From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:58104) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dpt1Q-00031V-VL for qemu-devel@nongnu.org; Thu, 07 Sep 2017 05:22:54 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dpt1L-00010c-UN for qemu-devel@nongnu.org; Thu, 07 Sep 2017 05:22:52 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40424) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dpt1L-00010B-LO for qemu-devel@nongnu.org; Thu, 07 Sep 2017 05:22:47 -0400 Date: Thu, 7 Sep 2017 10:22:35 +0100 From: "Daniel P. Berrange" Message-ID: <20170907092235.GC30609@redhat.com> Reply-To: "Daniel P. Berrange" References: <20170906104603.GK15510@redhat.com> <20170906104850.GB2215@work-vm> <20170906105414.GL15510@redhat.com> <20170906105704.GC2215@work-vm> <20170906110629.GM15510@redhat.com> <20170906113157.GD2215@work-vm> <20170906115428.GP15510@redhat.com> <20170907081341.GA23040@pxdev.xzpeter.org> <20170907085526.GA30609@redhat.com> <20170907091946.GC2098@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20170907091946.GC2098@work-vm> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: Peter Xu , qemu-devel@nongnu.org, Paolo Bonzini , Fam Zheng , Juan Quintela , mdroth@linux.vnet.ibm.com, Eric Blake , Laurent Vivier , Markus Armbruster On Thu, Sep 07, 2017 at 10:19:47AM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrange (berrange@redhat.com) wrote: > > On Thu, Sep 07, 2017 at 04:13:41PM +0800, Peter Xu wrote: > > > On Wed, Sep 06, 2017 at 12:54:28PM +0100, Daniel P. Berrange wrote: > > > > On Wed, Sep 06, 2017 at 12:31:58PM +0100, Dr. David Alan Gilbert = wrote: > > > > > * Daniel P. Berrange (berrange@redhat.com) wrote: > > > > > > This does imply that you need a separate monitor I/O processi= ng, from the > > > > > > command execution thread, but I see no need for all commands = to suddenly > > > > > > become async. Just allowing interleaved replies is sufficient= from the > > > > > > POV of the protocol definition. This interleaving is easy to = handle from > > > > > > the client POV - just requires a unique 'serial' in the reque= st by the > > > > > > client, that is copied into the reply by QEMU. > > > > >=20 > > > > > OK, so for that we can just take Marc-Andr=C3=A9's syntax and c= all it 'id': > > > > > https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg0363= 4.html > > > > >=20 > > > > > then it's upto the caller to ensure those id's are unique. > > > >=20 > > > > Libvirt has in fact generated a unique 'id' for every monitor com= mand > > > > since day 1 of supporting QMP. > > > >=20 > > > > > I do worry about two things: > > > > > a) With this the caller doesn't really know which commands co= uld be > > > > > in parallel - for example if we've got a recovery command tha= t's > > > > > executed by this non-locking thread that's OK, we expect that > > > > > to be doable in parallel. If in the future though we do > > > > > what you initially suggested and have a bunch of commands get > > > > > routed to the migration thread (say) then those would suddenl= y > > > > > operate in parallel with other commands that we're previously > > > > > synchronous. > > > >=20 > > > > We could still have an opt-in for async commands. eg default to e= xecuting > > > > all commands in the main thread, unless the client issues an expl= icit > > > > "make it async" command, to switch to allowing the migration thre= ad to > > > > process it async. > > > >=20 > > > > { "execute": "qmp_allow_async", > > > > "data": { "commands": [ > > > > "migrate_cancel", > > > > ] } } > > > >=20 > > > >=20 > > > > { "return": { "commands": [ > > > > "migrate_cancel", > > > > ] } } > > > >=20 > > > > The server response contains the subset of commands from the requ= est > > > > for which async is supported. > > > >=20 > > > > That gives good negotiation ability going forward as we increment= ally > > > > support async on more commands. > > >=20 > > > I think this goes back to the discussion on which design we'd like = to > > > choose. IMHO the whole async idea plus the per-command-id is indee= d > > > cleaner and nicer, and I believe that can benefit not only libvirt, > > > but also other QMP users. The problem is, I have no idea how long > > > it'll take to let us have such a feature - I believe that will incl= ude > > > QEMU and Libvirt to both support that. And it'll be a pity if the > > > postcopy recovery cannot work only because we cannot guarantee a > > > stable monitor. > >=20 > > This is not a blocker for having postcopy recovery feature merged. > > It merely means that in a situation where the mainloop is blocked, > > then we can't recover, in other situations we'll be able to recover > > fine. Sure it would be nice to fix that problem too, but I don't > > see it as a block. >=20 > It's probably OK to merge the recovery code before the monitor code; > but I don't think it's something you'd want to tell users about - > a 'postcopy recovery that only works rarely' isn't much use. I dunno. Compared to today where there's zero post-copy recovery, I think even an incremental improvement is useful. Its a choice between "your VM is dead" and "you've a 50/50 chance of life". Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberran= ge :| |: https://libvirt.org -o- https://fstop138.berrange.c= om :| |: https://entangle-photo.org -o- https://www.instagram.com/dberran= ge :|