From: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>
Date: Fri, 6 Oct 2017 13:22:31 -0400
> From: Willem de Bruijn <willemb@xxxxxxxxxx>
>
> Vhost-net has a hard limit on the number of zerocopy skbs in flight.
> When reached, transmission stalls. Stalls cause latency, as well as
> head-of-line blocking of other flows that do not use zerocopy.
>
> Instead of stalling, revert to copy-based transmission.
>
> Tested by sending two udp flows from guest to host, one with payload
> of VHOST_GOODCOPY_LEN, the other too small for zerocopy (1B). The
> large flow is redirected to a netem instance with 1MBps rate limit
> and deep 1000 entry queue.
>
> modprobe ifb
> ip link set dev ifb0 up
> tc qdisc add dev ifb0 root netem limit 1000 rate 1MBit
>
> tc qdisc add dev tap0 ingress
> tc filter add dev tap0 parent ffff: protocol ip \
> u32 match ip dport 8000 0xffff \
> action mirred egress redirect dev ifb0
>
> Before the delay, both flows process around 80K pps. With the delay,
> before this patch, both process around 400. After this patch, the
> large flow is still rate limited, while the small reverts to its
> original rate. See also discussion in the first link, below.
>
> Without rate limiting, {1, 10, 100}x TCP_STREAM tests continued to
> send at 100% zerocopy.
>
> The limit in vhost_exceeds_maxpend must be carefully chosen. With
> vq->num >> 1, the flows remain correlated. This value happens to
> correspond to VHOST_MAX_PENDING for vq->num == 256. Allow smaller
> fractions and ensure correctness also for much smaller values of
> vq->num, by testing the min() of both explicitly. See also the
> discussion in the second link below.
>
> Changes
> v1 -> v2
> - replaced min with typed min_t
> - avoid unnecessary whitespace change
>
> Link:http://lkml.kernel.org/r/CAF=yD-+Wk9sc9dXMUq1+x_hh=3ThTXa6BnZkygP3tgVpjbp93g@xxxxxxxxxxxxxx
> Link:http://lkml.kernel.org/r/20170819064129.27272-1-den@xxxxxxxxxxxxx
> Signed-off-by: Willem de Bruijn <willemb@xxxxxxxxxx>
Applied, thanks Willem.
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