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Message-Id: <1218903997.3665.6.camel@matrix>
Date: Sat, 16 Aug 2008 18:26:37 +0200
From: Stefani Seibold <stefani@...bold.net>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: SCHED_FIFO and SCHED_RR broken by cfs
I haved tried your suggestion on my 2.6.26 pentium notebook. Nothing is
changing.
After applying 'echo -1> /proc/sys/kernel/sched_rt_runtime_us' the
SCHED_FIFO jitter are still higher than with SCHED_OTHER.
Here are the results of my notebook
time chrt -f 99 /tmp/a.out time chrt -o 0 /tmp/a.out
average: 212 average: 13
min. jitter: 0 usec min. jitter: 0 usec
max. jitter: 50013 usec max. jitter: 33 usec
The kernel was startet with init=/bin/bash, so no other process is
running.
Thanx for supporting me.
Am Samstag, den 16.08.2008, 16:53 +0200 schrieb Peter Zijlstra:
> On Sat, 2008-08-16 at 11:55 +0200, Stefani Seibold wrote:
> > Hi kernel hackers,
> >
> > it seems that the new completely fair scheduler breaks the SCHED_RR and
> > SCHED_FIFO realtime scheduler.
> >
> > In my opinion a high priority real time user process with SCHED_FIFO
> > should be only interrupted by the kernel or a process with an higher
> > priority. So a user process running under SCHED_FIFO and priority 99
> > should never be interrupted by any other process. This was true under
> > kernel 2.6.20.
> >
> > On my pentium/celeron III/400 MHz system with kernel 2.6.20 a busy loop
> > using the "time stamp counter" of the x86 cpu for delaying, this was
> > very accurate. The max. jitter of the delaying was about 5 microseconds.
> >
> > With the new kernel 2.6.26 the jitter is about 51177 microseconds or in
> > other words 51 milliseconds or more the 10000 times greater than kernel
> > 2.6.20. This huge latency is far away from realtime.
> >
> > Below are the results of the attached test program. Maybe somebody else
> > can confirm this results. All measurements was done with no other
> > process running, only the busybox 1.11.1 shell and the init process was
> > there.
>
> Has nothing to do with CFS, but everything to do with the fact that we
> now have a 95% bandwidth control by default.
>
> Does doing:
>
> echo -1 > /proc/sys/kernel/sched_rt_runtime_us
>
> fix it?
>
> So, up to 95% cpu usage (per sched_rt_period_us) FIFO and RR behave like
> they always did, once they cross that line, they'll be throttled.
>
> 95% seemed like a sane default in that it leaves a little room to
> recover from a run-away rt process (esp handy now that !root users can
> also use RT scheduling classes), and should be enough for most
> applications as they usually don't consume all that much time.
>
>
>
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