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Fri, 3 Jan 2020 17:49:46 +0000 (UTC) Received: from redhat.com (ovpn-112-41.ams2.redhat.com [10.36.112.41]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8B8137D961; Fri, 3 Jan 2020 17:49:43 +0000 (UTC) Date: Fri, 3 Jan 2020 17:49:40 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Juan Quintela Subject: Re: [PATCH v2 01/10] migration: Increase default number of multifd channels to 16 Message-ID: <20200103174940.GX2753983@redhat.com> References: <20191218020119.3776-1-quintela@redhat.com> <20191218020119.3776-2-quintela@redhat.com> MIME-Version: 1.0 In-Reply-To: <20191218020119.3776-2-quintela@redhat.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-MC-Unique: GbzEKnbTPca1o9qfkmO7LA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Laurent Vivier , Thomas Huth , Eduardo Habkost , Markus Armbruster , qemu-devel@nongnu.org, Paolo Bonzini , "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Dec 18, 2019 at 03:01:10AM +0100, Juan Quintela wrote: > We can scale much better with 16, so we can scale to higher numbers. >=20 > Signed-off-by: Juan Quintela > --- > migration/migration.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) >=20 > diff --git a/migration/migration.c b/migration/migration.c > index 354ad072fa..e7f707e033 100644 > --- a/migration/migration.c > +++ b/migration/migration.c > @@ -86,7 +86,7 @@ > =20 > /* The delay time (in ms) between two COLO checkpoints */ > #define DEFAULT_MIGRATE_X_CHECKPOINT_DELAY (200 * 100) > -#define DEFAULT_MIGRATE_MULTIFD_CHANNELS 2 > +#define DEFAULT_MIGRATE_MULTIFD_CHANNELS 16 I think this can break compatibility for migration between new and old QEMU. Consider a deployment has enabled multifd, but not given an explicit number of channels, and now try to start the migration. In the method multifd_recv_initial_packet() there is a check if (msg.id > migrate_multifd_channels()) { error_setg(errp, "multifd: received channel version %d " "expected %d", msg.version, MULTIFD_VERSION); return -1; } which will fail if migrating from new QEMU to old QEMU because migrate_multifd_channels() will be 2 for old QEMU and new QEMU will be trying to open 16 channels. Regards, Daniel --=20 |: 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= :|