[dpdk-dev] [PATCH v2 0/5] maximize vector rx burst for PMDs

[Wang, Haiyue]

> -----Original Message-----
> From: dev <dev-bounces at dpdk.org> On Behalf Of Morten Brørup
> Sent: Thursday, August 27, 2020 20:38
> To: Guo, Jia <jia.guo at intel.com>; Yang, Qiming <qiming.yang at intel.com>; Xing, Beilei
> <beilei.xing at intel.com>; Zhao1, Wei <wei.zhao1 at intel.com>; Zhang, Qi Z <qi.z.zhang at intel.com>; Wu,
> Jingjing <jingjing.wu at intel.com>
> Cc: Richardson, Bruce <bruce.richardson at intel.com>; dev at dpdk.org; Zhang, Helin <helin.zhang at intel.com>;
> Yigit, Ferruh <ferruh.yigit at intel.com>; barbette at kth.se
> Subject: Re: [dpdk-dev] [PATCH v2 0/5] maximize vector rx burst for PMDs
> 
> > From: Jeff Guo [mailto:jia.guo at intel.com]
> > Sent: Thursday, August 27, 2020 12:10 PM
> >
> > The limitation of burst size in vector rx was removed, since it should
> > retrieve as much received packets as possible. And also the scattered
> > receive path should use a wrapper function to achieve the goal of
> > burst maximizing.
> >
> > This patch set aims to maximize vector rx burst for for
> > ixgbe/i40e/ice/iavf/fm10k PMDs.
> >
> > v2->v1:
> > 1:add fm10k driver case
> > 2:refine some doc
> >
> 
> I now noticed that the vector functions also does:
> nb_pkts = RTE_ALIGN_FLOOR(nb_pkts, RTE_I40E_DESCS_PER_LOOP);
> 
> I am not sure about this, but if I read it correctly, calling rte_eth_rx_burst() with nb_pkts = 33
> (not 32) would only return 32 packets, even if more packets are available. (I assume that
> RTE_I40E_DESCS_PER_LOOP is 32.) In this case, I guess that you need to read the remaining of the
> requested packets using the non-vector function in order to support any nb_pkts value.
> 

This is vector instruction handling design requirement, like for avx2: #define ICE_DESCS_PER_LOOP_AVX 8,
if deep into the real loop handling, you will get the point why doing RTE_ALIGN_FLOOR. ;-)

_ice_recv_raw_pkts_vec_avx2:
for (i = 0, received = 0; i < nb_pkts; i += ICE_DESCS_PER_LOOP_AVX, rxdp += ICE_DESCS_PER_LOOP_AVX)

Maybe it is worth to tell PMD to stop using vector by calling rte_eth_rx_queue_setup with the application
burst size it wants.

> That is, unless the rte_eth_rx_burst() API is extended with requirements to nb_pkts, as discussed in
> the other thread:
> http://inbox.dpdk.org/dev/20200827114117.GD569@bricha3-
> MOBL.ger.corp.intel.com/T/#mc8051e9022d6aeb20c51c5a226b2274d3d6d4266
> 
> 
> Med venlig hilsen / kind regards
> - Morten Brørup
> 
>