Messages in this thread | | | Subject | Re: [PATCH net-next RFC 5/5] vhost_net: basic tx virtqueue batched processing | From | Jason Wang <> | Date | Wed, 27 Sep 2017 10:04:18 +0800 |
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On 2017年09月27日 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@redhat.com> > 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
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