Re: [PATCH net-next RFC 5/5] vhost_net: basic tx virtqueue batched processing

[Jason Wang]

On 2017å09æ27æ 03:25, Michael S. Tsirkin wrote:
> On Fri, Sep 22, 2017 at 04:02:35PM +0800, Jason Wang wrote:
> > This patch implements basic batched processing of tx virtqueue by
> > prefetching desc indices and updating used ring in a batch. For
> > non-zerocopy case, vq->heads were used for storing the prefetched
> > indices and updating used ring. It is also a requirement for doing
> > more batching on top. For zerocopy case and for simplicity, batched
> > processing were simply disabled by only fetching and processing one
> > descriptor at a time, this could be optimized in the future.
> >
> > XDP_DROP (without touching skb) on tun (with Moongen in guest) with
> > zercopy disabled:
> >
> > Intel(R) Xeon(R) CPU E5-2650 0 @ 2.00GHz:
> > Before: 3.20Mpps
> > After:  3.90Mpps (+22%)
> >
> > No differences were seen with zerocopy enabled.
> >
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> So where is the speedup coming from? I'd guess the ring is
> hot in cache, it's faster to access it in one go, then
> pass many packets to net stack. Is that right?
>
> Another possibility is better code cache locality.

Yes, I think the speed up comes from:

- less cache misses
- less cache line bounce when virtqueue is about to be full (guest is 
faster than host which is the case of MoonGen)
- less memory barriers
- possible faster copy speed by using copy_to_user() on modern CPUs

> So how about this patchset is refactored:
>
> 1. use existing APIs just first get packets then
>     transmit them all then use them all

Looks like current API can not get packets first, it only support get 
packet one by one (if you mean vhost_get_vq_desc()). And used ring 
updating may get more misses in this case.

> 2. add new APIs and move the loop into vhost core
>     for more speedups

I don't see any advantages, looks like just need some e.g callbacks in 
this case.

Thanks