From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH] hpsa: scsi-mq support Date: Sat, 12 Nov 2016 09:32:23 -0800 Message-ID: <20161112173223.GA22158@infradead.org> References: <1478879194-32529-1-git-send-email-hare@suse.de> <20161111163532.GA14922@infradead.org> <39d1198d-eb49-71ed-9139-f0efd9eefd0d@suse.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from bombadil.infradead.org ([198.137.202.9]:50827 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966584AbcKLRcb (ORCPT ); Sat, 12 Nov 2016 12:32:31 -0500 Content-Disposition: inline In-Reply-To: <39d1198d-eb49-71ed-9139-f0efd9eefd0d@suse.com> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: Hannes Reinecke Cc: Christoph Hellwig , Hannes Reinecke , Christoph Hellwig , "Martin K. Petersen" , James Bottomley , Don Brace , linux-scsi@vger.kernel.org, Jens Axboe On Fri, Nov 11, 2016 at 07:22:05PM +0100, Hannes Reinecke wrote: > Well, it's the same as with megasas and mpt3sas. Each of those have > a single MMIO register where the driver writes the address of the > command into. What exactly the hardware does in the back doesn't > really matter here; the command is in memory and the hardware can > access it as it sees fit. So from that point of view we can assume > having a submission queue to match the completion queue; > With that setup we do have a contention point on the single command > register, but that's about it. > We still should benefit from scsi-mq, though. How do we benefit from scsi-mq in this case? We still hit global cachelines like commands_outstanding in the driver, and we lost the batching done by the ctx -> hw_ctx layering for the single queue blk-mq case. We also get much less efficient merging and will not have the chance of having and I/O schedule in the near future. But back to my question from the last mail: What workload is improved by using this patch?