linux-kernel - Re: [patch 6/9] signalfd/timerfd v1

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Message-ID: <Pine.LNX.4.64.0703102132240.10330@woody.linux-foundation.org>
Date:	Sat, 10 Mar 2007 21:35:54 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davide Libenzi <davidel@...ilserver.org>
cc:	Nicholas Miell <nmiell@...cast.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 6/9] signalfd/timerfd v1 - timerfd core ...

On Sat, 10 Mar 2007, Davide Libenzi wrote:

> On Sat, 10 Mar 2007, Linus Torvalds wrote:
> 
> > (That said, using "struct itimerspec" might be a good idea. That would 
> > also obviate the need for TFD_TIMER_SEQ, since an itimerspec automatically 
> > has both "base" and "incremental" parts).
> 
> But TFD_TIMER_SEQ is a simple auto-rearm case of TFD_TIMER_REL. So the 
> timespec is sufficent too (in all three cases we just need *one* time). 

Well, people actually do use itimers like "give me a timer every second, 
starting five seconds from now".

> Actually, the only place where I can find the itimerspec usefull, is 
> indeed with TFD_TIMER_SEQ. In cases where you want you clock starting at a 
> given time (it_value) *and* with the given frequency (it_interval).

.. and this is where itimerspec is even better: once you have absolute 
time, *and* a process that might miss ticks (because it does something 
else), the "absolute time start + interval" thing can avoid drifting 
(which a "relative interval" has a really hard time doing).

So if you want a "timer tick every second, *on* the second" kind of 
interface, you really do want a absolute time starting point, and then a 
fixed interval. Two different times.

		Linus
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