From: Robert Olsson <redacted>
Date: Fri, 29 Jul 2005 16:50:31 +0200

 My experiences from playing with prefetching eth_type_trans in this 
 case. One must look in the total performance not just were the 
 prefetching is done. In this case I was able to get eth_type_trans
 down in the profile list but other functions increased so performance
 was the same or lower. This needs to be sorted out... 

The problem is that if you just barely fit in the cache for a
workload, prefetches can hurt if done too early.  Let's say that your
code path needs to access data items A, B, and C, in that order.  If
you need to access A in order to know C, and subsequently if you
prefetch C before you use B, you might kick out B and end up making
performance worse (since you'll thus need to bring in C twice).

I really do not want to merge in any prefetch patches until
there is hard data showing an improvement, instead of some
shamanistic justification :-)

When the witch doctor comes to town and starts adding prefetches
all over the place without performance metrics, I become rightly
concerned.