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

Prentice Bisbal prentice at ias.edu
Fri Jun 25 10:57:41 PDT 2010

This may be naive, but I assumed that if the language supports real*16,
then the language and compiler would have to support all of the
functions that are native to the language (add, subtract, and whatever
else the Fortran standard specifies), would have to handle real*16
operands as well. Is that a correct assumption?

I can see how more complicated functions and libraries built off the
simpler functions would be a problem.

I forwarded some of the e-mails from this discussion to the programmer,
so he'd understand the possible issues.

I don't know exactly what problem this person is tackling with this
program, but I can say he is a theoretical physicist whose research
includes quantum mechanics and field theory.


Nathan Moore wrote:
> 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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Prentice Bisbal
Linux Software Support Specialist/System Administrator
School of Natural Sciences
Institute for Advanced Study
Princeton, NJ

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