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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id C84D8C6FD1D for ; Thu, 30 Mar 2023 14:58:22 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1phtih-0003tG-TA; Thu, 30 Mar 2023 10:57:43 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1phtif-0003sS-K9 for qemu-devel@nongnu.org; Thu, 30 Mar 2023 10:57:41 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1phtid-0000CG-2R for qemu-devel@nongnu.org; Thu, 30 Mar 2023 10:57:40 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1680188257; 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=VwtSGaZ4LV2gDM+QFf5Vdk3i8RLDDWFS5eIUvgzLfdE=; b=SiIv9nj3zbgonTHyBJA645QqdUZ89EuIxxl4b30rJXULjxnMsxRxy7V5qN7AfE714SiAcS HEETqe/jBQO3t5wbw1WKfwny3U2LSYw6aBMM4Ep96t4lxXlrH16qhvSqRFfVM787qbCiGn LCPmjNTjcVZ8xSVSSvMYN+StT4CUG10= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-159-ITC5jL1YMY-0aLboHSnjlQ-1; Thu, 30 Mar 2023 10:57:35 -0400 X-MC-Unique: ITC5jL1YMY-0aLboHSnjlQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 121508028AD for ; Thu, 30 Mar 2023 14:57:35 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.64]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0515018EC7; Thu, 30 Mar 2023 14:57:33 +0000 (UTC) Date: Thu, 30 Mar 2023 15:57:31 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: Leonardo Bras , Juan Quintela , "Dr. David Alan Gilbert" , qemu-devel@nongnu.org Subject: Re: [RFC PATCH v1 1/1] migration: Disable postcopy + multifd migration Message-ID: References: <20230327161518.2385074-1-leobras@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Thu, Mar 30, 2023 at 10:36:11AM -0400, Peter Xu wrote: > On Thu, Mar 30, 2023 at 03:20:14PM +0100, Daniel P. Berrangé wrote: > > On Mon, Mar 27, 2023 at 01:15:18PM -0300, Leonardo Bras wrote: > > > Since the introduction of multifd, it's possible to perform a multifd > > > migration and finish it using postcopy. > > > > > > A bug introduced by yank (fixed on cfc3bcf373) was previously preventing > > > a successful use of this migration scenario, and now it should be > > > working on most cases. > > > > > > But since there is not enough testing/support nor any reported users for > > > this scenario, we should disable this combination before it may cause any > > > problems for users. > > > > Clearly we don't have enough testing, but multifd+postcopy looks > > like a clearly useful scenario that we should be supporting. > > > > Every post-copy starts with at least one pre-copy iteration, and > > using multifd for that will be important for big VMs where single > > threaded pre-copy is going to be CPU bound. The greater amount we > > can transfer in the pre-copy phase, the less page faults / latency > > spikes postcopy is going to see. > > If we're using 1-round precopy + postcopy approach, the amount of memory > will be the same which is the guest mem size. > > Multifd will make the round shorter so more chance of getting less > re-dirtied pages during the iteration, but that effect is limited. E.g.: > > - For a very idle guest, finishing 1st round in 1min or 3min may not > bring a large difference because most of the pages will be constant > anyway, or > > - For a very busy guest, probably similar amount of pages will be dirtied > no matter in 1min / 3min. Multifd will bring a benefit here, but > busier the guest smaller the effect. I don't feel like that follows. If we're bottlenecking mostly on CPU but have sufficient network bandwidth, then multifd can be the difference between needing to switch to post-copy or being successful in converging in pre-copy. IOW, without multifd we can expect 90% of guests will get stuck and need a switch to post-copy, but with multifd 90% of the guest will complete while in precopy mode and only 10% need switch to post-copy. That's good because it means most guests will avoid the increased failure risk and the period of increased page fault latency from post-copy. > > In terms of migration usage, my personal recommendation to mgmt > > apps would be that they should always enable the post-copy feature > > when starting a migration. Even if they expect to try to get it to > > complete using exclusively pre-copy in the common case, its useful > > to have post-copy capability flag enabled, as a get out of jail > > free card. ie if migration ends up getting stuck in non-convergance, > > or they have a sudden need to urgently complete the migration it is > > good to be able to flip to post-copy mode. > > I fully agree. > > It should not need to be enabled only if not capable, e.g., the dest host > may not have privilege to initiate the userfaultfd (since QEMU postcopy > requires kernel fault traps, so it's very likely). Sure, the mgmt app (libvirt) should be checking support for userfaultfd on both sides before permitting / trying to enable the feature. > > I'd suggest that we instead add a multifd+postcopy test case to > > migration-test.c and tackle any bugs it exposes. By blocking it > > unconditionally we ensure no one will exercise it to expose any > > further bugs. > > That's doable. But then we'd better also figure out how to identify the > below two use cases of both features enabled: > > a. Enable multifd in precopy only, then switch to postcopy (currently > mostly working but buggy; I think Juan can provide more information here, > at least we need to rework multifd flush when switching, and test and > test over to make sure there's nothing else missing). > > b. Enable multifd in both precopy and postcopy phase (currently > definitely not supported) > > So that mgmt app will be aware whether multifd will be enabled in postcopy > or not. Currently we can't identify it. > > I assume we can say by default "mutlifd+postcopy" means a) above, but we > need to document it, and when b) is wanted and implemented someday, we'll > need some other flag/cap for it. As I've mentioned a few times, I think we need to throw away the idea of exposing capabilities that mgmt apps need to learn about, and make the migration protocol fully bi-directional so src + dst QEMU can directly negotiate features. Apps shouldn't have to care about the day-to-day improvements in the migration impl to the extent that they are today. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|