Re: Problem with the O(1) scheduler in 2.4.19

From: Mark Mielke (mark@mark.mielke.cc)
Date: Tue Sep 03 2002 - 10:58:37 EST

I wonder if it does not make sense to just give the process real time
priority? No scheduler will be excellent in all situations. I would not
consider a game, or game server, to be a standard application.

mark

On Tue, Sep 03, 2002 at 02:23:49PM +0200, Tobias Ringstrom wrote:
> On Tue, 3 Sep 2002, Ingo Molnar wrote:
>
> > do you expect a task that uses up 50% CPU time over an extended period of
> > time to be rated 'interactive'?
>
> Interactive is not the best word, but I would not expect a process like
> the one I described to be considedred a CPU hog. It's a deadline driven
> semi realtime process.
>
> > we might make the '50%' rule to be '100% / nr_running_avg', so that if
> > your task is the only one in the system then it gets rated interactive -
> > but i suspect it will still be rated a CPU hog if it keeps trying to use
> > up 50% of CPU time even during busier periods. I have tried the
> > (1/nr_running) rule in earlier incarnations of the scheduler, and it didnt
> > make much difference, but we obviously need a boundary case like yours to
> > see the differences.
>
> I think the problem I have (that I loose a lot of performance to processes
> such as crond, httpd, etc.) is common to the whole class of semi-realtime
> processes, at least if they use >50% CPU. This means that CPU intensive
> audio and video (e.g. DVD) playback programs might have the same problem.
>
> I see three simple ways to solve the problem without changing the
> scheduler. Either run the process with nice -20, use SCHED_RR, or use a
> dedicated server with no other processes (such as crond, httpd, etc).
> The first two might be OK, but you need root privilegies to run renice and
> to change the scheduler policy. The third one is not an option for all
> users, and definately not for the video playback case.
>
> A problem is that this new scheduler behaviour will hit people running
> semi realtime processes as a regression when they switch to 2.6. It would
> be nice to avoid that.
>
> One solution might be to teach the scheduler how to detect these deadline
> driven semi-realtime processes, and not punish them. It is not obvious to
> me how to do that.
>
> Another much simpler solution that might work just as well is be to change
> the CPU utilization threshold from 50% to 90%.
>
> You're the expert of course. I'm only fumbling in the dark... :-)
>
> > (it could in theory make a difference in some rare cases, in which the
> > frequency of sampling resonates with internal timings of the application -
> > i asked for this only to make sure there are no interactions.)
>
> I'll try it out and let you know if it does make a difference.
>
> /Tobias
>
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