Re: [dpdk-dev] [PATCH] ixgbe: avoid unnessary break when checking at the tail of rx hwring

[Jianbo Liu <jianbo.liu@linaro.org>]

On 16 March 2016 at 19:14, Bruce Richardson <bruce.richardson@intel.com> wrote:
> On Wed, Mar 16, 2016 at 03:51:53PM +0800, Jianbo Liu wrote:
>> Hi Wenzhuo,
>>
>> On 16 March 2016 at 14:06, Lu, Wenzhuo <wenzhuo.lu@intel.com> wrote:
>> > HI Jianbo,
>> >
>> >
>> >> -----Original Message-----
>> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Jianbo Liu
>> >> Sent: Monday, March 14, 2016 10:26 PM
>> >> To: Zhang, Helin; Ananyev, Konstantin; dev@dpdk.org
>> >> Cc: Jianbo Liu
>> >> Subject: [dpdk-dev] [PATCH] ixgbe: avoid unnessary break when checking at the
>> >> tail of rx hwring
>> >>
>> >> When checking rx ring queue, it's possible that loop will break at the tail while
>> >> there are packets still in the queue header.
>> > Would you like to give more details about in what scenario this issue will be hit? Thanks.
>> >
>>
>> vPMD will place extra RTE_IXGBE_DESCS_PER_LOOP - 1 number of empty
>> descriptiors at the end of hwring to avoid overflow when do checking
>> on rx side.
>>
>> For the loop in _recv_raw_pkts_vec(), we check 4 descriptors each
>> time. If all 4 DD are set, and all 4 packets are received.That's OK in
>> the middle.
>> But if come to the end of hwring, and less than 4 descriptors left, we
>> still need to check 4 descriptors at the same time, so the extra empty
>> descriptors are checked with them.
>> This time, the number of received packets is apparently less than 4,
>> and we break out of the loop because of the condition "var !=
>> RTE_IXGBE_DESCS_PER_LOOP".
>> So the problem arises. It is possible that there could be more packets
>> at the hwring beginning that still waiting for being received.
>> I think this fix can avoid this situation, and at least reduce the
>> latency for the packets in the header.
>>
> Packets are always received in order from the NIC, so no packets ever get left
> behind or skipped on an RX burst call.
>
> /Bruce
>

I knew packets are received in order, and no packets will be skipped,
but some will be left behind as I explained above.
vPMD will not received nb_pkts required by one RX burst call, and
those at the beginning of hwring are still waiting to be received till
the next call.

Thanks!
Jianbo