Re: [dpdk-dev] [PATCH v2] librte_pmd_packet: add PMD for AF_PACKET-based virtual devices

["Zhou, Danny" <danny.zhou@intel.com>]

> -----Original Message-----
> From: Neil Horman [mailto:nhorman@tuxdriver.com]
> Sent: Wednesday, July 16, 2014 4:31 AM
> To: John W. Linville
> Cc: Zhou, Danny; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v2] librte_pmd_packet: add PMD for
> AF_PACKET-based virtual devices
> 
> On Tue, Jul 15, 2014 at 10:01:11AM -0400, John W. Linville wrote:
> > On Tue, Jul 15, 2014 at 08:17:44AM -0400, Neil Horman wrote:
> > > On Tue, Jul 15, 2014 at 12:15:49AM +0000, Zhou, Danny wrote:
> > > > According to my performance measurement results for 64B small
> > > > packet, 1 queue perf. is better than 16 queues (1.35M pps vs.
> > > > 0.93M
> > > > pps) which make sense to me as for 16 queues case more CPU cycles
> > > > (16 queues' 87% vs. 1 queue' 80%) in kernel land needed for
> > > > NAPI-enabled ixgbe driver to switch between polling and interrupt
> > > > modes in order to service per-queue rx interrupts, so more context
> > > > switch overhead involved. Also, since the
> > > > eth_packet_rx/eth_packet_tx routines involves in two memory copies
> > > > between DPDK mbuf and pbuf for each packet, it can hardly achieve
> > > > high performance unless packet are directly DMA to mbuf which needs ixgbe
> driver to support.
> > >
> > > I thought 16 queues would be spread out between as many cpus as you
> > > had though, obviating the need for context switches, no?
> >
> > I think Danny is testing the single CPU case.  Having more queues than
> > CPUs probably does not provide any benefit.
> >
> Ah, yes, generally speaking, you never want nr_cpus < nr_queues.  Otherwise you'll
> just be fighting yourself.
> 

It is true for interrupt based NIC driver and this AF_PACKET based PMD because it depends 
on kernel NIC driver. But for poll-mode based DPDK native NIC driver, you can have a cpu pinning to
to a core polling multiple queues on a NIC or queues on different NICs, at the cost of more
power consumption or wasted CPU cycles busying waiting packets.

> > It would be cool to hack the DPDK memory management to work directly
> > out of the mmap'ed AF_PACKET buffers.  But at this point I don't have
> > enough knowledge of DPDK internals to know if that is at all
> > reasonable...
> >
> > John
> >
> > P.S.  Danny, have you run any performance tests on the PCAP driver?
> >
> > --
> > John W. Linville		Someday the world will need a hero, and you
> > linville@tuxdriver.com			might be all we have.  Be ready.
> >