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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Jianbo Liu [mailto:jianbo.liu@linaro.org]
> Sent: Monday, March 21, 2016 2:27 AM
> To: Richardson, Bruce
> Cc: Lu, Wenzhuo; Zhang, Helin; Ananyev, Konstantin; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] ixgbe: avoid unnessary break when checking at the tail of rx hwring
> 
> On 18 March 2016 at 18:03, Bruce Richardson <bruce.richardson@intel.com> wrote:
> > 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
> It will be a problem because the possibility could be high.
> Considering rx hwring size is 128 and rx burst is 32, the possiblity
> can be 32/128.
> I know this change is critical, so I want you (and maintainers) to do
> full evaluations about throughput/latency..before making conclusion.

I am still not sure what is a problem you are trying to solve here.
Yes recv_raw_pkts_vec() call wouldn't wrap around HW ring boundary,  
and yes can return less packets that are actually available by the HW.
Though as Bruce pointed, they'll be returned to the user by next call.
Actually recv_pkts_bulk_alloc() works in a similar way.
Why do you consider that as a problem?
Konstantin

> 
> Jianbo