166caf96ef
Motivation The recently-introduced event loop scheduling hooks can be exploited by the epoll transport to avoid waking the event loop when scheduling future tasks if there is a timer already set to wake up sooner. There is also a "default" timeout which will wake the event loop after 1 second if there are no pending future tasks. The performance impact of these wakeups themselves is likely negligible but there's significant overhead in having to re-arm the timer every time the event loop goes to sleep (see #7816). It's not 100% clear why this timeout was there originally but we're sure it's no longer needed. Modification Combine the existing volatile wakenUp and non-volatile prevDeadlineNanos fields into a single AtomicLong that stores the next scheduled wakeup time while the event loop is in epoll_wait, and is -1 while it is awake. Use this as a guard to debounce wakeups from both immediate scheduled tasks and future scheduled tasks, the latter using the new before/afterScheduledTaskSubmitted overrides and based on whether the new deadline occurs prior to an already-scheduled timer. A similar optimization was already added to NioEventLoop, but it still uses two separate volatiles. We should consider similar streamlining of that in a future update. Result Fewer event loop wakeups when scheduling future tasks, greatly reduced overhead when no future tasks are scheduled. |
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