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

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

On Wed, Jul 20, 2016 at 10:44:13AM +0800, Tan, Jianfeng wrote:
> Hi Yuanhan,
> 
> On 7/20/2016 9:44 AM, Yuanhan Liu wrote:
> >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.
> 
> If I remember it correctly, OVS will enable mergeable.

Yes, and actually, vhost-example also should have enabled it by default.
Meanwhile, all features could be enabled/disabled by user.

> >
> >- it doesn't even fix it when tso is enabled and mergeable-rx is disabled
> >   with this vhost-example.
> 
> But we'd better avoid users go into such doubt that performance drops
> because of that commit under the case tso=off,mergeable=off, right?

I doubt people would actually use vhost-example (besides developer like
us), meaning they can NOT see the benifit from this patch; it also means
that user __does__ go into doubt that performance drops for the case
tso=off,mergeable=off.

Actually, it looks wrong to me to fiddle with those flags in the vhost-example.
If you want to disable tso, you should go disable it on the qemu side,
with something like:

    csum=off,gso=off,guest_tso4=off,guest_tso6=off,...

	--yliu