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Tue, 14 Feb 2023 10:04:09 -0800 (PST) Received: from redhat.com ([46.136.252.173]) by smtp.gmail.com with ESMTPSA id f21-20020a05600c43d500b003dc522dd25esm17209133wmn.30.2023.02.14.10.04.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Feb 2023 10:04:09 -0800 (PST) From: Juan Quintela To: Markus Armbruster Cc: qemu-devel@nongnu.org, Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , "Dr. David Alan Gilbert" , Eduardo Habkost , Eric Blake , Yanan Wang , Marcel Apfelbaum Subject: Re: [PATCH v5 1/3] multifd: Create property multifd-sync-after-each-section In-Reply-To: <87mt5goctt.fsf@pond.sub.org> (Markus Armbruster's message of "Tue, 14 Feb 2023 09:45:02 +0100") References: <20230213091548.76444-1-quintela@redhat.com> <20230213091548.76444-2-quintela@redhat.com> <87mt5goctt.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) Date: Tue, 14 Feb 2023 19:04:08 +0100 Message-ID: <87zg9g5dk7.fsf@secure.mitica> MIME-Version: 1.0 Content-Type: text/plain Received-SPF: pass client-ip=170.10.129.124; envelope-from=quintela@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: quintela@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Markus Armbruster wrote: > Juan Quintela writes: > >> We used to synchronize all channels at the end of each RAM section >> sent. That is not needed, so preparing to only synchronize once every >> full round in latests patches. >> >> Notice that we initialize the property as true. We will change the >> default when we introduce the new mechanism. >> >> Signed-off-by: Juan Quintela >> Reviewed-by: Dr. David Alan Gilbert >> Signed-off-by: Juan Quintela >> >> --- >> >> Rename each-iteration to after-each-section >> >> Signed-off-by: Juan Quintela >> --- >> qapi/migration.json | 10 +++++++++- >> migration/migration.h | 1 + >> hw/core/machine.c | 1 + >> migration/migration.c | 15 +++++++++++++-- >> 4 files changed, 24 insertions(+), 3 deletions(-) >> >> diff --git a/qapi/migration.json b/qapi/migration.json >> index c84fa10e86..2907241b9c 100644 >> --- a/qapi/migration.json >> +++ b/qapi/migration.json >> @@ -478,6 +478,13 @@ >> # should not affect the correctness of postcopy migration. >> # (since 7.1) >> # >> +# @multifd-sync-after-each-section: Synchronize channels after each >> +# section is sent. > > What does it mean to synchronize channels? > > When would I want to, and why? > >> +# We used to do >> +# that in the past, but it is >> +# suboptimal. > > This isn't particularly helpful, I'm afraid. > >> +# Default value is true until all code is in. > > As far as I can tell, it's actually *unused* for now, and a later patch > will put it to use ... We (well, libvert preffers) want capabilities to be false by default. When I introduce a new capability/parameter: - Patch1: I introduce the capability/parameter, it does nothing yet. - Patch2: I conditionalize the old code on this capability. Default value is true (old code). - Patch3: I introduce the new code to implement the feature. At this point I change the default. Depending on complexity, Patch2 and 3 can be a series, but you get the idea O:-) >> +# (since 8.0) Retry. What about: # @multifd-sync-after-each-section: flush each channel after each # section sent. This assures that # we can't mix pages from one # iteration through the dirty bitmap # with pages for the following # iteration. We really only need to # do this flush after we have go # trhough all the dirty bitmap. For # historical reasons, we do that after # each section. This is suboptimal # (we flush too many times). # Default value is true until the code # to implement it is in tree. # (since 8.0) Better? >> +bool migrate_multifd_sync_after_each_section(void) >> +{ >> + MigrationState *s = migrate_get_current(); >> + >> + return true; >> + // We will change this when code gets in. >> + return s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION]; > > ... here. > > No warning about unreachable code? Checking... nope, gcc seems to not > to care. Yeap. Gcc thinks this is ok. In others try's I have done: return true || s->enabled_capabilities[MIGRATION_CAPABILITY_MULTIFD_SYNC_AFTER_EACH_SECTION]; If you preffer I can change to this, not strong opinions. Later, Juan.