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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 363FBC33C8C for ; Tue, 7 Jan 2020 14:46:26 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 009BD2077B for ; Tue, 7 Jan 2020 14:46:25 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="LnDHb9fr" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 009BD2077B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:50366 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ioq7l-0002hY-1T for qemu-devel@archiver.kernel.org; Tue, 07 Jan 2020 09:46:25 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:46826) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iooyH-0004ZE-GJ for qemu-devel@nongnu.org; Tue, 07 Jan 2020 08:32:35 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iooyF-0005kU-IO for qemu-devel@nongnu.org; Tue, 07 Jan 2020 08:32:33 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:50664 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iooyF-0005jq-Cr for qemu-devel@nongnu.org; Tue, 07 Jan 2020 08:32:31 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1578403950; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=fQ8YZ4kxjS/Wvi2iTyW3cG1BWPlzmL6gE4ayGKp9Syw=; b=LnDHb9frWaT1RU/FS/vbV0Sc31dm+qknQF/VPTJwNZXgGgE1qxFjkg8G6HTihHPbSrCybM LH98AZwH1lKvlQn2+mnRbl3GV1E6jI6cuLRHDdRDhKGnPXRqLqO6R19iUHVjbaZtXMnitd YJEU8wlJ8TVykfffbkq2P+wGq0scguQ= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-248-GYwiqeN3PdyEDhCVhyAzgA-1; Tue, 07 Jan 2020 08:32:29 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C1E31DBA3 for ; Tue, 7 Jan 2020 13:32:28 +0000 (UTC) Received: from redhat.com (ovpn-116-141.ams2.redhat.com [10.36.116.141]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3DABD5D9CA; Tue, 7 Jan 2020 13:32:28 +0000 (UTC) From: Juan Quintela To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Subject: Re: [PATCH v2 01/10] migration: Increase default number of multifd channels to 16 In-Reply-To: <20200107124934.GK3368802@redhat.com> ("Daniel P. =?utf-8?Q?B?= =?utf-8?Q?errang=C3=A9=22's?= message of "Tue, 7 Jan 2020 12:49:34 +0000") References: <20191218020119.3776-1-quintela@redhat.com> <20191218020119.3776-2-quintela@redhat.com> <20200103165832.GU2753983@redhat.com> <87mub4xurf.fsf@trasno.org> <20200107124934.GK3368802@redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) Date: Tue, 07 Jan 2020 14:32:24 +0100 Message-ID: <87muazpf2v.fsf@trasno.org> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-MC-Unique: GYwiqeN3PdyEDhCVhyAzgA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: quintela@redhat.com Cc: Laurent Vivier , Thomas Huth , Eduardo Habkost , Markus Armbruster , qemu-devel@nongnu.org, Paolo Bonzini , "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Daniel P. Berrang=C3=A9 wrote: > On Fri, Jan 03, 2020 at 07:25:08PM +0100, Juan Quintela wrote: >> Daniel P. Berrang=C3=A9 wrote: >> > On Wed, Dec 18, 2019 at 03:01:10AM +0100, Juan Quintela wrote: >> >> We can scale much better with 16, so we can scale to higher numbers. >> > >> > What was the test scenario showing such scaling ? >>=20 >> On my test hardware, with 2 channels we can saturate around 8Gigabit max= , >> more than that, and the migration thread is not fast enough to fill the >> network bandwidth. >>=20 >> With 8 that is enough to fill whatever we can find. >> We used to have a bug where we were getting trouble with more channels >> than cores. That was the initial reason why the default was so low. >>=20 >> So, pros/cons are: >> - have low value (2). We are backwards compatible, but we are not using >> all bandwith. Notice that we will dectect the error before 5.0 is >> out and print a good error message. >>=20 >> - have high value (I tested 8 and 16). Found no performance loss when >> moving to lower bandwidth limits, and clearly we were able to saturate >> the higher speeds (I tested on localhost, so I had big enough bandwidt= h) >>=20 >>=20 >> > In the real world I'm sceptical that virt hosts will have >> > 16 otherwise idle CPU cores available that are permissible >> > to use for migration, or indeed whether they'll have network >> > bandwidth available to allow 16 cores to saturate the link. >>=20 >> The problem here is that if you have such a host, and you want to have >> high speed migration, you need to configure it. My measumermets are >> that high number of channels don't affect performance with low >> bandwidth, but low number of channels affect performance with high >> bandwidth speed. > > I'm not concerned about impact on performance of migration on a > low bandwidth link, rather I'm concerned about impact on performance > of other guests on the host. It will cause migration to contend with > other guest's vCPUs and network traffic.=20 Two things here: - vcpus: If you want migration to consume all the bandwidth, you are happy with it using more vcpus. - bandwidth: It will only consume only the one that the guest has assigned, split (we hope evenly) between all the channels. My main reason to have a higher number of channels is: - test better the code with more than one channel - work "magically" well in all scenarios. With a low number of channels, we are not going to be able to saturate a big network pipe. > >> So, if we want to have something that works "automatically" everywhere, >> we need to put it to at least 8. Or we can trust that management app >> will do the right thing. > > Aren't we still setting the bandwidth limit to MB bandwidth out of the > box, so we already require mgmt app to change settings to use more > bandwidth ?=20 Yeap. This is the default bandwidth. #define MAX_THROTTLE (32 << 20) >> If you are using a low value of bandwidth, the only difference with 16 >> channels is that you are using a bit more memory (just the space for the >> stacks) and that you are having less contention for the locks (but with >> low bandwidth you are not having contention anyways). >>=20 >> So, I think that the question is: Note that my idea is to make multifd "default" in the near future (5.1 timeframe or so). >> - What does libvirt prefferes > > Libvirt doesn't really have an opinion in this case. I believe we'll > always set the number of channels on both src & dst, so we don't > see the defaults. What does libvirt does today for this value? >> - What does ovirt/openstack preffer > > Libvirt should insulate them from any change in defaults in QEMU > in this case, but always explicitly setting channels on src & dst > to match. I agree here, they should don't care by default. >> - Do we really want that the user "have" to configure that value > > Right so this is the key quesiton - for a user not using libvirt > or a libvirt based mgmt app, what we do want out out of the box > migration to be tuned for ? In my opinion, we should have something like: - multifd: enabled by default - max downtime: 300 ms (current) looks right to me - max bandwidth: 32MB/s (current) seems a bit low. 100MB/s (i.e. almost full gigabit ethernet) seems reasonable to me. Having a default for 10Gigabit ethernet or similar seems too high. > If we want to maximise migration performance, at cost of anything > else, then we can change the migration channels count, but probably > also ought to remove the 32MB bandwidth cap as no useful guest with > active apps will succeed migration with a 32MB cap. Will start another series with the current values to discuss all the defaults, ok? thanks for the comments, Juan.