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=-4.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 6EB26C56201 for ; Fri, 20 Nov 2020 17:27:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0CE8B2225B for ; Fri, 20 Nov 2020 17:27:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="Nsd+qdpX" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728587AbgKTR06 (ORCPT ); Fri, 20 Nov 2020 12:26:58 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59678 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728366AbgKTR05 (ORCPT ); Fri, 20 Nov 2020 12:26:57 -0500 Received: from mail-wm1-x343.google.com (mail-wm1-x343.google.com [IPv6:2a00:1450:4864:20::343]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2D368C0613CF; Fri, 20 Nov 2020 09:26:57 -0800 (PST) Received: by mail-wm1-x343.google.com with SMTP id a3so10485216wmb.5; Fri, 20 Nov 2020 09:26:57 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:from:to:cc:references:autocrypt:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=l35IrISFA/TkuzzKCjcWFPjo5RTIWN1CtrxKHY5P8hg=; b=Nsd+qdpXzZdy+EDwxskcdhga/+RpZhotCpILybcDR71RH09EoDYYvbsHQdrtfMSYot F9MFgEeO7FgJaHGDDgxokViEyRgP+o/XZOixnWZQPZFDBwbMZoRrJ3UgxW+6JhJzNVmp 2+52YTqUnwM8V5bAScPo4v7+RPvWh/QoRU5zv4pFKSu2l9the5GlbJDpi5Sh2ig5FECf ZWgSZVJb6EHpa9AsYtf8DA3AyUafquZiF//+V9fWtOp/KkMdh380J81/lRo4/OOyeIzC KIs/j+9OYC9xzCi4krC2Dq7HRxB0JoSbSfcuOelN8v8ck+v+UvcpTQHQNwrTENHQemHL rWOA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:from:to:cc:references:autocrypt :message-id:date:user-agent:mime-version:in-reply-to :content-language:content-transfer-encoding; bh=l35IrISFA/TkuzzKCjcWFPjo5RTIWN1CtrxKHY5P8hg=; b=gXtrYhtqWAbmIylIwXhaS3sLgox9+28rUv1YtNagZAy/Hmb6goETRTmbjI8COrYL3y Uz1VLPq7iP0ZimtrDkw0Ux6byquiuL1ZscAzpSS+eI+uIz5L3nkC2VhGliaWC+yUWPtB EJpRXx7fC2PYbxvR+//8qWwTKgzC8NKh3coxrTLeqkOGs3Kp0wCa86I4m75gFDIHwhU6 Z/2UB3BqUifZTn2Mmo8HnsHK4JwVLB+gRS+4MQu/BjWFUFCieTPPnlYuXT/CQplkEcW1 kSUxEPmUDhki0o9cUF7cHb8+kT7zCz8YU4M3oxTqf5w68QPtWGZXs4lYf5w7ikg5Ds8H oqQg== X-Gm-Message-State: AOAM531g0wv9A7ar7Iv6UW1B9i5b3ZMTLMdygROfU0T+mQYNz0XkLlwx yrB8vIV+urSGQ6zFs21U/B9iXq73ruKibA== X-Google-Smtp-Source: ABdhPJz5neR3wDbJo7caa+Mnr0nC5e4z1HxXy8XXXoLWCZeLSFTTc3EStaubn5uaO8O7SjM5+tjnnQ== X-Received: by 2002:a7b:ca43:: with SMTP id m3mr11221061wml.25.1605893215587; Fri, 20 Nov 2020 09:26:55 -0800 (PST) Received: from [192.168.1.31] (host109-152-100-189.range109-152.btcentralplus.com. [109.152.100.189]) by smtp.gmail.com with ESMTPSA id h15sm5872669wrw.15.2020.11.20.09.26.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Nov 2020 09:26:55 -0800 (PST) Subject: Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec From: Pavel Begunkov To: Ming Lei , Matthew Wilcox Cc: linux-fsdevel@vger.kernel.org, Alexander Viro , Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <20201120012017.GJ29991@casper.infradead.org> <35d5db17-f6f6-ec32-944e-5ecddcbcb0f1@gmail.com> <20201120014904.GK29991@casper.infradead.org> <3dc0b17d-b907-d829-bfec-eab96a6f4c30@gmail.com> <20201120020610.GL29991@casper.infradead.org> <20201120022426.GC333150@T590> <59329ec4-e894-e3ff-6f6e-7d89c34bebaf@gmail.com> Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Message-ID: <0326428b-19d2-4f82-a6b5-f7c7ad35d86e@gmail.com> Date: Fri, 20 Nov 2020 17:23:47 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <59329ec4-e894-e3ff-6f6e-7d89c34bebaf@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 20/11/2020 17:22, Pavel Begunkov wrote: > On 20/11/2020 02:24, Ming Lei wrote: >> On Fri, Nov 20, 2020 at 02:06:10AM +0000, Matthew Wilcox wrote: >>> On Fri, Nov 20, 2020 at 01:56:22AM +0000, Pavel Begunkov wrote: >>>> On 20/11/2020 01:49, Matthew Wilcox wrote: >>>>> On Fri, Nov 20, 2020 at 01:39:05AM +0000, Pavel Begunkov wrote: >>>>>> On 20/11/2020 01:20, Matthew Wilcox wrote: >>>>>>> On Thu, Nov 19, 2020 at 11:24:38PM +0000, Pavel Begunkov wrote: >>>>>>>> The block layer spends quite a while in iov_iter_npages(), but for the >>>>>>>> bvec case the number of pages is already known and stored in >>>>>>>> iter->nr_segs, so it can be returned immediately as an optimisation >>>>>>> >>>>>>> Er ... no, it doesn't. nr_segs is the number of bvecs. Each bvec can >>>>>>> store up to 4GB of contiguous physical memory. >>>>>> >>>>>> Ah, really, missed min() with PAGE_SIZE in bvec_iter_len(), then it's a >>>>>> stupid statement. Thanks! >>>>>> >>>>>> Are there many users of that? All these iterators are a huge burden, >>>>>> just to count one 4KB page in bvec it takes 2% of CPU time for me. >>>>> >>>>> __bio_try_merge_page() will create multipage BIOs, and that's >>>>> called from a number of places including >>>>> bio_try_merge_hw_seg(), bio_add_page(), and __bio_iov_iter_get_pages() >>>> >>>> I get it that there are a lot of places, more interesting how often >>>> it's actually triggered and if that's performance critical for anybody. >>>> Not like I'm going to change it, just out of curiosity, but bvec.h >>>> can be nicely optimised without it. >>> >>> Typically when you're allocating pages for the page cache, they'll get >>> allocated in order and then you'll read or write them in order, so yes, >>> it ends up triggering quite a lot. There was once a bug in the page >>> allocator which caused them to get allocated in reverse order and it >>> was a noticable performance hit (this was 15-20 years ago). >> >> hugepage use cases can benefit much from this way too. > > This didn't yield any considerable boost for me though. 1.5% -> 1.3% > for 1 page reads. I'll send it anyway though because there are cases > that can benefit, e.g. as Ming mentioned. And yeah, it just shifts my attention for optimisation to its callers, e.g. blkdev_direct_IO. > Ming would you want to send the patch yourself? After all you did post > it first. > -- Pavel Begunkov