linux-kernel - Re: High scheduler wake up times

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Date:	Sat, 30 Jan 2010 21:47:18 -0600
From:	Shawn Bohrer <shawn.bohrer@...il.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: High scheduler wake up times

On Sat, Jan 30, 2010 at 04:47:16PM -0800, Arjan van de Ven wrote:
> On Sat, 30 Jan 2010 18:35:49 -0600
> Shawn Bohrer <shawn.bohrer@...il.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

Of course that patch makes my situation worse, which was my point.  We
are depending on the _current_ epoll_wait() implementation which calls
schedule_timeout(1).  You do agree that the current epoll_wait()
implementation sleeps less than 1 msec with HZ == 1000 correct?  So as
long as:

work + scheduling_overhead < 1 msec

We _should_ be able to achieve 1000 iterations per second.  I also
realize that with multiple worker processes I need:

(total_work + scheduling_overhead)/number_cpus < 1 msec

With the old kernel I can run 500 of these processes, and I'm hoping
that I'm simply missing the knob I need to tweak to achieve similar
performance on a recent kernel.

Thanks,
Shawn
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