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

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

On Tue, Jul 19, 2016 at 01:53:11PM +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 root cause is that below commit adds support for
> VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6, and when
> mergeable is disabled, it triggers big_packets path of virtio-net
> driver. In this path, virtio driver uses 19 desc with 18 4K-sized
> pages to receive each packet, so that it can receive a big packet
> with size of 64K. 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: 859b480d5afd ("vhost: add guest offload setting")
> 
> Reported-by: Qian Xu <qian.q.xu@intel.com>
> Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>

We could apply this patch, but I don't think it actually fix anything:

- it doesn't fix other vhost applications, say OVS, which is for sure
  way more widly used than vhost-example.

- it doesn't even fix it when tso is enabled and mergeable-rx is disabled
  with this vhost-example.

Thanks for the good root-cause, btw!

	--yliu