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 ACA5FC433DB for ; Tue, 12 Jan 2021 14:07:03 +0000 (UTC) Received: from ml01.01.org (ml01.01.org [198.145.21.10]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 5090D22CE3 for ; Tue, 12 Jan 2021 14:07:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5090D22CE3 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=ACULAB.COM Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-nvdimm-bounces@lists.01.org Received: from ml01.vlan13.01.org (localhost [IPv6:::1]) by ml01.01.org (Postfix) with ESMTP id E26A1100EB83E; Tue, 12 Jan 2021 06:07:02 -0800 (PST) Received-SPF: Pass (mailfrom) identity=mailfrom; client-ip=207.82.80.151; helo=eu-smtp-delivery-151.mimecast.com; envelope-from=david.laight@aculab.com; receiver= Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [207.82.80.151]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id 45D47100EB83C for ; Tue, 12 Jan 2021 06:06:59 -0800 (PST) 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" 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 Message-ID-Hash: NGFCOWKCMLL3UXPWKZWHK5ETQDIVRF4V X-Message-ID-Hash: NGFCOWKCMLL3UXPWKZWHK5ETQDIVRF4V X-MailFrom: david.laight@aculab.com X-Mailman-Rule-Hits: nonmember-moderation X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation 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" X-Mailman-Version: 3.1.1 Precedence: list List-Id: "Linux-nvdimm developer list." Archived-At: List-Archive: List-Help: List-Post: List-Subscribe: List-Unsubscribe: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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) _______________________________________________ Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org To unsubscribe send an email to linux-nvdimm-leave@lists.01.org