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, URIBL_BLOCKED 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 78E5DC433E0 for ; Mon, 11 Jan 2021 10:11:27 +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 1087022288 for ; Mon, 11 Jan 2021 10:11:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1087022288 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 9A22E100EBBB4; Mon, 11 Jan 2021 02:11:26 -0800 (PST) Received-SPF: Pass (mailfrom) identity=mailfrom; client-ip=185.58.85.151; 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 [185.58.85.151]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id 7905A100EBBAF for ; Mon, 11 Jan 2021 02:11:23 -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-68-LSP2TgUWPoW5OYymkm_RvQ-1; Mon, 11 Jan 2021 10:11:18 +0000 X-MC-Unique: LSP2TgUWPoW5OYymkm_RvQ-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; Mon, 11 Jan 2021 10:11:17 +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; Mon, 11 Jan 2021 10:11:17 +0000 From: David Laight To: 'Matthew Wilcox' , Mikulas Patocka Subject: RE: Expense of read_iter Thread-Topic: Expense of read_iter Thread-Index: AQHW5xgPc3YVJbtxFkyYJqwIgIdF7KoiMj8g Date: Mon, 11 Jan 2021 10:11:17 +0000 Message-ID: <647fe440ab4640038a6ccaf59ab99685@AcuMS.aculab.com> References: <20210107151125.GB5270@casper.infradead.org> <20210110061321.GC35215@casper.infradead.org> In-Reply-To: <20210110061321.GC35215@casper.infradead.org> 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: E5YIUSKFQGLKZRNVVZN7OW5CI4SVU2HG X-Message-ID-Hash: E5YIUSKFQGLKZRNVVZN7OW5CI4SVU2HG 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: Andrew Morton , "Dave Jiang , Ira Weiny , Jan Kara" , Steven Whitehouse , Eric Sandeen , Dave Chinner , Theodore Ts'o , Wang Jianchao , "Tadakamadla,"@ml01.01.org, Rajesh@ml01.01.org, " , "@ml01.01.org, linux-kernel@vger.kernel.org, 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: Matthew Wilcox > Sent: 10 January 2021 06:13 ... > nvfs_rw_iter_locked() looks very complicated. I suspect it can > be simplified. Of course new_sync_read() needs to be improved too, > as do the other functions here, but fully a third of the difference > between read() and read_iter() is the difference between nvfs_read() > and nvfs_rw_iter_locked(). There is also the non-zero cost of import_iovec(). I've got some slight speedups, but haven't measured an old kernel yet to see how much slower 5.11-rc1 made it. Basic test is: fd = open("/dev/null", O_RDWR); for (1 = 0; 1 < 10000; i++) { start = rdtsc(); writev(fd, iovec, count); histogram[rdtsc() - start]++; } This doesn't actually copy any data - the iovec isn't iterated. I'm seeing pretty stable counts for most of the 10000 iterations. But different program runs can give massively different timings. I'm quessing that depends on cache collisions due to the addresses (virtual of physical?) selected for some items. For 5.11-rc2 -mx32 is slightly faster than 64bit. Whereas -m32 has a much slower syscall entry/exit path, but the difference between gettid() and writev() is lower. The compat code for import_iovec() is actually faster. This isn't really surprising since copy_from_user() is absolutely horrid these days - especially with userspace hardening. 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