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 D8855CA9EB5 for ; Mon, 4 Nov 2019 18:42:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B2762214D8 for ; Mon, 4 Nov 2019 18:42:21 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="pjIabtGE" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729312AbfKDSmV (ORCPT ); Mon, 4 Nov 2019 13:42:21 -0500 Received: from mail-qt1-f196.google.com ([209.85.160.196]:43271 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728377AbfKDSmU (ORCPT ); Mon, 4 Nov 2019 13:42:20 -0500 Received: by mail-qt1-f196.google.com with SMTP id l24so7160299qtp.10; Mon, 04 Nov 2019 10:42:20 -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=sEseK7rUIua31dfTM0MrVVdwejEK2arCI2kack4UXRI=; b=pjIabtGEseDUarzOCYLgDCwRqkYJ36wuaijAJoBjBphHPc85mt3DmboOrn9rKkcr1j kgVZDjk78rrYEsxp4aLBvLZq3VL23R0e99WaASg4+ILDyuL97CDSwA/TQtt2omjv8VDJ jVE8kcZ1QVuZ1JCwa7d+v4GaBcaBGdOU2SPYCGsyhqVYCZWALopMhR3Qb0eSr+209dtT UduEWyJPa82ykfbZnlKIl7OnfqwJq/mtHG7p+TcP5vcKRMzOV+yK/NKOUa1p6XlvgwgS JhKvN6PTawRrIm6x59yDmiON+oFfHIe/Joc3j1UKTeAC99qdUSHvaoyb6rb9bqRjIeZR +pcw== 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=sEseK7rUIua31dfTM0MrVVdwejEK2arCI2kack4UXRI=; b=ZpRlj3ABMuIWmdCbGTr3oqIF7HZXANQdJKGoI5ti6nQVA9wgN7R0eYGp71ek+foO8M hh03UDZi1nbZYSwQQEo3bYw4DOT/aOt7NLS7Z3q/cKc0NbWae/S57Z2K2zb+yh1HKDjo AhxTCvcO4qx0Ytl5y6VUyHAisEywwX06idlaVy45dqaIz6VR5a8NYA+U+J685kKegUo9 swuK2c2XhGoEfXPFYxq1RIwwh9WaOM0YVpbU7YswIIgi1eKyV91wHsDhRCB5S/1jv9YC U9OKUrtz20IkvlZan7p0qV747z3ksFH2FIqdIXQT/vxj6op64/2nAEj9gr/Lj6DzD2K7 Ab6Q== X-Gm-Message-State: APjAAAUWSghEPN7g0ZSFZXiCopuggEVvQpMeOz39Cl1mi9UJHasbs06h d89rGBTGGevi4wfQ44SPVA== X-Google-Smtp-Source: APXvYqwIcUUUIML523nErBcTIsdXb3cbr2Y/8+66qVi3T+zFkQ+CsjoNxFPmZp/OYNfo/SEAVw83Lg== X-Received: by 2002:ac8:2fba:: with SMTP id l55mr14080384qta.167.1572892939748; Mon, 04 Nov 2019 10:42:19 -0800 (PST) Received: from kmo-pixel ([65.183.151.50]) by smtp.gmail.com with ESMTPSA id i4sm7264030qtp.57.2019.11.04.10.42.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 04 Nov 2019 10:42:19 -0800 (PST) Date: Mon, 4 Nov 2019 13:42:17 -0500 From: Kent Overstreet To: Jens Axboe Cc: Christoph Hellwig , Ming Lei , 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: <20191104184217.GD8984@kmo-pixel> References: <20191102072911.24817-1-ming.lei@redhat.com> <20191104181403.GA8984@kmo-pixel> <20191104181541.GA21116@infradead.org> <20191104181742.GC8984@kmo-pixel> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.12.2 (2019-09-21) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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.