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Szmigiero" , =?utf-8?Q?C=C3=A9dric?= Le Goater Subject: Re: [PATCH 1/2] migration: Add some documentation for multifd In-Reply-To: References: <20250307134203.29443-1-farosas@suse.de> <20250307134203.29443-2-farosas@suse.de> <87tt84u0d2.fsf@suse.de> <87o6y9t14g.fsf@suse.de> <87ecz4adoi.fsf@suse.de> Date: Thu, 20 Mar 2025 10:38:47 -0300 Message-ID: <878qozbz4o.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[99.99%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_FIVE(0.00)[5]; RCVD_COUNT_TWO(0.00)[2]; FUZZY_BLOCKED(0.00)[rspamd.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo, suse.de:mid, suse.de:email] Received-SPF: pass client-ip=195.135.223.130; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Prasad Pandit writes: > On Tue, 11 Mar 2025 at 00:59, Fabiano Rosas wrote: >> Peter Xu writes: >> > To me, this is a fairly important question to ask. Fundamentally, the very >> > initial question is why do we need periodic flush and sync at all. It's >> > because we want to make sure new version of pages to land later than old >> > versions. > ... >> > Then v1 and v2 of the page P are ordered. >> > If without the message on the main channel: >> > Then I don't see what protects reorder of arrival of messages like: > ... >> That's all fine. As long as the recv part doesn't see them out of >> order. I'll try to write some code to confirm so I don't waste too much >> of your time. > > * Relying on this receive order seems like a passive solution. On one > side we are saying there is no defined 'requirement' on the network or > compute capacity/quality for migration. ie. compute and network can be > as bad as possible, yet migration shall always work reliably. > > * When receiving different versions of pages, couldn't multifd_recv > check the latest version present in guest RAM and accept the incoming > version only if it is fresher than the already present one? ie. if v1 > arrives later than v2 on the receive side, the receive side > could/should discard v1 because v2 is already received. > "in guest RAM" I don't think so, the performance would probably be affected. We could have a sequence number that gets bumped per iteration, but I'm not sure how much of a improvement that would be. Without a sync, we'd need some sort of per-page handling*. I have a gut feeling this would get costly. *- maybe per-iovec depending on how we queue pages to multifd. > Thank you. > --- > - Prasad