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Subject: Re: [OMPI users] Problems in OpenMPI
From: Ashley Pittman (ashley_at_[hidden])
Date: 2009-07-13 05:34:17


On Sun, 2009-07-12 at 19:49 -0500, Yin Feng wrote:
> Can you give me a further explanation about why results are different
> when it ran it on mutiprocessors against single processor?

Floating point number are problematical for a number of reasons, they
are only *approximations of real numbers because of the limited
precision. This means that when you do calculations with floating point
numbers the you end up with approximations of the answers (because you
only really had a approximation of the question).

In parallel computing you find that the route taken to reach an answer
is different to that taken in serial computing and hence you get
different errors so the eventual answer is different. Furthermore
you'll quite likely find that you get different answers running at
different scales, depending on how you spread out your job.

Unfortunately it's a fundamental limitation of classical computing and
one that people have learned to live with.

http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems

Ashley,

-- 
Ashley Pittman, Bath, UK.
Padb - A parallel job inspection tool for cluster computing
http://padb.pittman.org.uk