From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 29E5BC433B4 for ; Fri, 21 May 2021 11:53:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F22B96100C for ; Fri, 21 May 2021 11:53:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229610AbhEULzT (ORCPT ); Fri, 21 May 2021 07:55:19 -0400 Received: from mx2.suse.de ([195.135.220.15]:48546 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229519AbhEULzS (ORCPT ); Fri, 21 May 2021 07:55:18 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id CDC2EAAA6; Fri, 21 May 2021 11:53:54 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 923E71F2C73; Fri, 21 May 2021 13:53:54 +0200 (CEST) Date: Fri, 21 May 2021 13:53:54 +0200 From: Jan Kara To: Ming Lei Cc: Jan Kara , Jens Axboe , linux-block@vger.kernel.org, Khazhy Kumykov , Paolo Valente Subject: Re: [PATCH 1/2] block: Do not merge recursively in elv_attempt_insert_merge() Message-ID: <20210521115354.GJ18952@quack2.suse.cz> References: <20210520223353.11561-1-jack@suse.cz> <20210520223353.11561-2-jack@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri 21-05-21 08:42:16, Ming Lei wrote: > On Fri, May 21, 2021 at 12:33:52AM +0200, Jan Kara wrote: > > Most of the merging happens at bio level. There should not be much > > merging happening at request level anymore. Furthermore if we backmerged > > a request to the previous one, the chances to be able to merge the > > result to even previous request are slim - that could succeed only if > > requests were inserted in 2 1 3 order. Merging more requests in > > Right, but some workload has this kind of pattern. > > For example of qemu IO emulation, it often can be thought as single job, > native aio, direct io with high queue depth. IOs is originated from one VM, but > may be from multiple jobs in the VM, so bio merge may not hit much because of IO > emulation timing(virtio-scsi/blk's MQ, or IO can be interleaved from multiple > jobs via the SQ transport), but request merge can really make a difference, see > recent patch in the following link: > > https://lore.kernel.org/linux-block/3f61e939-d95a-1dd1-6870-e66795cfc1b1@suse.de/T/#t Oh, request merging definitely does make a difference. But the elevator hash & merge logic I'm modifying here is used only by BFQ and MQ-DEADLINE AFAICT. And these IO schedulers will already call blk_mq_sched_try_merge() from their \.bio_merge handler which gets called from blk_mq_submit_bio(). So all the merging that can happen in the code I remove should have already happened. Or am I missing something? Honza -- Jan Kara SUSE Labs, CR