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=-3.8 required=3.0 tests=BAYES_00, 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 47B2AC433E0 for ; Tue, 12 Jan 2021 14:08:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DE95B23102 for ; Tue, 12 Jan 2021 14:08:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727975AbhALOIe convert rfc822-to-8bit (ORCPT ); Tue, 12 Jan 2021 09:08:34 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:25105 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727700AbhALOId (ORCPT ); Tue, 12 Jan 2021 09:08:33 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-190-LrEcQ_I1My-Flm7q2b_3Gw-1; Tue, 12 Jan 2021 14:06:55 +0000 X-MC-Unique: LrEcQ_I1My-Flm7q2b_3Gw-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Tue, 12 Jan 2021 14:06:54 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Tue, 12 Jan 2021 14:06:54 +0000 From: David Laight To: 'Zhongwei Cai' , Mingkai Dong , Matthew Wilcox , "viro@zeniv.linux.org.uk" CC: Mikulas Patocka , Andrew Morton , Jan Kara , Steven Whitehouse , Eric Sandeen , Dave Chinner , Theodore Ts'o , Wang Jianchao , "Tadakamadla, Rajesh" , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-nvdimm@lists.01.org" Subject: RE: Expense of read_iter Thread-Topic: Expense of read_iter Thread-Index: Adbo7C52XXt2HSHdSaqZSwQZFrXi9Q== Date: Tue, 12 Jan 2021 14:06:53 +0000 Message-ID: <68567cb68fec4d79be79a655301effe1@AcuMS.aculab.com> References: <20210107151125.GB5270@casper.infradead.org> <17045315-CC1F-4165-B8E3-BA55DD16D46B@gmail.com> <2041983017.5681521.1610459100858.JavaMail.zimbra@sjtu.edu.cn> In-Reply-To: <2041983017.5681521.1610459100858.JavaMail.zimbra@sjtu.edu.cn> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org From: Zhongwei Cai > Sent: 12 January 2021 13:45 .. > The overhead mainly consists of two parts. The first is constructing > struct iov_iter and iterating it (i.e., new_sync, _copy_mc_to_iter and > iov_iter_init). The second is the dax io mechanism provided by VFS (i.e., > dax_iomap_rw, iomap_apply and ext4_iomap_begin). Setting up an iov_iter with a single buffer ought to be relatively cheap - compared to a file system read. The iteration should be over the total length calling copy_from/to_iter() for 'chunks' that don't depend on the size of the iov[] fragments. So copy_to/from_iter() should directly replace the copy_to/from_user() calls in the 'read' method. For a single buffer this really ought to be noise as well. Clearly is the iov[] has a lot of short fragments the copy will be more expensive. Access to /dev/null and /dev/zero are much more likely to show the additional costs of the iov_iter code than fs code. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)