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=-2.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 924CDCA9ED3 for ; Tue, 5 Nov 2019 02:11:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E5EC217F5 for ; Tue, 5 Nov 2019 02:11:36 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="VJZT7qBE" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729524AbfKECLf (ORCPT ); Mon, 4 Nov 2019 21:11:35 -0500 Received: from mail-qt1-f196.google.com ([209.85.160.196]:46222 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728987AbfKECLf (ORCPT ); Mon, 4 Nov 2019 21:11:35 -0500 Received: by mail-qt1-f196.google.com with SMTP id u22so27126600qtq.13; Mon, 04 Nov 2019 18:11:35 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=W0+7k1DRafJ+r2YfPcqo1EdMvigU/iVfY/W/noJefnI=; b=VJZT7qBEe0DMw1OYRW8Nbq7dkKMRGPSE+deEt5uexE8da96FsaX3djNlEquJFmw4zz Rm33SODNu8MpxIA2lRQB2IaHO3hk0FrSlTCZJHv1talWB+jWlhB3VJcklrm4fbDxO9ig 4dLOFvIPnMr2BjXbrK4u2ivTvAzvVilCKlBg1qeJgFLy+H37xi216SxmUvbjg0FmXjYo AGwBnw3k/Y58TPzfFBXq3l31WjtR9d3MoSIz/Nw6X1A8byaSbGp79MMgFkuPQsVzhrYy PqPFYAQ83wZ/+2p9BLhkYFu380iMG8r8m93yUwtj7KaXWx2AcAaLlihEQhQBHZRMKQJs SseQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=W0+7k1DRafJ+r2YfPcqo1EdMvigU/iVfY/W/noJefnI=; b=sYji6e8qp2bMql+ezSeOtAU0+msicRGT4MMhecHr/Lth3o0HFqNcrEXSiVlDKfBqHv RZLsvxVJgGW/0Qm2EhVelZGCBra1MNbtmqkuEOcIpjuGiWH2F2BIQEiO8cy2fvLFnFdR 3N4UlNKVrl4rncluAjjqEpRVJQzyAEZ4lxocGqYcmsKrQjYnIJxwhl3O7mp3sWuaXYpj cmmPcPvWzMY2IwHz0wS8Q0r5yjz4x7fbukTE9Ykqcncn5lsAKAFylgxHLxZj1a/ZDvMj 4oP4GRjW4NnFdxKBXbzg1gVUx9Xtwr+4ZONEMAg1EzQp6+2M9IvgzBB7omtXh42MmWOP qgJA== X-Gm-Message-State: APjAAAVoucxMO/blHTDifN88rpJTxZPmdrzayQdQEJvqmw+3ZRy5KGZ3 9H4h99F7j2d4xbngAwyRlg== X-Google-Smtp-Source: APXvYqyAHboqhEcn3AO/tqBd4xAJPyzUQF6B/TNyR/OU2id4uWSEPC5Q/L/Cp+4kXW79nZcRKjuR/A== X-Received: by 2002:ac8:89c:: with SMTP id v28mr15558195qth.156.1572919894330; Mon, 04 Nov 2019 18:11:34 -0800 (PST) Received: from moria.home.lan ([2601:19b:c500:a1:7285:c2ff:fed5:c918]) by smtp.gmail.com with ESMTPSA id 189sm9896682qki.10.2019.11.04.18.11.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 04 Nov 2019 18:11:32 -0800 (PST) Date: Mon, 4 Nov 2019 21:11:30 -0500 From: Kent Overstreet To: Ming Lei Cc: Jens Axboe , Christoph Hellwig , linux-block@vger.kernel.org, Coly Li , Keith Busch , linux-bcache@vger.kernel.org Subject: Re: [PATCH V4] block: optimize for small block size IO Message-ID: <20191105021130.GB18564@moria.home.lan> References: <20191102072911.24817-1-ming.lei@redhat.com> <20191104181403.GA8984@kmo-pixel> <20191104181541.GA21116@infradead.org> <20191104181742.GC8984@kmo-pixel> <20191104184217.GD8984@kmo-pixel> <20191105011135.GD11436@ming.t460p> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191105011135.GD11436@ming.t460p> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue, Nov 05, 2019 at 09:11:35AM +0800, Ming Lei wrote: > On Mon, Nov 04, 2019 at 01:42:17PM -0500, Kent Overstreet wrote: > > On Mon, Nov 04, 2019 at 11:23:42AM -0700, Jens Axboe wrote: > > > On 11/4/19 11:17 AM, Kent Overstreet wrote: > > > > On Mon, Nov 04, 2019 at 10:15:41AM -0800, Christoph Hellwig wrote: > > > >> On Mon, Nov 04, 2019 at 01:14:03PM -0500, Kent Overstreet wrote: > > > >>> On Sat, Nov 02, 2019 at 03:29:11PM +0800, Ming Lei wrote: > > > >>>> __blk_queue_split() may be a bit heavy for small block size(such as > > > >>>> 512B, or 4KB) IO, so introduce one flag to decide if this bio includes > > > >>>> multiple page. And only consider to try splitting this bio in case > > > >>>> that the multiple page flag is set. > > > >>> > > > >>> So, back in the day I had an alternative approach in mind: get rid of > > > >>> blk_queue_split entirely, by pushing splitting down to the request layer - when > > > >>> we map the bio/request to sgl, just have it map as much as will fit in the sgl > > > >>> and if it doesn't entirely fit bump bi_remaining and leave it on the request > > > >>> queue. > > > >>> > > > >>> This would mean there'd be no need for counting segments at all, and would cut a > > > >>> fair amount of code out of the io path. > > > >> > > > >> I thought about that to, but it will take a lot more effort. Mostly > > > >> because md/dm heavily rely on splitting as well. I still think it is > > > >> worthwhile, it will just take a significant amount of time and we > > > >> should have the quick improvement now. > > > > > > > > We can do it one driver at a time - driver sets a flag to disable > > > > blk_queue_split(). Obvious one to do first would be nvme since that's where it > > > > shows up the most. > > > > > > > > And md/md do splitting internally, but I'm not so sure they need > > > > blk_queue_split(). > > > > > > I'm a big proponent of doing something like that instead, but it is a > > > lot of work. I absolutely hate the splitting we're doing now, even > > > though the original "let's work as hard as we add add page time to get > > > things right" was pretty abysmal as well. > > > > Last I looked I don't think it was going to be that bad, just needed a bit of > > finesse. We just need to be able to partially process a request in e.g. > > nvme_map_data(), and blk_rq_map_sg() needs to be modified to only map as much as > > will fit instead of popping an assertion. > > I think it may not be doable. > > blk_rq_map_sg() is called by drivers and has to work on single request, however > more requests have to be involved if we delay the splitting to blk_rq_map_sg(). > Cause splitting means that two bios can't be submitted in single IO request. Of course it's doable, do I have to show you how?