[dpdk-dev] [PATCH v2] kni: add new mbuf in alloc_q only based on its empty slots

[gowrishankar muthukrishnan]

Hi Ferruh,

On Wednesday 31 May 2017 09:51 PM, Ferruh Yigit wrote:
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> I have sampled below data in x86_64 for KNI on ixgbe pmd. iperf server
>> runs on
>> remote interface connecting PMD and iperf client runs on KNI interface,
>> so as to
>> create more egress from KNI into DPDK (w/o and with this patch) for 1MB and
>> 100MB data. rx and tx stats are from kni app (USR1).
>>
>> 100MB w/o patch 1.28Gbps
>> rx      tx        alloc_call  alloc_call_mt1tx freembuf_call
>> 3933 72464 51042      42472              1560540
> Some math:
>
> alloc called 51042 times with allocating 32 mbufs each time,
> 51042 * 32 = 1633344
>
> freed mbufs: 1560540
>
> used mbufs: 1633344 - 1560540 = 72804
>
> 72804 =~ 72464, so looks correct.
>
> Which means rte_kni_rx_burst() called 51042 times and 72464 buffers
> received.
>
> As you already mentioned, for each call kernel able to put only 1-2
> packets into the fifo. This number is close to 3 for my test with KNI PMD.
>
> And for this case, agree your patch looks reasonable.
>
> But what if kni has more egress traffic, that able to put >= 32 packets
> between each rte_kni_rx_burst()?
> For that case this patch introduces extra cost to get allocq_free count.

Are there case(s) we see kernel thread writing txq faster at a rate 
higher than kni application
could dequeue it ?. In my understanding, KNI is suppose to be a slow 
path as it puts
packets back into network stack (control plane ?).

Regards,
Gowrishankar

> Overall I am not disagree with patch, but I have concern if this would
> cause performance loss some cases while making better for this one. That
> would help a lot if KNI users test and comment.
>
> For me, applying patch didn't give any difference in final performance
> numbers, but if there is no objection, I am OK to get this patch.
>
>

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