Fortran vs. C for numerical work (SUMMARY)

[Tony Warnock]

Dan Bernstein asks:
 
RE: >In article <7200 at lanl.gov> ttw at lanl.gov (Tony Warnock) writes:
    >>     With respect to speed, almost all machines that I have
    >>     used during the last 25 or so years have had faster
    >>     multiplications than memory accesses.
 
>Hmmm. What machines do you use? In my experience (mostly mainframes and
>supers, some micros, and only recently a bit with minis) local memory
>access is up to several times as fast as integer multiplication. (I
>don't like this situation; converting to floating point just to multiply
>quickly on a Cray seems rather silly.)
 
 
      Model        Multiplication Time     Memory Latency
 
      YMP          5  clock periods         18 clock periods
      XMP          4  clock periods         14 clock periods
      CRAY-1       6  clock periods         11 clock periods
 
      Compaq       25 clock periods         4  clock periods
      Zenith      120 clock periods        30  clock periods
 
    The times on the PC-clones are approximate depending on the type
    of variables being accessed and the sizes of the indices.
 
    Most of my work has been on CRAY type computers. It is always a
    win to avoid memory accesses. On the PC-type machines, memory
    access is faster, but in the particular cases that I have been
    programming, one does many more constant-stride access than random
    accesses, usually in a ratio of many thousand to one. For an LU
    decompositon with partial pivoting, one does rougly N/3 constant
    stride memory accesses for each "random" access. For small N, say
    100 by 100 size matrices or so, one would do about 30
    strength-reduced operations for each memory access. For medium
    (1000 by 1000) problems, the ratio is about 300 and for large
    (10000 by 10000) it is about 30000.