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

Nathan Moore ntmoore at gmail.com
Fri Jun 25 13:30:39 EDT 2010


I agree, it seems odd that the OS/compiler has a 128 bit math library
available. I certainly hope that your seemingly correct answer is not
being corrupted when you compute a 64-bit sine of a 128 bit number...

I used GMP's arbitrary precision library (rational number arithmetic)
for my thesis a few years back.  It was very easy to implement, but
not fast (better on x86 hardware than sun/sgi/power as I recall).  I
too am curious about the sort of algorithm that would require that
much precision.  (for me, it was inverting a probability distribution
that was piecewise-defined, described here,
http://arxiv.org/abs/cond-mat/0506786, sorry, nobody ever gets to talk
about their thesis...).

Nathan



On Fri, Jun 25, 2010 at 10:54 AM, Nifty Tom Mitchell
<niftyompi at niftyegg.com> wrote:
> On Fri, Jun 25, 2010 at 10:28:15AM -0400, 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 am surprised that it works as support in things like
> the math lib, log and trig functions could be missing.
> Which compiler is he using?
>
>
>
>
>
> --
>        T o m  M i t c h e l l
>        Found me a new hat, now what?
>
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Nathan Moore
Assistant Professor, Physics
Winona State University
AIM: nmoorewsu
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