From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56927) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dpsyi-0000oD-Do for qemu-devel@nongnu.org; Thu, 07 Sep 2017 05:20:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dpsye-00076l-He for qemu-devel@nongnu.org; Thu, 07 Sep 2017 05:20:04 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35004) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dpsye-00076Y-7l for qemu-devel@nongnu.org; Thu, 07 Sep 2017 05:20:00 -0400 Date: Thu, 7 Sep 2017 10:19:47 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20170907091946.GC2098@work-vm> References: <20170906094846.GA2215@work-vm> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20170907085526.GA30609@redhat.com> 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: "Daniel P. Berrange" Cc: Peter Xu , qemu-devel@nongnu.org, Paolo Bonzini , Fam Zheng , Juan Quintela , mdroth@linux.vnet.ibm.com, Eric Blake , Laurent Vivier , Markus Armbruster * 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 wr= ote: > > > > * Daniel P. Berrange (berrange@redhat.com) wrote: > > > > > This does imply that you need a separate monitor I/O processing= , from the > > > > > command execution thread, but I see no need for all commands to= suddenly > > > > > become async. Just allowing interleaved replies is sufficient f= rom the > > > > > POV of the protocol definition. This interleaving is easy to ha= ndle from > > > > > the client POV - just requires a unique 'serial' in the request= by the > > > > > client, that is copied into the reply by QEMU. > > > >=20 > > > > OK, so for that we can just take Marc-Andr=E9's syntax and call i= t 'id': > > > > https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg03634.= 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 comma= nd > > > since day 1 of supporting QMP. > > >=20 > > > > I do worry about two things: > > > > a) With this the caller doesn't really know which commands coul= d be > > > > in parallel - for example if we've got a recovery command that'= 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 suddenly > > > > 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 exe= cuting > > > all commands in the main thread, unless the client issues an explic= it > > > "make it async" command, to switch to allowing the migration thread= 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 reques= t > > > for which async is supported. > > >=20 > > > That gives good negotiation ability going forward as we incremental= ly > > > 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 indeed > > 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 includ= e > > 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. 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. Dave > I don't think the hacks proposed are a good tradeoff, compared to > fixing the fundamental problem with the monitor impl in QEMU. We > have discussed this monitor problem for years pretty much since > day 1 of QMP being designed, but it never gets serious attention. > IMHO it is well overdue to change that and focus attention on the > root problem and not just punt it down the road yet again by adding > short term hacks. >=20 > Adding an extra monitor channel, even as a short term hack, is > *not* short term from libvirt's POV - we'll have to carry that > code for many years into the future, even after QEMU provides > a real fix. So even if QEMU provides such a short term hack, I > would none the less be strongly against libvirt using it. >=20 >=20 > Regards, > Daniel > --=20 > |: https://berrange.com -o- https://www.flickr.com/photos/dberr= ange :| > |: https://libvirt.org -o- https://fstop138.berrange= .com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberr= ange :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK