fix: --cpu-priority not working on Windows
#283
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The current implementation works by setting the current thread's priority, which is inherited by spawned Stockfish workers on Unix but not on Windows, where it had no effect. Replace the implementation with a process-wide priority change, which is properly inherited on Windows. Resolves lichess-org#279
alvaro-cuesta
commented
Oct 10, 2025
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| // BELOW_NORMAL_PRIORITY_CLASS is the lowest priority that won't completely | ||
| // starve tasks of CPU time on high loads. The lowest IDLE_PRIORITY_CLASS | ||
| // is stricter than Linux's nice 19! | ||
| unsafe { SetPriorityClass(GetCurrentProcess(), BELOW_NORMAL_PRIORITY_CLASS) } |
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This choice was inherited from thread_priority but I think it makes sense.
IDLE_PRIORITY_CLASS will preempt aggressively and completely starve the process unless the CPU is really idle, which I think could lead to unintended issues (like timing out on API if preempted in the middle of handling a request).
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| // On Linux the priority range is -20 (highest) to 19 (lowest). On other | ||
| // Unixes the range is -20 to 20. | ||
| #[cfg(target_os = "linux")] | ||
| const MINIMUM_PRIORITY_NICENESS: libc::c_int = 19; | ||
| #[cfg(not(target_os = "linux"))] | ||
| const MINIMUM_PRIORITY_NICENESS: libc::c_int = 20; |
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I tested this and Linux clamps to 19, but I still think it makes sense to be explicit.
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Thanks! |
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The current implementation works by setting the current thread's priority, which is inherited by spawned Stockfish workers on Unix but not on Windows, where it had no effect.
Replace the implementation with a process-wide priority change, which is properly inherited on Windows.
Resolves #279