Re: [PATCH net-next] vhost_net: batch used ring update in rx

[David Miller <davem@davemloft.net>]

From: Jason Wang <jasowang@redhat.com>
Date: Tue,  9 Jan 2018 18:27:45 +0800

> This patch tries to batched used ring update during RX. This is pretty
> fit for the case when guest is much faster (e.g dpdk based
> backend). In this case, used ring is almost empty:
> 
> - we may get serious cache line misses/contending on both used ring
>   and used idx.
> - at most 1 packet could be dequeued at one time, batching in guest
>   does not make much effect.
> 
> Update used ring in a batch can help since guest won't access the used
> ring until used idx was advanced for several descriptors and since we
> advance used ring for every N packets, guest will only need to access
> used idx for every N packet since it can cache the used idx. To have a
> better interaction for both batch dequeuing and dpdk batching,
> VHOST_RX_BATCH was used as the maximum number of descriptors that
> could be batched.
> 
> Test were done between two machines with 2.40GHz Intel(R) Xeon(R) CPU
> E5-2630 connected back to back through ixgbe. Traffic were generated
> on one remote ixgbe through MoonGen and measure the RX pps through
> testpmd in guest when do xdp_redirect_map from local ixgbe to
> tap. RX pps were increased from 3.05 Mpps to 4.00 Mpps (about 31%
> improvement).
> 
> One possible concern for this is the implications for TCP (especially
> latency sensitive workload). Result[1] does not show obvious changes
> for most of the netperf test (RR, TX, and RX). And we do get some
> improvements for RX on some specific size.
 ...
> Signed-off-by: Jason Wang <jasowang@redhat.com>

Applied, thanks Jason.