improve text encoder encode performance by anonrig · Pull Request #5448 · cloudflare/workerd

anonrig · 2025-10-31T14:50:27Z

Improves Next.js benchmarks by 4-7% and WebAssembly applications by 5%

Important disclaimer: Codspeed will not show any improvement (although you can look into previous edits of the codspeed post) because this change is behind an autogate.

Sharing @mhart's benchmark results:

Before:

  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    17.63ms    7.86ms 102.29ms   87.71%
    Req/Sec    57.83      9.24    80.00     75.59%
  Latency Distribution
     50%   14.25ms
     75%   17.30ms
     90%   33.63ms
     99%   41.37ms
  1736 requests in 30.06s, 517.73MB read
Requests/sec:     57.75

After:

  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    16.96ms    9.48ms 145.37ms   88.17%
    Req/Sec    61.43      9.89    80.00     74.92%
  Latency Distribution
     50%   13.27ms
     75%   16.94ms
     90%   32.61ms
     99%   41.71ms
  1842 requests in 30.05s, 549.21MB read
Requests/sec:     61.30

This is pretty consistent over multiple runs. Seeing 1.04x - 1.07x

Claude is used to write documentation in encoding-test.c++.

src/workerd/api/encoding.c++

codspeed-hq · 2025-10-31T15:10:31Z

Merging this PR will degrade performance by 21.41%

❌ 1 regressed benchmark
✅ 69 untouched benchmarks
⏩ 129 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	`BASE`	`HEAD`	Efficiency
❌	`bodyWithHeaders[Response]`	30.2 µs	38.5 µs	-21.41%

_{Comparing yagiz/experiment-value-view (9dce7c9) with main (f3b58ab)}

129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

src/workerd/api/encoding.c++

jasnell · 2025-11-03T17:01:52Z

Looks like there are still some bugs to work out here with the failing CI... but even then, my preference would be to settle on #5449 first before landing any changes here. Also, since we do utf8 conversions everywhere, not just in TextEncoder::encode, my preference would be to address this more generally. That is, the optimized encoding path -- if we decide it's worthwhile -- should go into jsg::JsString so that it can be used everywhere rather than just in TextEncoder::encode.

src/workerd/api/encoding.c++

erikcorry · 2025-11-03T20:45:07Z

I don't think we need to speed optimize for the broken UTF-16 case unless and until someone shows it matters. The only reason to space-optimize would be to avoid throwing OOM, so if we can guarantee that doesn't happen I'm OK with some temporary blowup in space too.

erikcorry · 2025-11-03T21:05:55Z

https://paste.cfdata.org/GKKdyFGFqSks

anonrig · 2025-11-03T21:28:36Z

https://paste.cfdata.org/GKKdyFGFqSks

This helps a lot. I'll push with your changes. Thanks @erikcorry

src/workerd/api/encoding.c++

* Simplify * Tune slightly * Fix assert * Fix perf regression and OOM read * Handle very tiny output buffers * feedback

codecov-commenter · 2026-02-23T17:17:30Z

Codecov Report

❌ Patch coverage is 71.24464% with 67 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.58%. Comparing base (f3b58ab) to head (9dce7c9).

Files with missing lines	Patch %	Lines
src/workerd/api/encoding-test.c++	3.50%	0 Missing and 55 partials ⚠️
src/workerd/api/encoding.c++	92.94%	2 Missing and 10 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5448      +/-   ##
==========================================
- Coverage   70.59%   70.58%   -0.01%     
==========================================
  Files         413      414       +1     
  Lines      109986   110195     +209     
  Branches    18120    18196      +76     
==========================================
+ Hits        77644    77782     +138     
- Misses      21531    21546      +15     
- Partials    10811    10867      +56

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anonrig requested review from a team as code owners October 31, 2025 14:50

jasnell reviewed Oct 31, 2025

View reviewed changes