Re: [Qemu-devel] [QA-virtio]:Why vring size is limited to 1024?

[Michael S. Tsirkin]

On Wed, Oct 08, 2014 at 12:51:21PM +0300, Avi Kivity wrote:
> 
> On 10/08/2014 12:15 PM, Michael S. Tsirkin wrote:
> >On Wed, Oct 08, 2014 at 10:43:07AM +0300, Avi Kivity wrote:
> >>On 09/30/2014 12:33 PM, Michael S. Tsirkin wrote:
> >>>a single descriptor might use all of
> >>>the virtqueue. In this case we wont to be able to pass the
> >>>descriptor directly to linux as a single iov, since
> >>>
> >>You could separate maximum request scatter/gather list size from the
> >>virtqueue size.  They are totally unrelated - even now you can have a larger
> >>request by using indirect descriptors.
> >We could add a feature to have a smaller or larger S/G length limit.
> >Is this something useful?
> >
> 
> Having a larger ring size is useful, esp. with zero-copy transmit, and you
> would need the sglist length limit in order to not require linearization on
> linux hosts.  So the limit is not useful in itself, only indirectly.
> 
> Google cloud engine exposes virtio ring sizes of 16384.

OK this sounds useful, I'll queue this up for consideration.
Thanks!

> Even more useful is getting rid of the desc array and instead passing descs
> inline in avail and used.

You expect this to improve performance?
Quite possibly but this will have to be demonstrated.

-- 
MST