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

[Peter Xu]

On Mon, Sep 03, 2018 at 03:41:16PM +0100, Daniel P. Berrangé wrote:
> On Mon, Sep 03, 2018 at 09:30:52AM -0500, Eric Blake wrote:
> > On 09/03/2018 08:31 AM, Markus Armbruster wrote:
> > 
> > > Example:
> > > 
> > >      client sends in-band command #1
> > >      QEMU reads and queues
> > >      QEMU dequeues in-band command #1
> > >      in-band command #1 starts executing, but it's slooow
> > >      client sends in-band command #2
> > >      QEMU reads and queues
> > >      ...
> > >      client sends in-band command #8
> > >      QEMU reads, queues and suspends the monitor
> > >      client sends out-of-band command
> > > --> time passes...
> > >      in-band command #1 completes, QEMU sends reply
> > >      QEMU dequeues in-band command #2, resumes the monitor
> > >      in-band command #2 starts executing
> > >      QEMU reads and executes out-of-band command
> > >      out-of-band command completes, QEMU sends reply
> > >      in-band command #2 completes, QEMU sends reply
> > >      ... same for remaining in-band commands ...
> > > 
> > > The out-of-band command gets stuck behind the in-band commands.

(It's a shame of me to have just noticed that the out-of-band command
 will be stuck after we dropped the COMMAND_DROP event... so now I
 agree it's not that ideal any more to drop the event but maybe still
 preferable)

> > > 
> > > The client can avoid this by managing the flow of in-band commands: have
> > > no more than 7 in flight, so the monitor never gets suspended.
> > > 
> > > This is a potentially useful thing to do for clients, isn't it?
> > > 
> > > Eric, Daniel, is it something libvirt would do?
> > 
> > Right now, libvirt serializes commands - it never sends a QMP command until
> > the previous command's response has been processed. But that may not help
> > much, since libvirt does not send OOB commands.
> 
> Note that is not merely due to the QMP monitor restriction either.
> 
> Libvirt serializes all its public APIs that can change state of a running
> domain.  It usually aims to allow read-only APIs to be run in parallel with
> APIs that change state.
> 
> The exception to the rule right now are some of the migration APIs which
> we allow to be invoked to manage the migration process. 
> 
> > 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.
> 
> In terms of what libvirt would want to parallelize, I think it is reasonable
> to consider any of the query-XXXX commands desirable. Other stuff is likely
> to remain serialized from libvirt's side.

IMHO concurrency won't help much now even for query commands, since
our current concurrency is still "partly" - the executions of query
commands (which is the most time consuming part) will still be done
sequentially, so even if we send multiple query commands in parallel
(without waiting for a response of any sent commands), the total time
used for the list of commands would be mostly the same.

My understanding for why we have such a queue length now is that it
came from a security concern: after we have a queue, we need that
queue length to limit the memory usages for the QMP server.  Though
that might not help much for real users like Libvirt, it's majorly
serving as a way to protect QEMU QMP from being attacked or from being
turned down by a buggy QMP client.

But I agree now that the queue length information might still be
helpful some day.  Maybe, we can hide that until we support executing
commands in parallel for some of them.

Regards,

-- 
Peter Xu