[issue21302] time.sleep (floatsleep()) should use clock_nanosleep() on Linux

[Eryk Sun]

Eryk Sun <eryksun at gmail.com> added the comment:

> Absolute timeout using can reduce the overhead time of any variable 
> and object intialization cost before the WaitForMultipleObjects() 

Again, timer objects store the due time in interrupt time, not system time (i.e. InterruptTime vs SystemTime in the KUSER_SHARED_DATA record). The due time gets set as the current interrupt time plus a relative due time. If the due time is passed as absolute system time, the kernel just computes the delta from the current system time.

The timer object does record whether the requested due time is an absolute system time. This allows the kernel to recompute all absolute due times when the system time is changed manually. This is also the primary reason one wouldn't implement time.sleep() with absolute system time.

> using absolute timeout and GetSystemTimePreciseAsFileTime() can 
> improves the accuracy of the desired sleep time.

It would not improve the resolution. Timer objects are signaled when their due time is at or before the current interrupt time. The latter gets updated by the timer interrupt service routine, by default every 15.625 ms -- or at least that used to be the case.

The undocumented flag CREATE_WAITABLE_TIMER_HIGH_RESOLUTION creates a different timer type, called an "IRTimer" (implemented in Windows 8.1, but back then only accessible in the NT API). This timer type is based on precise interrupt time, which is interpolated using the performance counter. I don't know how the implementation of the timer interrupt has changed to support this increased resolution. It could be that the default 15.625 ms interrupt period is being simulated for compatibility with classic timers and Sleep(). I'd love for the CREATE_WAITABLE_TIMER_HIGH_RESOLUTION flag and the behavior of IRTimer objects to be documented.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue21302>
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