Re: [Qemu-devel] [RFC 00/15] QMP: out-of-band (OOB) execution support

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrange (berrange@redhat.com) wrote:
> 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-André Lureau wrote:
> > > > > > > There should be a limit in the number of requests the thread can
> > > > > > > queue. Before the patch, the limit was enforced by system socket
> > > > > > > buffering I think. Now, should oob commands still be processed even if
> > > > > > > the queue is full? If so, the thread can't be suspended.
> > > > > > 
> > > > > > I agree.
> > > > > > 
> > > > > > Memory usage must be bounded.  The number of requests is less important
> > > > > > than the amount of memory consumed by them.
> > > > > > 
> > > > > > Existing QMP clients that send multiple QMP commands without waiting for
> > > > > > replies need to rethink their strategy because OOB commands cannot be
> > > > > > processed if queued non-OOB commands consume too much memory.
> > > > > 
> > > > > Thanks for pointing out this.  Yes the memory usage problem is valid,
> > > > > as Markus pointed out as well in previous discussions (in "Flow
> > > > > Control" section of that long reply).  Hopefully this series basically
> > > > > can work from design prospective, then I'll add this flow control in
> > > > > next version.
> > > > > 
> > > > > 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 all,
> > > > > 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.
> > > > > 
> > > > > 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,
> > > > 
> > > > Another solution: the server reports available receive buffer space to
> > > > the client.  The server only guarantees immediate OOB processing when
> > > > the client stays within the receive buffer size.
> > > > 
> > > > Clients wishing to take advantage of OOB must query the receive buffer
> > > > size and make sure to leave enough room.
> > > 
> > > I don't think having to query it ahead of time is particularly nice,
> > > and of course it is inherantly racy.
> > > 
> > > I would just have QEMU emit an event when it pausing processing of the
> > > 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.
> > 
> > Hmm and now we're implementing flow control!
> > 
> > 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.

Lets just define it so that it can't - you send an OOB command and wait
for it's response before sending another on that channel.

> 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.

Can you justify 10 as opposed to 1?

Dave

> Regards,
> Daniel
> -- 
> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org         -o-            https://fstop138.berrange.com :|
> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK