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=-0.8 required=3.0 tests=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 43612C432C0 for ; Wed, 27 Nov 2019 16:04:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 253BE206F0 for ; Wed, 27 Nov 2019 16:04:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727105AbfK0QES convert rfc822-to-8bit (ORCPT ); Wed, 27 Nov 2019 11:04:18 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:34350 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726747AbfK0QEQ (ORCPT ); Wed, 27 Nov 2019 11:04:16 -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-18-Aa6kDxC7OPi54Qqo1k4BMw-1; Wed, 27 Nov 2019 16:04:12 +0000 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; Wed, 27 Nov 2019 16:04:12 +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; Wed, 27 Nov 2019 16:04:12 +0000 From: David Laight To: 'Jesper Dangaard Brouer' CC: 'Marek Majkowski' , linux-kernel , network dev , kernel-team , Paolo Abeni Subject: RE: epoll_wait() performance Thread-Topic: epoll_wait() performance Thread-Index: AdWgk3jgEIFNwcnRS6+4A+/jFPxTuQEdLCCAAAAn2qAADFPagAAAV68A Date: Wed, 27 Nov 2019 16:04:12 +0000 Message-ID: <5eecf41c7e124d7dbc0ab363d94b7d13@AcuMS.aculab.com> References: <5f4028c48a1a4673bd3b38728e8ade07@AcuMS.aculab.com> <20191127164821.1c41deff@carbon> In-Reply-To: <20191127164821.1c41deff@carbon> Accept-Language: en-GB, en-US Content-Language: 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 X-MC-Unique: Aa6kDxC7OPi54Qqo1k4BMw-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org From: Jesper Dangaard Brouer > Sent: 27 November 2019 15:48 > On Wed, 27 Nov 2019 10:39:44 +0000 David Laight wrote: > > > ... > > > > While using recvmmsg() to read multiple messages might seem a good idea, it is much > > > > slower than recv() when there is only one message (even recvmsg() is a lot slower). > > > > (I'm not sure why the code paths are so slow, I suspect it is all the copy_from_user() > > > > and faffing with the user iov[].) > > > > > > > > So using poll() we repoll the fd after calling recv() to find is there is a second message. > > > > However the second poll has a significant performance cost (but less than using recvmmsg()). > > > > > > That sounds wrong. Single recvmmsg(), even when receiving only a > > > single message, should be faster than two syscalls - recv() and > > > poll(). > > > > My suspicion is the extra two copy_from_user() needed for each recvmsg are a > > significant overhead, most likely due to the crappy code that tries to stop > > the kernel buffer being overrun. > > > > I need to run the tests on a system with a 'home built' kernel to see how much > > difference this make (by seeing how much slower duplicating the copy makes it). > > > > The system call cost of poll() gets factored over a reasonable number of sockets. > > So doing poll() on a socket with no data is a lot faster that the setup for recvmsg > > even allowing for looking up the fd. > > > > This could be fixed by an extra flag to recvmmsg() to indicate that you only really > > expect one message and to call the poll() function before each subsequent receive. > > > > There is also the 'reschedule' that Eric added to the loop in recvmmsg. > > I don't know how much that actually costs. > > In this case the process is likely to be running at a RT priority and pinned to a cpu. > > In some cases the cpu is also reserved (at boot time) so that 'random' other code can't use it. > > > > We really do want to receive all these UDP packets in a timely manner. > > Although very low latency isn't itself an issue. > > The data is telephony audio with (typically) one packet every 20ms. > > The code only looks for packets every 10ms - that helps no end since, in principle, > > only a single poll()/epoll_wait() call (on all the sockets) is needed every 10ms. > > I have a simple udp_sink tool[1] that cycle through the different > receive socket system calls. I gave it a quick spin on a F31 kernel > 5.3.12-300.fc31.x86_64 on a mlx5 100G interface, and I'm very surprised > to see a significant regression/slowdown for recvMmsg. > > $ sudo ./udp_sink --port 9 --repeat 1 --count $((10**7)) > run count ns/pkt pps cycles payload > recvMmsg/32 run: 0 10000000 1461.41 684270.96 5261 18 demux:1 > recvmsg run: 0 10000000 889.82 1123824.84 3203 18 demux:1 > read run: 0 10000000 974.81 1025841.68 3509 18 demux:1 > recvfrom run: 0 10000000 1056.51 946513.44 3803 18 demux:1 > > Normal recvmsg almost have double performance that recvmmsg. > recvMmsg/32 = 684,270 pps > recvmsg = 1,123,824 pps Can you test recv() as well? I think it might be faster than read(). ... > Found some old results (approx v4.10-rc1): > > [brouer@skylake src]$ sudo taskset -c 2 ./udp_sink --count $((10**7)) --port 9 --connect > recvMmsg/32 run: 0 10000000 537.89 1859106.74 2155 21559353816 > recvmsg run: 0 10000000 552.69 1809344.44 2215 22152468673 > read run: 0 10000000 476.65 2097970.76 1910 19104864199 > recvfrom run: 0 10000000 450.76 2218492.60 1806 18066972794 That is probably nearer what I am seeing on a 4.15 Ubuntu 18.04 kernel. recvmmsg() and recvmsg() are similar - but both a lot slower then recv(). David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)