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

["Dominik Moritz (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-16117?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Dominik Moritz resolved ARROW-16117.
------------------------------------
    Fix Version/s: 8.0.0
       Resolution: Fixed

Issue resolved by pull request 12793
[https://github.com/apache/arrow/pull/12793]

> [JS] Improve UTF8 decoding performance
> --------------------------------------
>
>                 Key: ARROW-16117
>                 URL: https://issues.apache.org/jira/browse/ARROW-16117
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: JavaScript
>         Environment: MacOS, Chrome, Safari
>            Reporter: Howard Zuo
>            Assignee: Dominik Moritz
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 8.0.0
>
>          Time Spent: 2.5h
>  Remaining Estimate: 0h
>
> 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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