From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754649AbbK0OaE (ORCPT ); Fri, 27 Nov 2015 09:30:04 -0500 Received: from mx2.suse.de ([195.135.220.15]:33804 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752164AbbK0OaB (ORCPT ); Fri, 27 Nov 2015 09:30:01 -0500 Message-ID: <565868E7.2010807@suse.de> Date: Fri, 27 Nov 2015 15:29:59 +0100 From: Hannes Reinecke User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 MIME-Version: 1.0 To: Ming Lei CC: Jens Axboe , Christoph Hellwig , "Martin K. Petersen" , Johannes Thumshirn , Linux Kernel , linux-nvme@lists.infradead.org, SCSI Mailing List Subject: Re: QUEUE_FLAG_NO_SG_MERGE and non-block-mq References: <5656BF25.3000407@suse.de> In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/26/2015 10:21 AM, Ming Lei wrote: > On Thu, Nov 26, 2015 at 4:13 PM, Hannes Reinecke wrote: >> Hi all, >> >> while investigating the crash in scsi_lib.c I found a rather curious >> behaviour for QUEUE_FLAG_NO_SG_MERGE. >> >> While the flag is evaluated in blk_recalc_rq_segments and >> blk_recount_segments (resulting in nr_phys_segments being >> computed based on that flag) it is completely ignored >> during blk_rq_map_sg() or the actual merging itself. > > Yes, I guess Jens introduced the flag for decreasing CPU > consumption when comuputing segments, but it is still > ignored by blk_rq_map_sg(), but it may not be used > by some drivers. > > After bio splitting is introduced, the flag is also ignored > when computing segments. > >> >> This typically shouldn't be an issue, seeing that with >> QUEUE_FLAG_NO_SG_MERGE nr_phys_segments will always be >> larger than the actual segment count. >> >> However, it still makes me wonder: >> What is the point of having a QUEUE_FLAG_NO_SG_MERGE >> which doesn't work as advertised? >> Or, to be precise, which only works for blk-mq? >> Should we make it work for non-block-mq, too? > > Thanks bio splitting, this flag has little effect on performance now, > so I think it can be removed if Jens has no objection. > As per your suggestion we've made some performance measurements, and 4k fio showed little if no impact: NO_SG_MERGE: IOPS R/W: 148097.7+-125.7 / 148124.1+-123.1 BW R/W: 592392.4+-502.7 / 592498.3+-492.3 SG_MERGE: IOPS R/W: 148054.4+-123.3 / 148082.6+-120.0 BW R/W: 592219.2+-493.5 / 592332.3+-479.7 So the performance benefit lies squarely within the error margin, making me wonder if it's worth bothering with having the NO_SG_MERGE flag at all. Thanks to Johannes for doing the measurements :-) Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@suse.de +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg)