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Tue, 22 Sep 2026 14:44:05 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id cvvTGjWUsmqzSQAAD6G6ig (envelope-from ); Tue, 22 Sep 2026 14:44:05 +0000 From: Fabiano Rosas To: Bin Guo , qemu-devel@nongnu.org Cc: peterx@redhat.com, pierrick.bouvier@oss.qualcomm.com Subject: Re: [PATCH 1/2] docs/migration: document that postcopy can be used with multifd In-Reply-To: <20260915063719.89031-2-guobin@linux.alibaba.com> References: <20260915063719.89031-1-guobin@linux.alibaba.com> <20260915063719.89031-2-guobin@linux.alibaba.com> Date: Tue, 22 Sep 2026 11:44:03 -0300 Message-ID: <87se319wi4.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; 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_HAS_DN(0.00)[]; RCPT_COUNT_THREE(0.00)[4]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email, suse.de:mid, imap1.dmz-prg2.suse.org:helo] Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:2; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de 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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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: qemu development 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 Bin Guo writes: > Since commit e27418861288 ("migration: enable multifd and postcopy > together") in the 10.1 release, the multifd and postcopy-ram capabilities > are no longer mutually exclusive, but the postcopy documentation does not > mention multifd at all, leaving users unaware that the two features can be > combined. > > Add a short section describing how they interact: multifd carries the > pages of the precopy phase, the channels are flushed and synced before the > switchover (because loading a page in a multifd receive thread is not > atomic with regard to a running vCPU), and the postcopy phase itself does > not use the multifd channels. > > Signed-off-by: Bin Guo > --- > docs/devel/migration/postcopy.rst | 26 ++++++++++++++++++++++++++ > 1 file changed, 26 insertions(+) > > diff --git a/docs/devel/migration/postcopy.rst b/docs/devel/migration/postcopy.rst > index e319388d8f..570a3a1ce6 100644 > --- a/docs/devel/migration/postcopy.rst > +++ b/docs/devel/migration/postcopy.rst > @@ -294,6 +294,32 @@ the background migration channel. Anyone who cares about latencies of page > faults during a postcopy migration should enable this feature. By default, > it's not enabled. > > +Postcopy with multifd > +--------------------- > + > +The ``multifd`` capability can be enabled together with ``postcopy-ram`` > +since the 10.1 QEMU release. The two features apply to different phases of > +the migration: > + > + - During the precopy phase, guest pages are sent over the multifd channels > + as usual, so the initial RAM transfer can use all of them. > + > + - Just before switching to postcopy, the source flushes and syncs the > + multifd channels. This guarantees that all the pages already queued on > + those channels are loaded on the destination *before* the destination > + CPUs are started, which is required because loading a page in a multifd > + receive thread is not atomic with regard to a running vCPU. > + > + - During the postcopy phase the multifd channels are no longer used for > + guest pages. Both the background stream and the pages requested by the > + destination go through the background migration channel instead, or > + through the preempt channel for the requested pages when postcopy > + preemption is enabled. > + > +Consequently, multifd only speeds up the precopy phase of a postcopy > +migration. The bandwidth available once postcopy has started is the same as > +without multifd. > + > Postcopy blocktime statistics > ----------------------------- Looks good to me, I'll wait a bit to allow Peter to comment. Acked-by: Fabiano Rosas