Re: [Xen-devel] [Hackathon minutes] PV block improvements

[Matt Wilson <msw@xxxxxxxxxx>]

On Tue, Jun 25, 2013 at 02:01:30PM +0100, Wei Liu wrote:
> On Mon, Jun 24, 2013 at 11:09:19PM -0700, Matt Wilson wrote:
> > On Sat, Jun 22, 2013 at 09:11:20AM +0200, Roger Pau Monné wrote:
> > > On 21/06/13 20:07, Matt Wilson wrote:
> > > > On Fri, Jun 21, 2013 at 07:10:59PM +0200, Roger Pau Monné wrote:
> > 
> > [...]
> > 
> > > >> 2. Switch to grant copy in blkback, and get rid of persistent grants (I
> > > >> have not benchmarked this solution, but I'm quite sure it will involve 
> > > >> a
> > > >> performance regression, specially when scaling to a high number of 
> > > >> domains).
> > > > 
> > > > Why do you think so?
> > > 
> > > First because grant_copy is done by the hypervisor, while when using
> > > persistent grants the copy is done by the guest. Also, grant_copy takes
> > > the grant lock, so when scaling to a large number of domains there's
> > > going to be contention around this lock. Persistent grants don't need
> > > any shared lock, and thus scale better.
> > 
> > It'd benefit xen-netback to make the locking in the copy path more
> > fine grained. That would help multi-vif domUs today, and multi-queue
> > vifs later on.
> > 
> 
> I'm not sure I follow. I presume you mean using persistent grant in
> xen-netback to help scale better?

No, I mean further scaling improvements in the GNTTABOP_copy path
would benefit xen-netback performance when a single guest has multiple
vifs, and will be needed for good multi-queue performance. Given we
might need to do some work there, would it make sense to change
blkback to use GNTTABOP_copy to avoid the problem he's identified with
persistent grants.

--msw

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