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To: se@zpr.uni-koeln.de (Stefan Esser)
cc: FreeBSD-scsi@freefall.freebsd.org, Ulrich.Windl@rz.uni-regensburg.de
Subject: Re: Simple Tagged Commands and Starvation 
In-reply-to: Your message of "Thu, 21 Mar 1996 16:16:21 +0100."
             <199603211516.AA19099@Sisyphos> 
Date: Thu, 21 Mar 1996 07:53:14 -0800
From: "Justin T. Gibbs" <gibbs@freefall.freebsd.org>
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>On Mar 20, 22:28, "Justin T. Gibbs" wrote:
>} Subject: Simple Tagged Commands and Starvation
>} I would like to hear other's opinions on this subject and possible ways to
>} avoid the problem without arbitrarily penalizing commands entered into the
>} queue.  The problem may be agravated in Linux by using a large number of
>} tags and a short timeout (Ulrich, FreeBSD uses a 10sec timeout and the two
>} drivers that use tagged queueing default to 4 tags max per target which
>} seems to offer the best performance under various benchmarks -- I have no
>} idea what Linux uses).  Since we are planning to push the dictation of
>} queue message types up into the generic SCSI layer, it may turn out that we
>} can solve this problem solely by enforcing ordered tags for syncronous
>} writes.  I'm also wading through the SCSI spec again in the hopes of an
>} alternate way out, but it doesn't look good.
>
>I've received mail from Gerard Roudier 
>(who ported the FreeBSD NCR driver to 
>Linux) on this topic, a few weeks ago, 
>and he suggested a fix.

This is the same "fix" the Linux Buslogic driver uses and I really
don't like it since it's extra overhead and may be totally unecessary
for everything but bonnie test runs with a large number of tags
(probably larger than the four we're using).  Perhaps a "soft" timeout
at 5 seconds that causes the next queued command to that target to
use an ordered tag would be better.

>Regards, STefan
>-- 
> Stefan Esser, Zentrum fuer Paralleles Rechnen		Tel:	+49 221 4706021
> Universitaet zu Koeln, Weyertal 80, 50931 Koeln	FAX:	+49 221 4705160
> =============================================================================
>=
> http://www.zpr.uni-koeln.de/~se			  <se@ZPR.Uni-Koeln.DE>

--
Justin T. Gibbs
===========================================
  FreeBSD: Turning PCs into workstations
===========================================