linux-kernel - Re: [LTP] [patch V2 00/20] timer: Refactor the timer wheel

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Date:	23 Jun 2016 09:58:03 -0400
From:	"George Spelvin" <linux@...encehorizons.net>
To:	chrubis@...e.cz, tglx@...utronix.de
Cc:	arjan@...radead.org, clm@...com, edumazet@...gle.com,
	fweisbec@...il.com, lenb@...nel.org, linux-kernel@...r.kernel.org,
	linux@...encehorizons.net, ltp@...ts.linux.it, mingo@...nel.org,
	paulmck@...ux.vnet.ibm.com, peterz@...radead.org, riel@...hat.com,
	rt@...utronix.de, torvalds@...ux-foundation.org,
	umgwanakikbuti@...il.com
Subject: Re: [LTP] [patch V2 00/20] timer: Refactor the timer wheel

Cyril Hrubis wrote:
> Thomas Gleixner wrote:
>> Err. You know that the timer expired because sigtimedwait() returns
>> EAGAIN. And the only thing you can reliably check for is that the timer did
>> not expired to early. Anything else is guesswork and voodoo programming.

> But seriously is there a reason
> why OS that is not under heavy load cannot expire timers with reasonable
> overruns? I.e. if I ask for a second of sleep and expect it to be woken
> up not much more than half of a second later?

> If we stick only to guarantees that are defined in POSIX playing music
> with mplayer would not be possible since it sleeps in futex() and if it
> wakes too late it will fail to fill buffers. In practice this worked
> fine for me for years.

Two points:
1) sigtimedwait() is unusual in that it uses the jiffies timer.  Most
   system call timeouts (including specifically the one in FUTEX_WAIT)
   use the high-resolution timer subsystem, which is a whole different
   animal with tighter guarantees, and
2) The worst-case error in tglx's proposal is 1/8 of the requested
   timeout: the wakeup is after 112.5% of the requested time, plus
   one tick.  This is well within your requested accuracy.  (For very
   short timeouts, the "plus one tick" can dominate the percentage error.)