Re: [PATCH v3] examples/vhost: fix perf regression

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2016-07-21 09:34, Yuanhan Liu:
> On Thu, Jul 21, 2016 at 12:42:45AM +0000, Jianfeng Tan wrote:
> > We find significant perfermance drop introduced by below commit,
> > when vhost example is started with --mergeable 0 and inside vm,
> > kernel virtio-net driver is used to do ip based forwarding.
> > 
> > The commit, 859b480d5afd ("vhost: add guest offload setting"), adds
> > support for VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6,
> > in vhost lib. But inside vhost example, the way to disable tso only
> > excludes the direction from virtio to vhost, but not the opposite
> > direction. When mergeable is disabled, it triggers big_packets path
> > of virtio-net driver to prepare to receive possible big packets with
> > size of 64K. Because mergeable is off, for each entry of avail ring,
> > virtio driver uses 19 desc chained together, with one desc pointing
> > to header, other 18 desc pointing to 4K-sized pages. But QEMU only
> > creates 256 desc entries for each vq, which results in that only 13
> > packets can be received. VM kernel can quickly handle those packets
> > and go to sleep (HLT).
> > 
> > As QEMU has no option to set the desc entries of a vq, so here,
> > we disable VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6
> > with VIRTIO_NET_F_HOST_TSO4 and VIRTIO_NET_F_HOST_TSO6 when we
> > disable tso of vhost example, to avoid VM kernel virtio driver
> > go into big_packets path.
> > 
> > Fixes: 9fd72e3cbd29 ("examples/vhost: add virtio offload")
> > 
> > Reported-by: Qian Xu <qian.q.xu@intel.com>
> > Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
> > ---
> > v3: reword commit log.
> 
> Yes, much better. One minor nit: you forgot to carry the Tested-by from
> Qian.
> 
> Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>

Applied, thanks