linux-kernel - Re: High scheduler wake up times

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Message-ID: <20100130164716.230dfe31@infradead.org>
Date:	Sat, 30 Jan 2010 16:47:16 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Shawn Bohrer <shawn.bohrer@...il.com>
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, 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

<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, 
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