linux-kernel - Re: [patch] sched: accurate user accounting

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Date:	Thu, 14 Jun 2007 23:18:28 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	malc <av1474@...tv.ru>
Cc:	linux-kernel@...r.kernel.org,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [patch] sched: accurate user accounting


* malc <av1474@...tv.ru> wrote:

> > the alternating balancing might be due to an uneven number of tasks 
> > perhaps? If you have 3 tasks on 2 cores then there's no other 
> > solution to achieve even performance of each task but to rotate them 
> > amongst the cores.
> 
> One task, one thread. I have also tried to watch fairly demanding 
> video (Elephants Dream in 1920x1080/MPEG4) with mplayer, and CFS moves 
> the only task between cores almost every second.

hm, mplayer is not running alone when it does video playback: Xorg is 
also pretty active. Furthermore, the task you are using to monitor 
mplayer counts too. The Core2Duo has a shared L2 cache between cores, so 
it is pretty cheap to move tasks between the cores.

> > well, precise/finegrained accounting patches have been available for 
> > years, the thing with CFS is that there we get them 'for free', 
> > because CFS needs those metrics for its own logic. That's why this 
> > information is much closer to reality now. But note: right now what 
> > is affected by the changes in the CFS patches is /proc/PID/stat 
> > (i.e. the per-task information that 'top' and 'ps' displays, _not_ 
> > /proc/stat) - but more accurate /proc/stat could certainly come 
> > later on too.
> 
> Aha. I see, it's just that integral load for hog is vastly improved 
> compared to vanilla 2.6.21 [...]

hm, which ones are improved? Could this be due to some other property of 
CFS? If your app relies on /proc/stat then there's no extra precision in 
those cpustat values yet.

i've Cc:-ed Balbir Singh and Dmitry Adamushko who are the main authors 
of the current precise accounting code in CFS. Maybe i missed some 
detail :-)

	Ingo
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