Why 256?

[Brian Glendenning]

I've just been reading the document "IBM RISC SYSTEM/6000 PERFORMANCE
TUNING FOR NUMERICALLY INTENSIVE FORTRAN AND C PROGRAMS" (hey - would
I invent all those caps!). Very nice.

The only thing I think I don't understand is the tuning summary entry
that says "Make sure leading dimension of arrays is not a multiple of
2 greater than or equal to 256."

I first thought this had to do with cases where you had to stride
through the array in other than the first dimension and the fact that
you would have increased cache misses, but I can't see why the number
"256" is picked on. What is the reasoning behind this statement? And
does this mean that FFTs and convolutions etc are doomed to be "slow"
on these machines?

(I would greatly appreciate an example of a hot FFT routine for the
RS/6000 if anyone has one).

Thanks!

Brian
--
       Brian Glendenning - National Radio Astronomy Observatory
bglenden at nrao.edu          bglenden at nrao.bitnet          (804) 296-0286