Re: [Xen-devel] [PATCH] xen: add persistent grants to xbd_xenbus

[Roger Pau Monné <roger.pau%citrix.com@localhost>]

On 12/11/12 16:34, Ian Campbell wrote:
> On Mon, 2012-11-12 at 09:30 +0000, Roger Pau Monné wrote:
>>
>> I will try to perform a similar test with a NetBSD Dom0, but with the
>> patch it will probably be a little bit slower than without it (because
>> we have to perform the memcpy in the frontend, 
> 
> Doesn't the frontend fallback to the same old code as always if the b/e
> doesn't advertise the new feature?

No, the frontend always uses persistent grants, since it's compatible
with a backend not using persistent grants, the frontend will reuse the
same grants, and the backend will use the old mechanism of mapping and
unmapping them.

Actually it seems I was wrong on that, using a persistent frontend with
a non-persistent backend lets to a more consistent performance, see the
following graph:

http://xenbits.xen.org/people/royger/persistent_grants/nonpers_read.png