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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

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.

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.

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.