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Date: Wed, 16 Mar 2016 11:14:54 +0000
From: Bruce Richardson <bruce.richardson@intel.com>
To: Jianbo Liu <jianbo.liu@linaro.org>
Cc: "Lu, Wenzhuo" <wenzhuo.lu@intel.com>,
 "Zhang, Helin" <helin.zhang@intel.com>,
 "Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
 "dev@dpdk.org" <dev@dpdk.org>
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Subject: Re: [dpdk-dev] [PATCH] ixgbe: avoid unnessary break when checking
 at the tail of rx hwring
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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