linux-kernel - Re: realtime-preempt and arm

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Date:	Wed, 13 Dec 2006 23:21:26 -0800 (PST)
From:	tike64 <tike64@...oo.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: realtime-preempt and arm

Steven Rostedt <rostedt@...dmis.org> wrote:
> Also, have you tried this with a nanosleep instead of a select.
> Select's timeout is just that, a timeout. It's not suppose to be
> accurate, as long as it doesn't expire early.  The reason I state
> this, is that select uses a different mechanism than nanosleep, and
> that can indeed affect the jitter.

Ok, understood; I tried this:

	t = raw_timer();
	ts.tv_nsec = 5000000;
	ts.tv_sec = 0;
	nanosleep(&ts, 0);
	t = raw_timer() - t;

It is better but I still see 8ms occasional delays when listing
nfs-mounted directories onto FB. And, what is funny, also this version
makes the average delay 20ms as if it made the jiffy 20ms.

> Although without the high res enabled, you can't get better than
jiffy
> resolution, you shouldn't get a large jitter either.  BTW, using high
> res won't help the select anyway. The select uses a normal
> schedule_timeout, which means that it's not really expected to
> timeout, but something should wake it up before hand. Which means
that
> the good old timer wheel (non-hrtimer) is going to do the waking of
> the process. This means that you need to wait for the timer softirq
to
> be scheduled before your process wakes up. If there's a process with
a
> higher priority than the timer softirq running, then you need to
wait.
> 
> Using nansleep uses the hrtimer code (available with out the high
> resolutions).  The hrtimer uses its own timer softirq
> (softirq-hrtimer), and it is special.  It inherits the priority of
the
> task that created the timer when the timer goes off.  Also, something
> like nanosleep, won't even use the softirq, and will bypass the
> softirq all together, and wake your process up from the interrupt.
> 
> So basically, don't use select for timing.

Thanks a lot for a thorough explanation. While we are at it, is it then
the only option to use threads to wait for IO and use ms-accurate
timing? Formerly I have used select with timeouts for this task but
timing requirement have not been this accurate back then, of course.

--

tike

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