Date: Thu, 21 Mar 1996 07:53:14 -0800 From: "Justin T. Gibbs" <gibbs@freefall.freebsd.org> 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 Message-ID: <199603211553.HAA09602@freefall.freebsd.org> In-Reply-To: Your message of "Thu, 21 Mar 1996 16:16:21 %2B0100." <199603211516.AA19099@Sisyphos>
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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 ===========================================
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