From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56286) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dpsxC-00007I-A8 for qemu-devel@nongnu.org; Thu, 07 Sep 2017 05:18:40 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dpsx7-0006Yp-78 for qemu-devel@nongnu.org; Thu, 07 Sep 2017 05:18:30 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43790) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dpsx6-0006YM-Tr for qemu-devel@nongnu.org; Thu, 07 Sep 2017 05:18:25 -0400 Date: Thu, 7 Sep 2017 10:18:16 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20170907091815.GB2098@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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: 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: Stefan Hajnoczi Cc: Peter Xu , "Daniel P. Berrange" , Laurent Vivier , Fam Zheng , Juan Quintela , qemu-devel , Markus Armbruster , Michael Roth , Paolo Bonzini * Stefan Hajnoczi (stefanha@gmail.com) wrote: > On Thu, Sep 7, 2017 at 9:13 AM, 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 wro= te: > >> > * 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 fr= om the > >> > > POV of the protocol definition. This interleaving is easy to han= dle from > >> > > the client POV - just requires a unique 'serial' in the request = by the > >> > > client, that is copied into the reply by QEMU. > >> > > >> > OK, so for that we can just take Marc-Andr=E9's syntax and call it= 'id': > >> > https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg03634.h= tml > >> > > >> > then it's upto the caller to ensure those id's are unique. > >> > >> Libvirt has in fact generated a unique 'id' for every monitor comman= d > >> since day 1 of supporting QMP. > >> > >> > I do worry about two things: > >> > a) With this the caller doesn't really know which commands could= 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. > >> > >> We could still have an opt-in for async commands. eg default to exec= uting > >> all commands in the main thread, unless the client issues an explici= t > >> "make it async" command, to switch to allowing the migration thread = to > >> process it async. > >> > >> { "execute": "qmp_allow_async", > >> "data": { "commands": [ > >> "migrate_cancel", > >> ] } } > >> > >> > >> { "return": { "commands": [ > >> "migrate_cancel", > >> ] } } > >> > >> The server response contains the subset of commands from the request > >> for which async is supported. > >> > >> That gives good negotiation ability going forward as we incrementall= y > >> support async on more commands. > > > > 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 > Please don't rush in a hack, they often introduce new bugs that we > have to support long-term when they are part of the QMP API. >=20 > In your original email you mentioned "info cpus". Have you considered > modifying this command so it does not sync the CPU? I'm not sure > callers really need to sync the CPU, typically they just want to know > the vcpu numbers, thread IDs, and current state (halted, running, > etc). But it has the pc as well, so that's actual state. Dave > The next step after that would be to audit other monitor commands for > unnecessary vcpu synchronization. >=20 > Stefan -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK