On 16 March 2016 at 19:14, Bruce Richardson <bruce.richardson at 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 at intel.com> wrote: >> > HI Jianbo, >> > >> > >> >> -----Original Message----- >> >> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Jianbo Liu >> >> Sent: Monday, March 14, 2016 10:26 PM >> >> To: Zhang, Helin; Ananyev, Konstantin; dev at 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
