Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Sep 07, 2017 at 07:41:29PM +0200, Markus Armbruster wrote:
> "Daniel P. Berrange" <berrange@redhat.com> writes:
> 
> > On Thu, Sep 07, 2017 at 02:59:28PM +0200, Markus Armbruster wrote:
> >> So, what exactly is going to drain the command queue?  If there's more
> >> than one consumer, how exactly are commands from the queue dispatched to
> >> the consumers?
> >
> > In terms of my proposal, for any single command there should only ever
> > be a single consumer. The default consumer would be the main event loop
> > thread, such that we have no semantic change to QMP operation from today.
> >
> > Some commands that are capable of being made "async", would have a
> > different consumer. For example, if the client requested the 'migrate-cancel'
> > be made async, this would change things such that the migration thread is
> > now responsible for consuming the "migrate-cancel" command, instead of the
> > default main loop.
> >
> >> What are the "no hang" guarantees (if any) and conditions for each of
> >> these consumers?
> >
> > The non-main thread consumers would have to have some reasonable
> > guarantee that they won't block on a lock held by the main loop,
> > otherwise the whole feature is largely useless.
> 
> Same if they block indefinitely on anything else, actually.  In other
> words, we need to talk about liveness.
> 
> Threads by themselves don't buy us liveness.  Being careful with
> operations that may block does.  That care may lead to farming out
> certain operations to other threads, where they may block without harm.
> 
> You only talk about "the non-main thread consumers".  What about the
> main thread?  Is it okay for the main thread to block?  If yes, why?

It isn't ok, but I feel that challenge is intractable in the short to
medium term. Agree that having separate threads doesn't automatically
give liveness, but I think it makes the problem tractble to solve for
at least a subset of scenarios.

> > No, that is not what I described. All synchronous commands are
> > serialized wrt each other, just as today. An asychronous command
> > can run as soon as it is received, regardless of whether any
> > earlier sent sync commands are still executing or pending. This
> > is trivial to achieve when you separate monitor I/O from command
> > execution in separate threads, provided of course the async
> > command consumers are not in the main loop.
> 
> So, a synchronous command is synchronous with respect to other commands,
> except for certain non-blocking commands.  The distinctive feature of
> the latter isn't so much an asynchronous reply, but out-of-band
> dispatch.

The terminology synchronous vs asynchronous is not a great fit for
what I was describing. The distinction is really closer to being
serialized vs parallelizable commands.

> >                                                     This allows the
> > migration operation to be cancelled immediately, regardless of whether
> > there are earlier monitor commands blocked in the main loop.
> 
> The necessary part is moving all operations that can block out of
> whatever loop runs the monitor, be it the main loop, some other event
> loop, or a dedicated monitor thread's monitor loop.
> 
> Moving out non-blocking operations isn't necessary.  migrate-cancel
> could communicate with the migration thread by any suitable mechanism or
> protocol.  It doesn't have to be QMP.  Why would we want it to be QMP?

I don't think we really want to invent yet another way of controlling
QEMU, that isn't QMP do we, particularly not if it is special cased
to just one operationg ?

Regards,
Daniel
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