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

[Dr. David Alan Gilbert]

* Stefan Hajnoczi (address@hidden) wrote:
> On Fri, Sep 15, 2017 at 01:29:13PM +0100, Daniel P. Berrange wrote:
> > On Fri, Sep 15, 2017 at 01:19:56PM +0100, Dr. David Alan Gilbert wrote:
> > > * Daniel P. Berrange (address@hidden) wrote:
> > > > On Fri, Sep 15, 2017 at 01:06:44PM +0100, Dr. David Alan Gilbert wrote:
> > > > > * Daniel P. Berrange (address@hidden) 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?
> > 
> > Semantically I don't think it makes a difference if the OOB commands are
> > being processed sequentially by their thread. A >1 length queue would only
> > matter for non-OOB commands if an app was filling the pipeline with non-OOB
> > requests, as then that could block reading of OOB commands. 
> 
> To summarize:
> 
> The QMP server has a lookahead of 1 command so it can dispatch
> out-of-band commands.  If 2 or more non-OOB commands are queued at the
> same time then OOB processing will not occur.
> 
> Is that right?

I think my view is slightly more complex;
  a) There's a pair of queues for each channel
  b) There's a central pair of queues on the QMP server
    one for OOB commands and one for normal commands.
  c) Each queue is only really guaranteed to be one deep.

  That means that each one of the channels can send a non-OOB
command without getting in the way of a channel that wants
to send one.

Dave

> Stefan
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK