linux-kernel - Re: Sched_autogroup and niced processes

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Date:	Fri, 13 May 2011 10:05:50 +0200
From:	Mike Galbraith <efault@....de>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	Carl-Johan Kjellander <carl-johan@...rna.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Sched_autogroup and niced processes

On Fri, 2011-05-13 at 15:53 +0800, Yong Zhang wrote:
> Cc'ing more people.
> 
> On Fri, May 13, 2011 at 3:39 PM, Carl-Johan Kjellander
> <carl-johan@...rna.com> wrote:
> > I've been running seti@...e niced to 19 in the background since 1999
> > without any problems. No noticeable effect even when playing a movie
> > or a game. But since 2.6.38 the new fix-all-problems automatic
> > grouping has been messing a bit with me. These are some timed compiles
> > on my 8 cores.

Heh, it's not a fix-all-problems thingy, and was never intended to be.
It's also not enabled by default.

> > time make -j12 # with seti@...e running
> > real    4m16.753s
> > user    10m33.770s
> > sys     1m39.710s
> >
> > time make -j12 # without seti@...e running
> > real    2m12.480s
> > user    10m11.580s
> > sys     1m39.980s
> >
> > echo 0 > /proc/sys/kernel/sched_autogroup_enabled
> > time make -j12 # no autogroup, seti@...e running again
> > real    2m33.276s
> > user    10m37.540s
> > sys     1m43.190s
> >
> > All compiles already had all files cached in RAM.
> >
> > Now I can take the 10% performance hit, but not the 100% hit of
> > running stuff super niced in the background. Processes niced to 19
> > should only use spare cycles and not take up half of the cores even
> > with autogroup. I would really like to run autogroup since it is a
> > neat idea, but it can't mess up running niced processes in the
> > background which have been working fine for 12 years.
> 
> Then how about change the nice value of seti@...e->autogroup?
> echo 19 > /proc/'pid of seti@...e'/autogroup

Yup.  Overhead and whatnot is the dark side of group scheduling.  The
thing to do is to turn group scheduling off if you don't like what it
does for/to you.

	-Mike

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