Re: [PATCH] kvm tools: Support virtio indirect buffers

[Sasha Levin <levinsasha928@gmail.com>]

On Mon, 2011-11-28 at 22:17 +0200, Sasha Levin wrote:
> On Mon, 2011-11-28 at 20:49 +0200, Pekka Enberg wrote:
> > On Mon, Nov 28, 2011 at 7:54 PM, Sasha Levin <levinsasha928@gmail.com> wrote:
> > > Indirect buffers are ring descriptors which point to more (even more)
> > > descriptors.
> > >
> > > This can be used to increase the effective ring capacity, which helps the
> > > guest to batch large requests - very useful for blk devices.
> > >
> > > This patch also enables indirect buffers for virtio-net and virtio-blk.
> > >
> > > The patch is based on the lguest's code which does the same.
> > >
> > > Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
> > 
> > In what exact way is it useful? Improved throughput? Will this have
> > negative impact on virtio block or virtio net latency?
> 
> The total size of requests is limited by the size of the virtio ring.
> This makes it hard to squeeze large requests (like the ones you usually
> see with blk devices) into the ring. This patch simply makes each entry
> in the virtio ring point to another descriptor list, thus it allows to
> squeeze much more requests into a singe virtio ring.
> 
> It shouldn't hurt latency.
> 
> I tried getting benchmarks with it, but the results I get from fio are
> all over the place and I can't seem to get a steady result (bad sign for
> my poor spindle disk).

btw, on an unrelated subject, I think that with this patch we've fully
covered the virtio spec, and as far as I know it's the first userspace
implementation which covers the entire spec :)

-- 

Sasha.