Re: [Qemu-devel] [RFC v2 4/8] QAPI: new QMP command option "without-bql"

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

* Peter Xu (peterx@redhat.com) wrote:
> On Wed, Aug 23, 2017 at 06:44:12PM +0100, Dr. David Alan Gilbert wrote:
> 
> [...]
> 
> > > +Most of the commands require the Big QEMU Lock (BQL) be held during
> > > +execution.  However, there is a small subset of the commands that may
> > > +not really need BQL at all.  To mark out this kind of commands, we can
> > > +specify "without-bql" to "true".  This parameter is only a hint for
> > > +internal QMP implementation to provide possiblility to allow commands
> > > +be run in parallel, or reduce the contention of the lock.  Users of QMP
> > > +should not really be aware of such information.
> > 
> > Well, I think users of these commands might select them specifically
> > because they know that they won't block.  Those who care about latency might
> > look to use commands that don't take the lock because of a reduced
> > effect on the performance as well.
> 
> What would be the best way to tell user?  I think again this should
> mostly for HMP only, right?

It needs to be docuemnted for QMP users as well so that those developing
management code know what's safe.

> Maybe we can add a new command to list these lock-free commands.  Or,
> I can dump something in "help" and "help info" like:
> 
> (qemu) help migrate_incoming
> migrate_incoming uri -- Continue an incoming migration from an -incoming defer (BQL-less)

'lock free' might be better?

Dave

> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK