Re: [patch] sched: accurate user accounting

From: Ingo Molnar
Date: Thu Jun 14 2007 - 17:19:24 EST

* malc <av1474@xxxxxxxx> 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 :-)

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