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

["Xu, Qian Q" <qian.q.xu@intel.com>]

My comments below. 

-----Original Message-----
From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Tan, Jianfeng
Sent: Wednesday, July 20, 2016 10:44 AM
To: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Cc: dev@dpdk.org; Wang, Zhihong <zhihong.wang@intel.com>
Subject: Re: [dpdk-dev] [PATCH] examples/vhost: fix perf regression

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.

>
> - 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?

Normally, when people enable TSO, they should turn on mergeable, if they don't turn on mergeable, then please don't expect high performance, 
so this is not a problem. They may get low performance due to the improper settings. 

As to a complete fix for the issue, we may need go back to the TSO feature design for vhost, currently, the feature negotiation code is in the application, 
but it's better to be considered in the vhost/virtio library so that application doesn't need to check/set the feature. But now it's too late for the complete fix, 
so the workaround is ok for this release from my view. 

Thanks,
Jianfeng

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