On 2017年09月28日 06:19, Michael S. Tsirkin wrote:
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:
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.
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.
If you mean vhost_add_used_n(), then more barriers and userspace memory
access as well. And you still need to cache vring_used_elem in vq->heads
or elsewhere. Since you do not batch reading avail ring indexes, more
write miss will happen if the ring is about to be full. So it looks
slower than what is done in this patch?
And if we want to indo more batching on top (do we want this?), we still
need new APIs anyway.
Thanks
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.
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