Re: [dpdk-dev] [PATCH] ixgbe: prefetch packet headers in vector PMD receive function

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

> -----Original Message-----
> From: Zoltan Kiss [mailto:zoltan.kiss@linaro.org]
> Sent: Monday, September 7, 2015 3:15 PM
> To: Richardson, Bruce; dev@dpdk.org
> Cc: Ananyev, Konstantin
> Subject: Re: [PATCH] ixgbe: prefetch packet headers in vector PMD receive
> function
> 
> 
> 
> On 07/09/15 13:57, Richardson, Bruce wrote:
> >
> >
> >> -----Original Message-----
> >> From: Zoltan Kiss [mailto:zoltan.kiss@linaro.org]
> >> Sent: Monday, September 7, 2015 1:26 PM
> >> To: dev@dpdk.org
> >> Cc: Ananyev, Konstantin; Richardson, Bruce
> >> Subject: Re: [PATCH] ixgbe: prefetch packet headers in vector PMD
> >> receive function
> >>
> >> Hi,
> >>
> >> I just realized I've missed the "[PATCH]" tag from the subject. Did
> >> anyone had time to review this?
> >>
> >
> > Hi Zoltan,
> >
> > the big thing that concerns me with this is the addition of new
> > instructions for each packet in the fast path. Ideally, this
> > prefetching would be better handled in the application itself, as for
> > some apps, e.g. those using pipelining, the core doing the RX from the
> > NIC may not touch the packet data at all, and the prefetches will
> instead cause a performance slowdown.
> >
> > Is it possible to get the same performance increase - or something
> > close to it - by making changes in OVS?
> 
> OVS already does a prefetch when it's processing the previous packet, but
> apparently it's not early enough. At least for my test scenario, where I'm
> forwarding UDP packets with the least possible overhead. I guess in tests
> where OVS does more complex processing it should be fine.
> I'll try to move the prefetch earlier in OVS codebase, but I'm not sure if
> it'll help.

I would suggest trying to prefetch more than one packet ahead. Prefetching 4 or
8 ahead might work better, depending on the processing being done.

/Bruce