Re: nanosleep resolution, jiffies vs microseconds

[Paul E. McKenney]

Hello, Darren,

Thank you very much for getting to the bottom of this!

This is mostly an issue when sleeping for small numbers of ticks,
so if HZ was 10000, a nanosleep(1000000) would get bumped by
a couple hundred microseconds rather than the current milliseconds,
right?

Further, if one were to do nanosleep(900000) given HZ of 1024,
the expected sleep time would be 2 milliseconds, right?

						Thanx, Paul

On Wed, Dec 08, 2004 at 08:47:48AM -0800, Darren Hart wrote:
> I am looking at trying to improve the latency of nanosleep for short
> sleep times (~1ms).  After reading Martin Schwidefsky's post for cputime
> on s390 (Message-ID:
> <20041111171439.GA4900@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>), it seems to me
> that we may be able to accomplish this by storing the expire time in
> microseconds rather than jiffies.  Here is an example for context:
> 
> Say we want to sleep for 1ms on i386, we call nanosleep(1000000).
> Unfortunately on i386 a jiffy is slightly less than 1ms (as one might
> expect with HZ = 1000).  So when sys_nanosleep calls
> timespec_to_jiffies, it returns 2.  Now to allow for the corner case
> when my 1ms sleep request gets called at the very tail end of a clock
> period (see ascii diagram below), nanosleep adds 1 to that and calls
> schedule_timeout with 3.  So a 1 ms sleep correctly turns into 3
> jiffies.
> 
> If we were to store the expire value in microseconds, this corner case
> would still exist and still span two full tick periods.  However, the
> large majority of the time, nanosleep(1000000) could pause for only 2
> jiffies, instead of 3.  Before I dug to deep into the relevant code I
> wanted to hear some opinions on this approach.
> 
> 
> Worst case scenario for a 1ms sleep:
> 
> TICK @ 1000000000 ns ------------------------   (X jiffies)
> 
> 
>     nanosleep(1000000) // this can't correctly wake until 1001999849
> TICK @ 1000999849 ns ------------------------   (X jiffies + 1)
> 
> 
> 
> TICK @ 1001999698 ns ------------------------   (X jiffies + 2)
>     at 1001999849 nanosleep call can wake up
>     (but since this is after X jiffies + 2, we can't actually wake
>      until X jiffies + 3)
> 
> TICK @ 1002999547 ns ------------------------   (X jiffies + 3)
>     wake from nanosleep
> 
> 
> Thanks,
> 
> -- 
> Darren Hart <darren@xxxxxxxxxx>
> 
> 
> 
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