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

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Mar 17, 2016 at 10:20:01AM +0800, Jianbo Liu wrote:
> 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
HI Jianbo,

ok, I understand now. I'm not sure that this is a significant problem though,
since we are working in polling mode. Is there a performance impact to your
change, because I don't think that we can reduce performance just to fix this?

Regards,
/Bruce