▁▁▁
64-byte chunks 465.56 µs/iter 463.33 µs █▅
(445.42 µs … 650.08 µs) 588.88 µs ▅██
( 14.05 kb … 1.09 mb) 552.85 kb ███▆▄▂▂▂▂▂▂▁▂▁▁▁▁▁▁▁▁
8-byte chunks 1.01 ms/iter 1.00 ms █
(976.13 µs … 1.22 ms) 1.13 ms ▆▄█
( 1.08 mb … 2.11 mb) 1.83 mb ▆████▄▃▂▂▂▁▁▁▁▁▁▂▂▂▂▁
1-byte chunks 5.10 ms/iter 5.10 ms █
(4.84 ms … 6.46 ms) 6.43 ms █ ▃
( 11.93 mb … 11.93 mb) 11.93 mb ▄███▄▃▁▁▂▁▁▁▂▁▁▁▂▁▂▁▁
• EventSourceStream parsing with BOM
------------------------------------------- -------------------------------
BOM + 8-byte chunks 1.02 ms/iter 1.02 ms █
(993.00 µs … 1.33 ms) 1.19 ms █
( 1.84 mb … 1.88 mb) 1.84 mb ▄▃██▃▃▂▂▂▁▁▁▁▁▁▁▁▁▁▁▁
BOM + 1-byte chunks 5.13 ms/iter 5.20 ms ▆█
(4.96 ms … 5.37 ms) 5.36 ms ██▄ ▄ ▃
( 11.93 mb … 11.93 mb) 11.93 mb ▅██▅█████▇█▆█▆▇▇▅▅▅▅▂ |
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #5032 +/- ##
==========================================
+ Coverage 93.14% 93.16% +0.01%
==========================================
Files 110 110
Lines 36117 36212 +95
==========================================
+ Hits 33642 33736 +94
- Misses 2475 2476 +1 ☔ View full report in Codecov by Sentry. 🚀 New features to boost your workflow:
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KhafraDev
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Apr 30, 2026
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EventSource isn't really my area of expertise.
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This relates to...
Fixes: #2630
Rationale
EventSourceStreamwas doing avoidable work on long-lived SSE streams by concatenating the buffered input for every chunk, allocating strings for every parsed field name, and replacing the internal event object after each dispatch. That creates unnecessary allocation churn in the hot path, especially when servers deliver heavily fragmented chunks.Changes
Buffer.concat([buffer, chunk])approach with chunk-list buffering plus cursors.data,event,id, andretryfield names from bytes instead of decoding field strings first.retryandiddirectly from bytes before decoding their values.EventSourceStreambenchmarks covering realistic payloads, fragmented chunking, and BOM-prefixed input.Features
N/A
Bug Fixes
Reduced allocation pressure in the
EventSourceStreamparser hot path for fragmented SSE streams.Breaking Changes and Deprecations
processEvent(event)now receives a reused parser event object instead of a freshly allocated object each time.Status