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Date: Tue, 3 May 2016 11:20:21 +0100
From: Bruce Richardson <bruce.richardson@intel.com>
To: Ivan Boule <ivan.boule@6wind.com>
Cc: Jerin Jacob <jerin.jacob@caviumnetworks.com>, dev@dpdk.org,
 pablo.de.lara.guarch@intel.com
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Subject: Re: [dpdk-dev] [PATCH] app/testpmd: add packet data prefetch in
 macswap loop
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On Tue, May 03, 2016 at 11:50:31AM +0200, Ivan Boule wrote:
> On 05/03/2016 11:45 AM, Bruce Richardson wrote:
> >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.
> >>
> >>...
> >>  	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.
> 
> This is an interesting point.
> Bruce, are you suggesting that prefetching at an invalid [virtual] address
> won't trigger a CPU exception?
> 

Yep. For example, adding "rte_prefetch0(NULL)" at the start of main in testpmd
causes no ill effects when running the app.

/Bruce