[llvm] Reland "[lit] Migrate lit to ProcessPoolExecutor (#202681)" (PR #209076)

[Prasoon Kumar via llvm-commits]

prasoon054 wrote:

I'm using Python 3.8.20.
Thanks for narrowing this down. 56s is still above the 47s baseline, so I don't think this is resolved yet. Can you try `LIT_SUBMISSION_WINDOW` values of 64, 96, 128, and 256 and share the timings? My current guess is that `wait()` scans every in-flight future each time it's called, so runtime grows with the window size. If the timings flatten before 256, the default window is probably larger than necessary. If they're flat even at 64, then the window size likely isn't the issue.
Also, if you noticed any workers going idle with `SUBMISSION_WINDOW_PER_WORKER=1` (window=64 on your machine), that would be useful to know.

https://github.com/llvm/llvm-project/pull/209076