Message-Id: <5.2.1.1.2.20050115080420.00bf2c90@pop.gmx.net>
Date: Sat, 15 Jan 2005 09:06:14 +0100
To: "Jack O'Quin" <joq@io.com>
From: Mike Galbraith <efault@gmx.de>
Subject: Re: [PATCH] [request for inclusion] Realtime LSM
Cc: Arjan van de Ven <arjanv@redhat.com>, Lee Revell <rlrevell@joe-job.com>,
       Chris Wright <chrisw@osdl.org>, Paul Davis <paul@linuxaudiosystems.com>,
       Matt Mackall <mpm@selenic.com>, Christoph Hellwig <hch@infradead.org>,
       Andrew Morton <akpm@osdl.org>, mingo@elte.hu, alan@lxorguk.ukuu.org.uk,
       linux-kernel@vger.kernel.org, Con Kolivas <kernel@kolivas.org>
In-Reply-To: <87llav7atz.fsf@sulphur.joq.us>
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 <87y8ezzake.fsf@sulphur.joq.us>
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At 07:14 PM 1/14/2005 -0600, Jack O'Quin wrote:
>Mike Galbraith <efault@gmx.de> writes:
>
> > At 05:31 PM 1/13/2005 -0600, Jack O'Quin wrote:
> >>Yes.  However, my tests have so far shown a need for "actual FIFO as
> >>long as the task behaves itself."
> >
> > I for one wonder why that appears to be so.  What happens if you use
> > SCHED_RR instead of SCHED_FIFO?
> >
> > (ie is the problem just one of running out of slice at a bad time, or
> > is it the dynamic priority adjustment)
>
>I have no quick and easy test for that.
>
>If it's important, I can modify a version of JACK to use SCHED_RR,
>instead.

I think the problem you're seeing is strange enough to consider trying the 
(possibly odd sounding) test.  I haven't seen an explanation of why nice 
-20 doesn't work for you.

>I very much doubt it would make any difference, since we normally only
>run one realtime thread at a time.  Each client taps the next on the
>shoulder when it is time for it to run, so there is essentially no
>concurrency among them.

It may not make any difference.  Seeing that would at least be an 
additional datapoint.  The only significant difference I see between a 
gaggle of SCHED_FIFO tasks and one of nice -20 tasks, who are alone in 
their top-of-the-heap queue, and who are not cpu hogs, is the timeslice.  I 
don't recall there being any wakeup/preempt logic differences, ergo the 
SCHED_RR suggestion.

         -Mike 

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