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


From: Peter Xu
Subject: Re: [Qemu-devel] [RFC v2 4/8] QAPI: new QMP command option "without-bql"
Date: Mon, 28 Aug 2017 16:26:08 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

On Fri, Aug 25, 2017 at 10:06:27AM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (address@hidden) 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.

I see.  What's the corresponding QMP documentation I should touch up?

> 
> > 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?

I'm ok with it.  But would the word "lock" too general?

-- 
Peter Xu



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