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

[Neil Horman <nhorman@tuxdriver.com>]

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 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.
>