GeezerVison is showing github.com — perf: reduce EventSourceStream parser allocations by trivikr · Pull Request #5032 · nodejs/undici
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This relates to...
Fixes: #2630
Rationale
EventSourceStream was 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
- Replaced the single-buffer
Buffer.concat([buffer, chunk]) approach with chunk-list buffering plus cursors.
- Kept line parsing lazy so buffers are only concatenated when a logical line spans multiple chunks.
- Matched
data, event, id, and retry field names from bytes instead of decoding field strings first.
- Validated
retry and id directly from bytes before decoding their values.
- Reset the parser event state in place instead of allocating a fresh event object on every event boundary.
- Added focused
EventSourceStream benchmarks covering realistic payloads, fragmented chunking, and BOM-prefixed input.
Features
N/A
Bug Fixes
Reduced allocation pressure in the EventSourceStream parser hot path for fragmented SSE streams.
Breaking Changes and Deprecations
- No public API changes.
- Internally,
processEvent(event) now receives a reused parser event object instead of a freshly allocated object each time.
Status
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Benchmarks
Before
$ 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 ▅██▅█████▇█▆█▆▇▇▅▅▅▅▂ |
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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.16%. Comparing base (2eb9ddc) to head (f660a6e).
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.
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EventSource isn't really my area of expertise.
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This was referenced Sep 4, 2026
Reviewers
mcollina
mcollina approved these changes
Uzlopak
Awaiting requested review from Uzlopak
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