From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41702) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dpuqR-0000AO-Cr for qemu-devel@nongnu.org; Thu, 07 Sep 2017 07:19:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dpuqO-0002y4-6t for qemu-devel@nongnu.org; Thu, 07 Sep 2017 07:19:39 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46026) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dpuqN-0002xi-Tj for qemu-devel@nongnu.org; Thu, 07 Sep 2017 07:19:36 -0400 From: Markus Armbruster 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> <20170907091946.GC2098@work-vm> Date: Thu, 07 Sep 2017 13:19:28 +0200 In-Reply-To: <20170907091946.GC2098@work-vm> (David Alan Gilbert's message of "Thu, 7 Sep 2017 10:19:47 +0100") Message-ID: <87fubybvsv.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 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: "Daniel P. Berrange" , Laurent Vivier , Fam Zheng , Juan Quintela , mdroth@linux.vnet.ibm.com, Peter Xu , qemu-devel@nongnu.org, Paolo Bonzini "Dr. David Alan Gilbert" writes: > * 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 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. >> > > >=20 >> > > > OK, so for that we can just take Marc-Andr=C3=A9's syntax and call= it 'id': >> > > > https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg03634.h= tml >> > > >=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 command >> > > since day 1 of supporting QMP. >> > >=20 >> > > > 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. >> > >=20 >> > > 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 explicit >> > > "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 request >> > > for which async is supported. >> > >=20 >> > > That gives good negotiation ability going forward as we incrementally >> > > 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 include >> > 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. "Rarely"? Are main loop hangs *that* common? Can we quantify the problem to help gauge urgency?