Re: [virtio-dev] Re: packed ring layout proposal v3
From: Jason Wang <hidden>
Date: 2017-09-13 01:26:32
On 2017年09月13日 00:23, Willem de Bruijn wrote:
On Mon, Sep 11, 2017 at 3:47 AM, Jason Wang [off-list ref] wrote:quoted
On 2017年09月10日 13:06, Michael S. Tsirkin wrote:quoted
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 _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization