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

[Tan, Jianfeng]

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 at intel.com>
>> Signed-off-by: Jianfeng Tan <jianfeng.tan at 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.

>
> - 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?

Thanks,
Jianfeng

>
> Thanks for the good root-cause, btw!
>
> 	--yliu