Re: [virtio-dev] Re: packed ring layout proposal v3

[Jason Wang <jasowang@redhat.com>]

On 2017年09月13日 00:23, Willem de Bruijn wrote:
> On Mon, Sep 11, 2017 at 3:47 AM, Jason Wang <jasowang@redhat.com> wrote:
> > On 2017年09月10日 13:06, Michael S. Tsirkin wrote:
> > > This is an update from v2 version.
> > > Changes:
> > > - update event suppression mechanism
> > > - add wrap counter: DESC_WRAP flag in addition to
> > >     DESC_DRIVER flag used for validity so device does not have to
> > >     write out all used descriptors.
> >
> > Do we have benchmark result to show the advantage of DESC_DRIVER over e.g
> > avail/used index?
> The KVM forum presentation has some numbers.

Yes. My question may be not accurate. I meant maybe we should benchmark 
packed ring layout without DESC_DRIVER but something like queue 
tail/head or producer/consumer (or whatever it called). Looks like most 
more nic does not use a flag inside descriptor to exam the descriptor 
ownership.

> I'm not sure that synthetic benchmarks will provide much value, as we
> understand the trade-off quite well.
>
> The benefit of this model is improved best case performance, by having
> a single cacheline read instead of two for the indirect used/avail ring model.
>
> The drawback is worse worst case, as scanning the ring of descriptors
> introduces more cacheline misses than scanning the compressed
> used/avail ring.

Like I've replied, looks like the scanning is not friendly to batching 
or prefetching and can cause extra overheads.

> This model is easier to implement in hardware and the common case is
> likely close to the best case, so I think it makes sense.

Maybe, but we probably need inputs from hardware vendor guys.

Thanks