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

Joe Landman landman at scalableinformatics.com
Fri Jun 25 10:51:52 EDT 2010


Prentice Bisbal wrote:
> Beowulfers,
> 
> 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?

At least 4x more work is often the case. 32x doesn't sound unreasonable.


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