[Beowulf] Peformance penalty when using 128-bit reals on AMD64

Prentice Bisbal prentice at ias.edu
Fri Jun 25 07:28:15 PDT 2010


One of my Fortran programmers had to increase the precision of his
program so he switched from REAL*8 to REAL*16 which changes the size of
his variables from 64 bits to 128 bits. The program now takes 32x longer
to run.

I'm not an expert on processor archtitecture, etc., but I do know that
once the size of a variable exceeds the size of the processors
registers, things will slow down considerably. Is his 32x performance
degradation in line with this?

Is there any way to reduce this degradation? Would The GNU GMP library
(or some other library) help speed things up?


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