[Webkit-unassigned] [Bug 252981] useConcurrentJIT=true makes JS2-wasm slower on higher core counts

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=252981

--- Comment #3 from Mark Lam <mark.lam at apple.com> ---
> Running the wasm parts of JS2 (wasm-cli.js) with useConcurrentJIT=true on an 80-core ARM64 box results in a performance slowdown. Specifically, with --useBBQJIT=false --useOMGJIT=false, the total score for wasm-cli.js hovers around 10 with useConcurrentJIT=true and between 13-14 for useConcurrentJIT=false.

1. What does it mean to run on an 80-core ARM64 box?
   1.1 How many automatic threads are we actually spinning up in response to this?
   1.2 How many of these threads are actually active during the slowdown?
   1.3 What are these threads working on at that time?
2. What is nature of the workload?
   2.1 Is the slowness due to more contention now?
   2.2 Is the slowness due to memory bandwidth saturation?

Instead of jumping to some solution prematurely, I think you should profile the workload to see what's changed in the high core count profile vs the low core count profile.  Since not everyone else has access to this 80-core ARM64 box, you'll have to do this profiling to determine what's change in the performance profile.

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