Commits on Oct 12, 2019

1. Avoid unnecessary epoll event loop wake-ups …

   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 netty#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.

   

   njhill committed Oct 12, 2019
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