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

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

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>

Thanks.

	--yliu