On Wed, Sep 27, 2017 at 10:04:18AM +0800, Jason Wang wrote:

On 2017年09月27日 03:25, Michael S. Tsirkin wrote:

quoted

On Fri, Sep 22, 2017 at 04:02:35PM +0800, Jason Wang wrote:

quoted

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

quoted

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.

Right. So if you do

for (...)
	vhost_get_vq_desc


then later

for (...)
	vhost_add_used


then you get most of benefits except maybe code cache misses
and copy_to_user.

quoted

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

IUC callbacks pretty much destroy the code cache locality advantages,
IP is jumping around too much.


-- 
MST