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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