[dpdk-dev] [PATCH 0/6] vhost: add Tx zero copy support

[Maxime Coquelin]

On 08/23/2016 04:53 PM, Yuanhan Liu wrote:
> BTW, I really appreicate your efforts on reviewing this patchset.
>
> It would be great if you could take some time to review my another
> patchset :)
>
>     [PATCH 0/7] vhost: vhost-cuse removal and code path refactoring
>
> It touchs a large of code base, that I wish I could apply it ASAP.
> So that the chance a later patch will introduce conflicts is small.

Sure, I will try to review it by tomorrow morning (CET).

REgards,
Maxime

>
> 	--yliu
>
> On Tue, Aug 23, 2016 at 10:42:11PM +0800, Yuanhan Liu wrote:
>> On Tue, Aug 23, 2016 at 04:18:40PM +0200, Maxime Coquelin wrote:
>>>
>>>
>>> On 08/23/2016 10:10 AM, Yuanhan Liu wrote:
>>>> This patch set enables vhost Tx zero copy. The majority work goes to
>>>> patch 4: vhost: add Tx zero copy.
>>>>
>>>> The basic idea of Tx zero copy is, instead of copying data from the
>>>> desc buf, here we let the mbuf reference the desc buf addr directly.
>>>>
>>>> The major issue behind that is how and when to update the used ring.
>>>> You could check the commit log of patch 4 for more details.
>>>>
>>>> Patch 5 introduces a new flag, RTE_VHOST_USER_TX_ZERO_COPY, to enable
>>>> Tx zero copy, which is disabled by default.
>>>>
>>>> Few more TODOs are left, including handling a desc buf that is across
>>>> two physical pages, updating release note, etc. Those will be fixed
>>>> in later version. For now, here is a simple one that hopefully it
>>>> shows the idea clearly.
>>>>
>>>> I did some quick tests, the performance gain is quite impressive.
>>>>
>>>> For a simple dequeue workload (running rxonly in vhost-pmd and runnin
>>>> txonly in guest testpmd), it yields 40+% performance boost for packet
>>>> size 1400B.
>>>>
>>>> For VM2VM iperf test case, it's even better: about 70% boost.
>>>
>>> This is indeed impressive.
>>> Somewhere else, you mention that there is a small regression with small
>>> packets. Do you have some figures to share?
>>
>> It could be 15% drop for PVP case with 64B packet size. The test topo is:
>>
>> 	 nic 0 --> VM Rx --> VM Tx --> nic 0
>>
>> Put simply, I run vhost-switch example in the host and run testpmd in
>> the guest.
>>
>> Though the number looks big, I don't think it's an issue. First of all,
>> it's disabled by default. Secondly, if you want to enable it, you should
>> be certain that the packet size is normally big, otherwise, you should
>> not bother to try with zero copy.
>>
>>> Also, with this feature OFF, do you see some regressions for both small
>>> and bigger packets?
>>
>> Good question. I didn't check it on purpose, but I did try when it's
>> disabled, the number I got is pretty the same as the one I got without
>> this feature. So, I would say I don't see regressions. Anyway, I could
>> do more tests to make sure.
>> 	
>> 	--yliu