[dpdk-dev] [PATCH] virtio: tx with can_push when VERSION_1 is set

[Pierre Pfister (ppfister)]

> Le 22 nov. 2016 à 14:17, Maxime Coquelin <maxime.coquelin at redhat.com> a écrit :
> 
> Hi Pierre,
> 
> On 11/22/2016 10:54 AM, Pierre Pfister (ppfister) wrote:
>> Hello Maxime,
>> 
>>> Le 9 nov. 2016 à 15:51, Maxime Coquelin <maxime.coquelin at redhat.com> a écrit :
>>> 
>>> Hi Pierre,
>>> 
>>> On 11/09/2016 01:42 PM, Pierre Pfister (ppfister) wrote:
>>>> Hello Maxime,
>>>> 
>>>> Sorry for the late reply.
>>>> 
>>>> 
>>>>> Le 8 nov. 2016 à 10:44, Maxime Coquelin <maxime.coquelin at redhat.com> a écrit :
>>>>> 
>>>>> Hi Pierre,
>>>>> 
>>>>> On 11/08/2016 10:31 AM, Pierre Pfister (ppfister) wrote:
>>>>>> Current virtio driver advertises VERSION_1 support,
>>>>>> but does not handle device's VERSION_1 support when
>>>>>> sending packets (it looks for ANY_LAYOUT feature,
>>>>>> which is absent).
>>>>>> 
>>>>>> This patch enables 'can_push' in tx path when VERSION_1
>>>>>> is advertised by the device.
>>>>>> 
>>>>>> This significantly improves small packets forwarding rate
>>>>>> towards devices advertising VERSION_1 feature.
>>>>> I think it depends whether offloading is enabled or not.
>>>>> If no offloading enabled, I measured significant drop.
>>>>> Indeed, when no offloading is enabled, the Tx path in Virtio
>>>>> does not access the virtio header before your patch, as the header is memset to zero at device init time.
>>>>> With your patch, it gets memset to zero at every transmit in the hot
>>>>> path.
>>>> 
>>>> Right. On the virtio side that is true, but on the device side, we have to access the header anyway.
>>> No more now, if no offload features have been negotiated.
>>> I have done a patch that landed in v16.11 to skip header parsing in
>>> this case.
>>> That said, we still have to access its descriptor.
>>> 
>>>> And accessing two descriptors (with the address resolution and memory fetch which comes with it)
>>>> is a costy operation compared to a single one.
>>>> In the case indirect descriptors are used, this is 1 desc access instead or 3.
>>> I agree this is far from being optimal.
>>> 
>>>> And in the case chained descriptors are used, this doubles the number of packets that you can put in your queue.
>>>> 
>>>> Those are the results in my PHY -> VM (testpmd) -> PHY setup
>>>> Traffic is flowing bidirectionally. Numbers are for lossless-rates.
>>>> 
>>>> When chained buffers are used for dpdk's TX: 2x2.13Mpps
>>>> When indirect descriptors are used for dpdk's TX: 2x2.38Mpps
>>>> When shallow buffers are used for dpdk's TX (with this patch): 2x2.42Mpps
>>> When I tried it, I also did PVP 0% benchmark, and I got opposite results. Chained and indirect cases were significantly better.
>>> 
>>> My PVP setup was using a single NIC and single Virtio PMD, and NIC2VM
>>> forwarding was IO mode done with testpmd on host, and Rx->Tx forwarding
>>> was macswap mode on guest side.
>>> 
>>> I also saw some perf regression when running simple tespmd test on both
>>> ends.
>>> 
>>> Yuanhan, did you run some benchmark with your series enabling
>>> ANY_LAYOUT?
>> 
>> It was enabled. But the specs specify that VERSION_1 includes ANY_LAYOUT.
>> Therefor, Qemu removes ANY_LAYOUT when VERSION_1 is set.
>> 
>> We can keep arguing about which is fastest. I guess we have different setups and different results, so we probably are deadlocked here.
>> But in any case, the current code is inconsistent, as it uses single descriptor when ANY_LAYOUT is set, but not when VERSION_1 is set.
>> 
>> I believe it makes sense to use single-descriptor any time it is possible, but you are free to think otherwise.
>> Please make a call and make the code consistent (removes single-descriptors all together, or use them when VERSION_1 is set too). Otherwise it just creates yet-another testing headache.
> I also think it makes sense to have a single descriptor, but I had to
> highlight that I noticed a significant performance degradation when
> using single descriptor on my setup.
> 
> I'm fine we take your patch in virtio-next, so that more testing is conducted.
> 

Thanks,

I just realised there was an indentation error in the patch.
Meaning that this patch didn't make it to 16.11 ...
I will send a new version.

- Pierre


> Thanks,
> Maxime
> 
>> 
>> Thanks,
>> 
>> - Pierre
>> 
>>> 
>>>> 
>>>> I must also note that qemu 2.5 does not seem to deal with VERSION_1 and ANY_LAYOUT correctly.
>>>> The patch I am proposing here works for qemu 2.7, but with qemu 2.5, testpmd still behaves as if ANY_LAYOUT (or VERSION_1) was not available. This is not catastrophic. But just note that you will not see performance in some cases with qemu 2.5.
>>> 
>>> Thanks for the info.
>>> 
>>> Regards,
>>> Maxime
>>