Pentium SSE prefetcht0 instruction... How do you make it work

["Bulent Abali" <abali@us.ibm.com>]

I have a system with a large external L3 cache.   Uses Pentium III
Coppermine (stepping 3) processor.   L3 block size is relatively long.  I
am trying to increase the L3 performance by prefetching L3 lines.  I
thought prefetcht0, t1, t2, nta instructions may help.  These instructions
prefetch L1/L2 lines in to the processor therefore they should prefetch L3
as well.   However I see no benefit.  It is as if prefetch never make it to
the front side bus.  I took the example of arch/i386/lib/mmx.c to implement
the asm level routines.    Perhaps I am overlooking something.  I'd
appreciate any suggestions.   /Bulent

//This should prefetch an L2 line at addr (hence L3 line prefetch)
inline void L3_prefetch (char * addr)
{
     asm volatile("prefetcht1 %0" :: "m" (addr));
}