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

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Message-ID: <20160623151127.GA20808@rei.lan>
Date:	Thu, 23 Jun 2016 17:11:27 +0200
From:	Cyril Hrubis <chrubis@...e.cz>
To:	George Spelvin <linux@...encehorizons.net>
Cc:	tglx@...utronix.de, arjan@...radead.org, clm@...com,
	edumazet@...gle.com, fweisbec@...il.com, lenb@...nel.org,
	linux-kernel@...r.kernel.org, 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

Hi!
> 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

That is likely POSIX conformance bug, since POSIX explicitly states that
sigtimedwait() shall use CLOCK_MONOTONIC to measure the timeout.

"If the Monotonic Clock option is supported, the CLOCK_MONOTONIC clock
shall be used to measure the time interval specified by the timeout
argument."

> 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.)

Hmm, that still does not add up to the number in the original email
where it says time_elapsed: 1.197057. As far as I can tell the worst
case for a tick is CONFIG_HZ=100 so one tick is 0.01s and even after
that we get 118.7% since we requested 1s. But that may be caused by the
fact that the test uses gettimeofday() to measure the elapsed time, it
should use CLOCK_MONOTONIC instead.

-- 
Cyril Hrubis
chrubis@...e.cz