linux-kernel - Re: [RFC patch 1/2] sched: dynamically adapt granularity with nr

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Message-ID: <1284392368.17152.44.camel@gandalf.stny.rr.com>
Date:	Mon, 13 Sep 2010 11:39:28 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Lindgren <tony@...mide.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC patch 1/2] sched: dynamically adapt granularity with
 nr_running

On Mon, 2010-09-13 at 11:25 -0400, Mathieu Desnoyers wrote:

> > > The more divs we put it, the more expensive it all becomes.
> > > 
> > 
> > What about:
> > 
> > 	if (delta_exec < sysctl_sched_min_granularity ||
> > 	    delta_exec < __sched_gran(cfs_rq->nr_running))
> > 
> > This way we avoid the div when delta_exec is less than the
> > min_granularity, and then we can do the div to perhaps avoid a needless
> > resched?
> 
> Please note that my patch does not even take the 32-bit div when there are less
> than 4 threads running on the system.
> 
> So I'm not at all sure we're trying to optimize an important case here.

If we have 5 threads running and hit this where delta_exec is <
sched_min_granularity, then we do have that div, right? Question is, how
often do we hit this condition where delta_exec is less while running
more that 4 threads?

-- Steve


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