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=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 359AEC47247 for ; Sat, 9 May 2020 09:43:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0F8C3217BA for ; Sat, 9 May 2020 09:43:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="PAfZrfR3" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728146AbgEIJn3 (ORCPT ); Sat, 9 May 2020 05:43:29 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:21345 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727867AbgEIJn1 (ORCPT ); Sat, 9 May 2020 05:43:27 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1589017406; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=l8D8Ae345rAYDqawjkcSExOxW2YprJe5XqaST1kNQ6s=; b=PAfZrfR31L6LmVFMx1IzzHBXCi0T+ddQIG0EBnys+jasXqD87xlb5fFglZZGITpE3RvT6E MlQLl/lBIxRayuZrbkZUa/6ibZAQvjtmT6gyLXn7J7LxBhL3KcSMxPJmHcDNPylsbVEYTj Yz0jKeJvWCMmuc2Ei55idL29FV0G+Yg= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-386-rAyD8qh5NLyXbbJ-2MeO7Q-1; Sat, 09 May 2020 05:43:22 -0400 X-MC-Unique: rAyD8qh5NLyXbbJ-2MeO7Q-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3763C107ACCA; Sat, 9 May 2020 09:43:20 +0000 (UTC) Received: from T590 (ovpn-12-90.pek2.redhat.com [10.72.12.90]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 15F1D5D9CD; Sat, 9 May 2020 09:43:10 +0000 (UTC) Date: Sat, 9 May 2020 17:43:06 +0800 From: Ming Lei To: Baolin Wang Cc: Sagi Grimberg , Christoph Hellwig , axboe@kernel.dk, Ulf Hansson , Adrian Hunter , Arnd Bergmann , Linus Walleij , Paolo Valente , Orson Zhai , Chunyan Zhang , linux-mmc , linux-block , LKML Subject: Re: [RFC PATCH v2 1/7] block: Extand commit_rqs() to do batch processing Message-ID: <20200509094306.GA1414369@T590> References: <20200427154645.GA1201@infradead.org> <20200508214639.GA1389136@T590> <20200508232222.GA1391368@T590> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Sat, May 09, 2020 at 04:57:48PM +0800, Baolin Wang wrote: > On Sat, May 9, 2020 at 7:22 AM Ming Lei wrote: > > > > Hi Sagi, > > > > On Fri, May 08, 2020 at 03:19:45PM -0700, Sagi Grimberg wrote: > > > Hey Ming, > > > > > > > > Would it make sense to elevate this flag to a request_queue flag > > > > > (QUEUE_FLAG_ALWAYS_COMMIT)? > > > > > > > > request queue flag usually is writable, however this case just needs > > > > one read-only flag, so I think it may be better to make it as > > > > tagset/hctx flag. > > > > > > I actually intended it to be writable. > > > > > > > > I'm thinking of a possibility that an I/O scheduler may be used > > > > > to activate this functionality rather than having the driver set > > > > > it necessarily... > > > > > > > > Could you explain a bit why I/O scheduler should activate this > > > > functionality? > > > > > > Sure, I've recently seen some academic work showing the benefits > > > of batching in tcp/ip based block drivers. The problem with the > > > approaches taken is that I/O scheduling is exercised deep down in the > > > driver, which is not the direction I'd like to go if we are want > > > to adopt some of the batching concepts. > > > > > > I spent some (limited) time thinking about this, and it seems to > > > me that there is an opportunity to implement this as a dedicated > > > I/O scheduler, and tie it to driver specific LLD stack optimizations > > > (net-stack for example) relying on the commit_rq/bd->last hints. > > > > > > When scanning the scheduler code, I noticed exactly the phenomenon that > > > this patchset is attempting to solve and Christoph referred me to it. > > > Now I'm thinking if we can extend this batching optimization for both > > > use-cases. > > > > Got it, thanks for the sharing. > > > > > > > > > batching submission may be good for some drivers, and currently > > > > we only do it in limited way. One reason is that there is extra > > > > cost for full batching submission, such as this patch requires > > > > one extra .commit_rqs() for each dispatch, and lock is often needed > > > > in this callback. > > > > > > That is not necessarily the case at all. > > > > So far, all in-tree .commit_rqs() implementation requires lock. > > > > > > > > > IMO it can be a win for some slow driver or device, but may cause > > > > a little performance drop for fast driver/device especially in workload > > > > of not-batching submission. > > > > > > You're mostly correct. This is exactly why an I/O scheduler may be > > > applicable here IMO. Mostly because I/O schedulers tend to optimize for > > > something specific and always present tradeoffs. Users need to > > > understand what they are optimizing for. > > > > > > Hence I'd say this functionality can definitely be available to an I/O > > > scheduler should one exist. > > > > > > > I guess it is just that there can be multiple requests available from > > scheduler queue. Actually it can be so for other non-nvme drivers in > > case of none, such as SCSI. > > > > Another way is to use one per-task list(such as plug list) to hold the > > requests for dispatch, then every drivers may see real .last flag, so they > > may get chance for optimizing batch queuing. I will think about the > > idea further and see if it is really doable. > > How about my RFC v1 patch set[1], which allows dispatching more than > one request from the scheduler to support batch requests? > > [1] > https://lore.kernel.org/patchwork/patch/1210034/ > https://lore.kernel.org/patchwork/patch/1210035/ Basically, my idea is to dequeue request one by one, and for each dequeued request: - we try to get a budget and driver tag, if both succeed, add the request to one per-task list which can be stored in stack variable, then continue to dequeue more request - if either budget or driver tag can't be allocated for this request, marks the last request in the per-task list as .last, and send the batching requests stored in the list to LLD - when queueing batching requests to LLD, if one request isn't queued to driver successfully, calling .commit_rqs() like before, meantime adding the remained requests in the per-task list back to scheduler queue or hctx->dispatch. One issue is that this way might degrade sequential IO performance if the LLD just tells queue busy to blk-mq via return value of .queue_rq(), so I guess we still may need one flag, such as BLK_MQ_F_BATCHING_SUBMISSION. thanks, Ming