[jira] [Created] (ARROW-16117) [JS] Improve UTF8 decoding performance

["Howard Zuo (Jira)" <ji...@apache.org>]

Howard Zuo created ARROW-16117:
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             Summary: [JS] Improve UTF8 decoding performance
                 Key: ARROW-16117
                 URL: https://issues.apache.org/jira/browse/ARROW-16117
             Project: Apache Arrow
          Issue Type: Improvement
         Environment: MacOS, Chrome, Safari

            Reporter: Howard Zuo


While profiling the performance of decoding TPC-H Customer and Part in-browser, datasets where there are a lot of UTF8s, it turned out that much of the time was being spent in {{getVariableWidthBytes}} rather than in {{TextDecoder}} itself. Ideally all the time should be spent in {{{}TextDecoder{}}}.

On Chrome {{getVariableWidthBytes}} took up to ~15% of the e2e decoding latency, and on Safari it was close to ~40% (Safari's TextDecoder is much faster than Chrome's, so this took up relatively more time).

This is likely because the code in this PR is more amenable to V8/JSC's JIT, since {{x}} and {{y}} now are guaranteed to be SMIs ("small integers") instead of Object, allowing the JIT to emit efficient machine instructions that only deal in 32-bit integers. Once V8 discovers that a {{x}} and {{y}} can potentially be null (upon iterating past the bounds), it "poisons" the codepath forever, since it has to deal with the null case.

See this V8 post for a more in-depth explanation (in particular see the examples underneath "Performance tips"):
[https://v8.dev/blog/elements-kinds]

Doing the bounds check explicitly instead of implicitly basically eliminates this function from showing up in the profiling. Empirically, on my machine decoding TPC-H Part dropped from 1.9s to 1.7s on Chrome, and Customer dropped from 1.4s to 1.2s.

[https://github.com/apache/arrow/pull/12793]

 



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