Path: utzoo!attcan!uunet!wuarchive!rex!ukma!rutgers!mcnc!uvaarpa!murdoch!murdoch.acc.virginia.edu!bglenden From: bglenden@mandrill.cv.nrao.edu (Brian Glendenning) Newsgroups: comp.unix.aix Subject: Why 256? Message-ID: Date: 25 Oct 90 01:19:53 GMT Sender: news@murdoch.acc.Virginia.EDU Organization: National Radio Astronomy Observatory Lines: 25 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@nrao.edu bglenden@nrao.bitnet (804) 296-0286