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[82.30.61.225]) by smtp.gmail.com with ESMTPSA id bg23sm2732873wmb.5.2022.02.08.05.23.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 08 Feb 2022 05:23:31 -0800 (PST) Date: Tue, 8 Feb 2022 13:23:29 +0000 From: "Dr. David Alan Gilbert" To: Peter Xu Subject: Re: [PATCH RFC 14/15] migration: Postcopy preemption on separate channel Message-ID: References: <20220119080929.39485-1-peterx@redhat.com> <20220119080929.39485-15-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.1.5 (2021-12-30) Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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: , Cc: Juan Quintela , qemu-devel@nongnu.org, Leonardo Bras Soares Passos Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Peter Xu (peterx@redhat.com) wrote: > On Tue, Feb 08, 2022 at 11:24:14AM +0000, Dr. David Alan Gilbert wrote: > > > The current model is we only have 1 postcopy channel and 1 precopy channel, but > > > it should be easier if we want to make it N post + 1 pre base on this series. > > > > It's not clear to me if we need to be able to do N post + M pre, or > > whether we have a rule like always at least 1 post, but if there's more > > pagefaults in the queue then you can steal all of the pre channels. > > Right, >1 queue length should easily happen with workloads in real cloud > environment. Though even with only 1post channel we can already hit at least > <~1ms with this series even if there're 16 pending requests per my test. I > think that may cover quite some real workloads. > > One thing to mention is that we should always assume the pre-channels are > filled up with tons of pages already in the NIC send buffer, so they won't be > good candidate for postcopy requests, IMHO. So I'm not sure whether we can > mixly use the pre/post channels - we may need to leave the post channels idle. No I'm not sure either; even with separate channels do we have problems with contention on the NIC? Dave > Then, if we keep some of the multifd channels idle, then it will become some > other thing rather than the existing multifd, since we will start to treat > threads and channels differently and break the "equality" rule in the strict > version of multifd world. > > > > This also reminded me that, instead of a new capability, should I simply expose > > > a parameter "postcopy-channels=N" to CLI so that we can be prepared with multi > > > postcopy channels? > > > > I'm not sure we know enough yet about what configuration it would have; > > I'd be tempted to just make it work for the user by enabling both > > multifd and preemption and then using this new mechanism rather than > > having to add yet another parameter. > > Let me stick with the current capability bit then, so as to make it 1pre+1post. > And we can leave Npre+1post for later. > > Thanks, > > -- > Peter Xu > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK