move to semaphore #165
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move to semaphore #165
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Summary: sched_yield looping results in much higher latency in highly concurrent pipe I/O. It's not strictly busy wait, but relinquish CPU and move to the end of priority task queue while no immediately available data. http://man7.org/linux/man-pages/man2/sched_yield.2.html. The overhead of re-scheduling and context switch could be high. Conversely, modern semaphore/mutex is kind hybrid locking of spinlock and sleep-lock - performs spinklock for the first part of remained scheduled CPU time and sleep after if lock is unavailable. It avoids both frequent re-scheduling and eating up CPU with spin. In practice it's always performs better than pure spinlock or re-schedule, even to protect lightweight operations like counter update. Differential Revision: D21446104 fbshipit-source-id: b49bb1bc6b5576142344ed83d964ff6df5b21477
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This pull request was exported from Phabricator. Differential Revision: D21446104 |
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I want to take a close look at this, as it goes against what I had observed, so it may take a bit to get back to you, sorry... |
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Summary: sched_yield looping results in much higher latency in highly concurrent pipe I/O. It's not strictly busy wait, but relinquish CPU and move to the end of priority task queue while no immediately available data. http://man7.org/linux/man-pages/man2/sched_yield.2.html. The overhead of re-scheduling and context switch could be high. Conversely, modern semaphore/mutex is kind hybrid locking of spinlock and sleep-lock - performs spinklock for the first part of remained scheduled CPU time and sleep after if lock is unavailable. It avoids both frequent re-scheduling and eating up CPU with spin. In practice it's always performs better than pure spinlock or re-schedule, even to protect lightweight operations like counter update.
Differential Revision: D21446104