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

["Tan, Jianfeng" <jianfeng.tan@intel.com>]

On 7/20/2016 2:13 PM, Yuanhan Liu wrote:
> On Wed, Jul 20, 2016 at 01:50:34PM +0800, Tan, Jianfeng wrote:
>>
>> On 7/20/2016 12:38 PM, 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")
>>> And here you are patching vhost example to try to fix an "issue"
>>> in vhost lib, this is __logically__ wrong.
>>>
>>> 	--yliu
>> This is not an issue from vhost lib's perspective, vhost lib should provide
>> all features it supports by default.
> Bingo.., that's why "Fixes: 859b480d5afd ... " is wrong to me.
>    
>> Applications can enable/disable
>> features according to their own requirements.
> Yes, application can, but application normally doesn't do that. And
> as stated in my early reply, the qemu is the place you should go for
> all those options enabling/disabling, but not vhost (not vhost-example).
>
> I think it's sometimes more handy if we can do that by introducing
> some vhost-example options, and I guess that's why those options are
> given.
>
> In another word, there is nothing wrong about the commit 859b480d5afd,
> if you want to "fix" anything here, following commit is something
> we need fix:
>
>      Fixes: 9fd72e3cbd29 ("examples/vhost: add virtio offload")
>
> Because that commit just partially disables some TSO related features,
> letting the virtio net driver goes to the slow path.

Great, I see. And thanks for detailed clarification. I'll send v2.

>
>> And the vhost example after
>> this commit just triggers a slow path of virtio driver. So this fix just
>> makes sure vhost example does not go into the slow path by default.
> I have made a statement in the first time, that I am not object to
> have this patch at all.
>
> Meanwhile, the right "fix" is you need disable all TSO related features
> from QEMU, in such way, we should see no such issue from all vhost
> application, but not only this one, the one we used mostly internally.
>
> As you can see, it's more about the usage.

Yes, I agree this is the BKM we should adopt and recommend users to use.

Thanks,
Jianfeng

>
>> By the way, if a fix patch should only involve those commits it will change?
> IMO, logically, yes.
>
> 	--yliu