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

Joe Landman landman at scalableinformatics.com
Fri Jun 25 07:51:52 PDT 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.

Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics Inc.
email: landman at scalableinformatics.com
web  : http://scalableinformatics.com
phone: +1 734 786 8423 x121
fax  : +1 866 888 3112
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