Fix thread priority problems on windows

[matklad]

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[lnicola]

Would it make sense to reduce the priority of the thread pool threads instead? This way it would work on other platforms, too.

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• Rust (windows-latest)

[matklad]

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I would have lowered thread pool priority, but there’s no easy api to customize thread creation. But it seems to be orthogonal to being cross platform?

Ideally, I’d prefer to not tweak propiorities at all and just rely on fairness, but on windows we specifically hit an unfair case.

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• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript

[lnicola]

I would have lowered thread pool priority, but there’s no easy api to customize thread creation.

Ah, I missed that. I know that some crates like rayon have a thread creation callback, but threadpool doesn't. I was under the impression that you've recently switched to a different thread pool implementation, but no, it's the same one.

But it seems to be orthogonal to being cross platform?

On Linux you can decrease a thread's priority (nice), but not increase it unless you have CAP_SYS_NICE.

In general, I wouldn't expect schedulers to be fair. They're a mess of heuristics (like the priority boosting that Windows does in some cases) that either improve responsiveness or throughput. In the recent discussion about user-space spinlocks, Torvalds pointed out that fairness is bad for throughput, on smaller systems, but:

Pretty much every time we picked an unfair - but fast - locking model in the kernel, we ended up regretting it eventually, and had to add fairness.