From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52417) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dspWG-0004iT-IC for qemu-devel@nongnu.org; Fri, 15 Sep 2017 08:14:53 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dspWC-0003l8-HD for qemu-devel@nongnu.org; Fri, 15 Sep 2017 08:14:52 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52786) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dspWC-0003kY-8K for qemu-devel@nongnu.org; Fri, 15 Sep 2017 08:14:48 -0400 Date: Fri, 15 Sep 2017 13:14:33 +0100 From: "Daniel P. Berrange" Message-ID: <20170915121433.GI13610@redhat.com> Reply-To: "Daniel P. Berrange" References: <1505375436-28439-1-git-send-email-peterx@redhat.com> <20170914151911.GB7370@stefanha-x1.localdomain> <20170915035057.GM3617@pxdev.xzpeter.org> <20170915104926.GA14994@stefanha-x1.localdomain> <20170915113428.GF13610@redhat.com> <20170915120643.GN2170@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20170915120643.GN2170@work-vm> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [RFC 00/15] QMP: out-of-band (OOB) execution support List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: Stefan Hajnoczi , Peter Xu , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , QEMU , Paolo Bonzini , Stefan Hajnoczi , Fam Zheng , Juan Quintela , Michael Roth , Eric Blake , Laurent Vivier , Markus Armbruster On Fri, Sep 15, 2017 at 01:06:44PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrange (berrange@redhat.com) wrote: > > On Fri, Sep 15, 2017 at 11:49:26AM +0100, Stefan Hajnoczi wrote: > > > On Fri, Sep 15, 2017 at 11:50:57AM +0800, Peter Xu wrote: > > > > On Thu, Sep 14, 2017 at 04:19:11PM +0100, Stefan Hajnoczi wrote: > > > > > On Thu, Sep 14, 2017 at 01:15:09PM +0200, Marc-Andr=C3=A9 Lurea= u wrote: > > > > > > There should be a limit in the number of requests the thread = can > > > > > > queue. Before the patch, the limit was enforced by system soc= ket > > > > > > buffering I think. Now, should oob commands still be processe= d even if > > > > > > the queue is full? If so, the thread can't be suspended. > > > > >=20 > > > > > I agree. > > > > >=20 > > > > > Memory usage must be bounded. The number of requests is less i= mportant > > > > > than the amount of memory consumed by them. > > > > >=20 > > > > > Existing QMP clients that send multiple QMP commands without wa= iting for > > > > > replies need to rethink their strategy because OOB commands can= not be > > > > > processed if queued non-OOB commands consume too much memory. > > > >=20 > > > > Thanks for pointing out this. Yes the memory usage problem is va= lid, > > > > as Markus pointed out as well in previous discussions (in "Flow > > > > Control" section of that long reply). Hopefully this series basi= cally > > > > can work from design prospective, then I'll add this flow control= in > > > > next version. > > > >=20 > > > > Regarding to what we should do if the limit is reached: Markus > > > > provided a few options, but the one I prefer most is that we don'= t > > > > respond, but send an event showing that a command is dropped. > > > > However, I would like it not queued, but a direct reply (after al= l, > > > > it's an event, and we should not need to care much on ordering of= it). > > > > Then we can get rid of the babysitting of those "to be failed" > > > > requests asap, meanwhile we don't lose anything IMHO. > > > >=20 > > > > I think I also missed at least a unit test for this new interface= . > > > > Again, I'll add it after the whole idea is proved solid. Thanks, > > >=20 > > > Another solution: the server reports available receive buffer space= to > > > the client. The server only guarantees immediate OOB processing wh= en > > > the client stays within the receive buffer size. > > >=20 > > > Clients wishing to take advantage of OOB must query the receive buf= fer > > > size and make sure to leave enough room. > >=20 > > I don't think having to query it ahead of time is particularly nice, > > and of course it is inherantly racy. > >=20 > > I would just have QEMU emit an event when it pausing processing of th= e > > incoming commands due to a full queue. If the event includes the ID > > of the last queued command, the client will know which (if any) of > > its outstanding commands are delayed. Another even can be sent when > > it restarts reading. >=20 > Hmm and now we're implementing flow control! >=20 > a) What exactly is the current semantics/buffer sizes? > b) When do clients send multiple QMP commands on one channel without > waiting for the response to the previous command? > c) Would one queue entry for each class of commands/channel work > (Where a class of commands is currently 'normal' and 'oob') I do wonder if we need to worry about request limiting at all from the client side. For non-OOB commands clients will wait for a reply before sending a 2nd non-OOB command, so you'll never get a deep queue for. OOB commands are supposed to be things which can be handled quickly without blocking, so even if a client sent several commands at once without waiting for replies, they're going to be processed quickly, so whether we temporarily block reading off the wire is a minor detail. IOW, I think we could just have a fixed 10 command queue and apps just pretend that there's an infinite queue and nothing bad would happen from the app's POV. 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 :|