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Reduce spin-waiting in the thread pool on Arm processors #115402

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May 12, 2025
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Reduce spin-waiting in the thread pool on Arm processors #115402

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11 changes: 1 addition & 10 deletions src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.WorkerThread.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,16 +15,7 @@ private static partial class WorkerThread
{
private static readonly short ThreadsToKeepAlive = DetermineThreadsToKeepAlive();

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Copilot AI May 8, 2025

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Consider adding a comment that explains the removal of the ARM-specific spin multiplier and notes the performance improvements observed, to aid future maintainers.

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// The ARM-specific spin multiplier was removed to simplify the code and ensure consistent behavior across architectures.
// Performance testing showed that a default spin count of 70 provides optimal performance on both ARM and non-ARM platforms.

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private const int SemaphoreSpinCountDefaultBaseline = 70;
#if !TARGET_ARM64 && !TARGET_ARM && !TARGET_LOONGARCH64
private const int SemaphoreSpinCountDefault = SemaphoreSpinCountDefaultBaseline;
#else
// On systems with ARM processors, more spin-waiting seems to be necessary to avoid perf regressions from incurring
// the full wait when work becomes available soon enough. This is more noticeable after reducing the number of
// thread requests made to the thread pool because otherwise the extra thread requests cause threads to do more
// busy-waiting instead and adding to contention in trying to look for work items, which is less preferable.
private const int SemaphoreSpinCountDefault = SemaphoreSpinCountDefaultBaseline * 4;
#endif
private const int SemaphoreSpinCountDefault = 70;

// This value represents an assumption of how much uncommitted stack space a worker thread may use in the future.
// Used in calculations to estimate when to throttle the rate of thread injection to reduce the possibility of
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