From: Arjan van de Ven <arjan@infradead.org>
To: Shawn Bohrer <shawn.bohrer@gmail.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: High scheduler wake up times
Date: Sat, 30 Jan 2010 16:47:16 -0800 [thread overview]
Message-ID: <20100130164716.230dfe31@infradead.org> (raw)
In-Reply-To: <20100131003549.GC27390@mediacenter.gateway.2wire.net>
On Sat, 30 Jan 2010 18:35:49 -0600
Shawn Bohrer <shawn.bohrer@gmail.com> wrote:
\
>
> I agree that we are currently depending on a bug in epoll. The epoll
> implementation currently rounds up to the next jiffie, so specifying a
> timeout of 1 ms really just wakes the process up at the next timer
> tick. I have a patch to fix epoll by converting it to use
> schedule_hrtimeout_range() that I'll gladly send, but I still need a
> way to achieve the same thing.
it's not going to help you; your expectation is incorrect.
you CANNOT get 1000 iterations per second if you do
<wait 1 msec>
<do a bunch of work>
<wait 1 msec>
etc in a loop
the more accurate (read: not rounding down) the implementation, the
more not-1000 you will get, because to hit 1000 the two actions
<wait 1 msec>
<do a bunch of work>
combined are not allowed to take more than 1000 microseconds wallcock
time. Assuming "do a bunch of work" takes 100 microseconds, for you to
hit 1000 there would need to be 900 microseconds in a milliseconds...
and sadly physics don't work that way.
(and that's even ignoring various OS, CPU wakeup and scheduler
contention overheads)
--
Arjan van de Ven Intel Open Source Technology Centre
For development, discussion and tips for power savings,
visit http://www.lesswatts.org
next prev parent reply other threads:[~2010-01-31 0:46 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-01-30 23:45 High scheduler wake up times Shawn Bohrer
2010-01-30 23:56 ` Arjan van de Ven
2010-01-31 0:29 ` Shawn Bohrer
2010-01-31 0:02 ` Arjan van de Ven
2010-01-31 0:11 ` Arjan van de Ven
2010-01-31 0:35 ` Shawn Bohrer
2010-01-31 0:46 ` Shawn Bohrer
2010-01-31 0:47 ` Arjan van de Ven [this message]
2010-01-31 3:47 ` Shawn Bohrer
2010-01-31 18:28 ` Arjan van de Ven
2010-01-31 20:50 ` Shawn Bohrer
2010-02-01 8:51 ` Peter Zijlstra
2010-02-01 19:37 ` Shawn Bohrer
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