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Date: Tue, 18 Jun 2002 06:55:48 +0200 (CEST)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: Ingo Molnar <mingo@elte.hu>
To: David Schwartz <davids@webmaster.com>
Cc: mgix@mgix.com, <linux-kernel@vger.kernel.org>
Subject: Re: Question about sched_yield()
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On Mon, 17 Jun 2002, David Schwartz wrote:

> >I am seeing some strange linux scheduler behaviours,
> >and I thought this'd be the best place to ask.
> >
> >I have two processes, one that loops forever and
> >does nothing but calling sched_yield(), and the other
> >is basically benchmarking how fast it can compute
> >some long math calculation.

> [...] It is not a replacement for blocking or a scheduling priority
> adjustment. It simply lets other ready-to-run tasks be scheduled before
> returning to the current task.

and this is what the scheduler didnt do properly, it actually didnt
schedule valid ready-to-run processes for long milliseconds, it switched
between two sched_yield processes, starving the CPU-intensive process. I
posted a patch for 2.5 that fixes this, and the 2.4.19-pre10-ac2 backport
i did includes this fix as well:

    http://redhat.com/~mingo/O(1)-scheduler/sched-2.4.19-pre10-ac2-A4

a good sched_yield() implementation should give *all* other tasks a chance
to run, instead of switching between multiple sched_yield()-ing tasks. I
don think this is specified anywhere, but it's a quality of implementation
issue.

	Ingo

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