[dpdk-dev] [PATCH 1/6] cxgbe: Optimize forwarding performance for 40G

[Rahul Lakkireddy]

Hi Aaron,

On Friday, October 10/02/15, 2015 at 14:48:28 -0700, Aaron Conole wrote:
> Hi Rahul,
> 
> Rahul Lakkireddy <rahul.lakkireddy at chelsio.com> writes:
> 
> > Update sge initialization with respect to free-list manager configuration
> > and ingress arbiter. Also update refill logic to refill mbufs only after
> > a certain threshold for rx.  Optimize tx packet prefetch and free.
> <<snip>>
> >  			for (i = 0; i < sd->coalesce.idx; i++) {
> > -				rte_pktmbuf_free(sd->coalesce.mbuf[i]);
> > +				struct rte_mbuf *tmp = sd->coalesce.mbuf[i];
> > +
> > +				do {
> > +					struct rte_mbuf *next = tmp->next;
> > +
> > +					rte_pktmbuf_free_seg(tmp);
> > +					tmp = next;
> > +				} while (tmp);
> >  				sd->coalesce.mbuf[i] = NULL;
> Pardon my ignorance here, but rte_pktmbuf_free does this work. I can't
> actually see much difference between your rewrite of this block, and
> the implementation of rte_pktmbuf_free() (apart from moving your branch
> to the end of the function). Did your microbenchmarking really show this
> as an improvement? 
> 
> Thanks for your time,
> Aaron

rte_pktmbuf_free calls rte_mbuf_sanity_check which does a lot of
checks.  This additional check seems redundant for single segment
packets since rte_pktmbuf_free_seg also performs rte_mbuf_sanity_check.

Several PMDs already prefer to use rte_pktmbuf_free_seg directly over
rte_pktmbuf_free as it is faster.

The forwarding perf. improvement with only this particular block is
around 1 Mpps for 64B packets when using l3fwd with 8 queues.

Thanks,
Rahul