Why 256?

[Stephen Linam]

In article <BGLENDEN.90Oct24211953 at mandrill.cv.nrao.edu>,
bglenden at mandrill.cv.nrao.edu (Brian Glendenning) writes:
|> 
|> 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. 

Bits 18-24 (I think) of the virtual address control which entry
in the cache is used for a particular address.  If you are accessing 
data which is 256, 512, 1028 etc. double words apart, you are reducing the 
effective size of the cache, causing more frequent cache misses.  The document 
you refer to mentions the pathological case, 16k bytes or 2048
double words, where the cache is effectively only 4 lines (512 bytes).

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