Re: [Qemu-devel] [RFC v6 18/27] monitor: send event when command queue full

[Peter Xu <peterx@redhat.com>]

On Mon, Dec 25, 2017 at 03:13:49PM +0800, Fam Zheng wrote:
> On Mon, 12/25 14:18, Peter Xu wrote:
> > On Mon, Dec 25, 2017 at 01:55:56PM +0800, Fam Zheng wrote:
> > > On Mon, 12/25 13:18, Peter Xu wrote:
> > > > On Thu, Dec 21, 2017 at 07:42:46PM +0800, Fam Zheng wrote:
> > > > > On Tue, 12/19 16:45, Peter Xu wrote:
> > > > > > Set maximum QMP command queue length to 8.  If queue full, instead of
> > > > > > queue the command, we directly return a "command-dropped" event, telling
> > > > > > client that specific command is dropped.
> > > > > > 
> > > > > > Note that this flow control mechanism is only valid if OOB is enabled.
> > > > > > If it's not, the effective queue length will always be 1, which strictly
> > > > > > follows original behavior of QMP command handling (which never drop
> > > > > > messages).
> > > > > > 
> > > > > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > > > > ---
> > > > > >  monitor.c | 17 ++++++++++++++++-
> > > > > >  1 file changed, 16 insertions(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/monitor.c b/monitor.c
> > > > > > index ed9a741d06..b571866659 100644
> > > > > > --- a/monitor.c
> > > > > > +++ b/monitor.c
> > > > > > @@ -4038,6 +4038,8 @@ static void monitor_qmp_bh_dispatcher(void *data)
> > > > > >      }
> > > > > >  }
> > > > > >  
> > > > > > +#define  QMP_REQ_QUEUE_LEN_MAX  (8)
> > > > > 
> > > > > Is this limit introspectable on QMP?
> > > > 
> > > > Not yet.  IMHO it's really QMP internal stuff, and I see no benefit so
> > > > far for a client to know about this...
> > > 
> > > A client may need this number to batch (non-oob) commands without worrying about
> > > getting command-dropped event.
> > 
> > IMHO QMP batching will only be useful when performance matters, but
> > for now IMHO we don't need to worry much about QMP performance?  When
> > we do, I suspect we need to do more things to make sure of it, and
> > exposing this single parameter may not really help much, say, for now
> > even the client batches the stuff, they still need to wait for the
> > completion of commands.
> 
> I think when we declare that commands can be dropped when the queue is full, we
> should be clear about in what condition a queue is full, and don't make users
> guess. If we want is this incompleteness, and I'm the only one who doesn't like
> it, that may be fine. It's just that, like you said, this seems a bit useless.

Just to clarify that this maximum queue length is still useful to us
(not the clients) in the sense that we'll have limited memory usage
for QMP command queues.  Otherwise we have risk on using up all the
memory of the host or reach the limitation of existing QEMU process,
either of which is bad.

-- 
Peter Xu