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
