Message-Id: <5.2.0.9.2.20030518103757.00ce93e8@pop.gmx.net>
Date: Sun, 18 May 2003 10:55:17 +0200
To: "David Schwartz" <davids@webmaster.com>
From: Mike Galbraith <efault@gmx.de>
Subject: RE: Scheduling problem with 2.4?
Cc: "Andrea Arcangeli" <andrea@suse.de>, <dak@gnu.org>,
       <linux-kernel@vger.kernel.org>
In-Reply-To: <MDEHLPKNGKAHNMBLJOLKIELBDAAA.davids@webmaster.com>
References: <20030517235048.GB1429@dualathlon.random>
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At 05:16 PM 5/17/2003 -0700, David Schwartz wrote:

> > I see what you mean, but I still don't think it is a problem. If
> > bandwidth matters you will have to use large writes and reads anyways,
> > if bandwidth doesn't matter the number of ctx switches doesn't matter
> > either and latency usually is way more important with small messages.
>
> > Andrea
>
>         This is the danger of pre-emption based upon dynamic priorities. 
> You can
>get cases where two processes each are permitted to make a very small amount
>of progress in alternation. This can happen just as well with large writes
>as small ones, the amount of data is irrelevent, it's the amount of CPU time
>that's important, or to put it another way, it's how far a process can get
>without suffering a context switch.
>
>         I suggest that a process be permitted to use up at least some 
> portion of
>its timeslice exempt from any pre-emption based solely on dynamic
>priorities.

Cool.

Thank you for the spiffy idea.  I implemented this in my (hack/chop) mm5 
tree in about 30 seconds, and it works just fine.  Very simple 
time_after(jiffies, p->last_run + MIN_TIMESLICE) checks in wake_up_cpu() 
plus the obvious dinky change to schedule(), and it worked 
instantly.  Hmm.. I wonder if this might help some of the other scheduler 
(seemingly) bad behavior as well.

Is there any down-side to not preempting quite as often?  It seems like 
there should be a bandwidth gain.

         -Mike 

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