From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: PATCH [5/15] qla2xxx: SG tablesize update Date: Sun, 14 Mar 2004 16:15:27 +0100 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <20040314151527.GM6955@suse.de> References: <20040314082444.GA3416@linux.local.home> <1079275768.2022.1.camel@mulgrave> <20040314145142.GL6955@suse.de> <1079276396.2022.8.camel@mulgrave> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from ns.virtualhost.dk ([195.184.98.160]:50916 "EHLO virtualhost.dk") by vger.kernel.org with ESMTP id S261980AbUCNPQJ (ORCPT ); Sun, 14 Mar 2004 10:16:09 -0500 Content-Disposition: inline In-Reply-To: <1079276396.2022.8.camel@mulgrave> List-Id: linux-scsi@vger.kernel.org To: James Bottomley Cc: Andrew Vasquez , SCSI Mailing List On Sun, Mar 14 2004, James Bottomley wrote: > On Sun, 2004-03-14 at 09:51, Jens Axboe wrote: > > The explanation of why this was done (request queue limitations, huh?) > > sounded pretty bogus as well. > > Actually, to be fair, I think he means the qlogic request queue, not the > block one. > > The Qla chips are rather weird in that they have a single issue queue > (whose size you can vary) but whose entry formats are fixed. If I > remember correctly, an initial command can have 4 SG elements, but a > follow on entry can have 7 (not sure of the figures). But anyway, large > SG commands end up having to find multiple entries in this queue (and > being a single issue queue for the entire card, it has to be mutexed > while you search). The more resources you need, the more difficult the > search and the more contention you generate on the resource mutex. Ah, makes a lot more sense now, thanks :-) -- Jens Axboe