Re: [dpdk-dev] [PATCH] app/testpmd: add packet data prefetch in macswap loop

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, May 03, 2016 at 10:48:34AM +0100, De Lara Guarch, Pablo wrote:
> 
> 
> > -----Original Message-----
> > From: Richardson, Bruce
> > Sent: Tuesday, May 03, 2016 10:45 AM
> > To: Jerin Jacob
> > Cc: dev@dpdk.org; De Lara Guarch, Pablo
> > Subject: Re: [dpdk-dev] [PATCH] app/testpmd: add packet data prefetch in
> > macswap loop
> > 
> > On Mon, May 02, 2016 at 05:29:37PM +0530, Jerin Jacob wrote:
> > > prefetch the next packet data address in advance in macswap loop
> > > for performance improvement.
> > >
> > > Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > > ---
> > >  app/test-pmd/macswap.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/app/test-pmd/macswap.c b/app/test-pmd/macswap.c
> > > index 154889d..c10f4b5 100644
> > > --- a/app/test-pmd/macswap.c
> > > +++ b/app/test-pmd/macswap.c
> > > @@ -113,6 +113,9 @@ pkt_burst_mac_swap(struct fwd_stream *fs)
> > >  	if (txp->tx_ol_flags & TESTPMD_TX_OFFLOAD_INSERT_QINQ)
> > >  		ol_flags |= PKT_TX_QINQ_PKT;
> > >  	for (i = 0; i < nb_rx; i++) {
> > > +		if (likely(i < nb_rx - 1))
> > > +			rte_prefetch0(rte_pktmbuf_mtod(pkts_burst[i + 1],
> > > +						       void *));
> > 
> > At least on IA platforms, there is no issue with prefetching beyond the end of
> > the array, since it's only a hint to the cpu. If this is true for other platforms,
> > then I suggest we just drop the conditional and just always prefetch.
> 
> That's what I thought when I saw this patch, but the problem is that the prefetch is not for pkts_burst,
> but for rte_pktmbuf_mtod(pkts_burst...), so there must be a limit in nb_rx - 2, or there will be a seg fault.
> 
> Pablo
Good point, Pablo, I missed that subtlety here.

/Bruce
> > 
> > /Bruce
> > 
> > >  		mb = pkts_burst[i];
> > >  		eth_hdr = rte_pktmbuf_mtod(mb, struct ether_hdr *);
> > >
> > > --
> > > 2.1.0
> > >