Re: [Qemu-devel] [PATCH v7 4/7] qapi: remove COMMAND_DROPPED event

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

* Markus Armbruster (armbru@redhat.com) wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > On Tue, Sep 04, 2018 at 08:39:27AM +0200, Markus Armbruster wrote:
> >> Daniel P. Berrangé <berrange@redhat.com> writes:
> >> 
> >> > On Mon, Sep 03, 2018 at 09:30:52AM -0500, Eric Blake wrote:
> >> >> On 09/03/2018 08:31 AM, Markus Armbruster wrote:

> >> >> I guess when we are designing what libvirt should do, and deciding WHEN it
> >> >> should send OOB commands, we have the luxury of designing libvirt to enforce
> >> >> how many in-flight in-band commands it will ever have pending at once
> >> >> (whether the current 'at most 1', or even if we make it more parallel to 'at
> >> >> most 7'), so that we can still be ensured that the OOB command will be
> >> >> processed without being stuck in the queue of suspended in-band commands.
> >> >> If we never send more than one in-band at a time, then it's not a concern
> >> >> how deep the qemu queue is; but if we do want libvirt to start parallel
> >> >> in-band commands, then you are right that having a way to learn the qemu
> >> >> queue depth is programmatically more precise than just guessing the maximum
> >> >> depth.  But it's also hard to argue we need that complexity if we don't have
> >> >> an immediate use envisioned for it.
> >> 
> >> True.
> >> 
> >> Options for the initial interface:
> >> 
> >> (1) Provide means for the client to determine the queue length limit
> >>     (introspection or configuration).  Clients that need the monitory to
> >>     remain available for out-of-band commands can keep limit - 1 in-band
> >>     commands in flight.
> >> 
> >> (2) Make the queue length limit part of the documented interface.
> >>     Clients that need the monitory to remain available for out-of-band
> >>     commands can keep limit - 1 in-band commands in flight.  We can
> >>     increase the limit later, but not decrease it.  We can also switch
> >>     to (1) as needed.
> >> 
> >> (3) Treat the queue length limit as implementation detail (but tacitly
> >>     assume its at least 2, since less makes no sense[*]).  Clients that
> >>     need the monitory to remain available for out-of-band commands
> >>     cannot safely keep more than one in-band command in flight.  We can
> >>     switch to (2) or (1) as needed.
> >> 
> >> Opinions?
> >
> > If you did (3), effectively apps will be behaving as if you had done
> > (2) with a documented queue limit of 2, so why not just do (2) right
> > away.
> 
> Yup, that's what I thought, too.
> 
> I append a proposed replacement for the patch to qmp-spec.txt.  It
> assumes the current queue size 8.  That value is of course open to
> debate.

Could you return that size in the response to qmp_capabilities
at the start of the connection?

> 
> diff --git a/docs/interop/qmp-spec.txt b/docs/interop/qmp-spec.txt
> index 67e44a8120..1fc6a507e2 100644
> --- a/docs/interop/qmp-spec.txt
> +++ b/docs/interop/qmp-spec.txt
> @@ -130,9 +130,11 @@ to pass "id" with out-of-band commands.  Passing it with all commands
>  is recommended for clients that accept capability "oob".
>  
>  If the client sends in-band commands faster than the server can
> -execute them, the server will stop reading the requests from the QMP
> -channel until the request queue length is reduced to an acceptable
> -range.
> +execute them, the server's queue will fill up, and the server will
> +stop reading commands from the QMP channel until the queue has space
> +again.  Clients that need the server to remain responsive to
> +out-of-band commands should avoid filling up the queue by limiting the
> +number of in-band commands in flight to seven.

If I understand what you're saying then this is a shared limit; i.e.
if you've got two QMP connections then you have to be aware of how many
the other connection is queuing, which is tricky.

Dave

>  Only a few commands support out-of-band execution.  The ones that do
>  have "allow-oob": true in output of query-qmp-schema.
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK