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

[Matt Wilson <msw@xxxxxxxxxx>]

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.

Thoughts?

--msw


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