Diego,
There aren't many linear solvers that are bit-consistent, where the
answer is the same no matter how many cores or processes you use.
Intel's version of Pardiso is bit-consistent and I think MUMPS 5.0 might
be, but that's all. You should assume your answer will not be exactly
the same as you change the number of cores or processes, although you
should reach the same overall error tolerance in approximately the same
number of iterations.
Damien
On 2015-10-28 3:51 PM, Diego Avesani wrote:
dear Andreas, dear all,
The code is quite long. It is a conjugate gradient algorithm to solve
a complex system.
I have noticed that when a do cycle is small, let's say
do i=1,3
enddo
the results are identical. If the cycle is big, let's say do i=1,20,
the results are different and the difference increase with the number
of iterations.
What do you think?
Diego
On 28 October 2015 at 22:32, Andreas Schäfer <gent...@gmx.de
<mailto:gent...@gmx.de>> wrote:
On 22:03 Wed 28 Oct , Diego Avesani wrote:
> When I use a single CPU a get a results, when I use 4 CPU I get
another
> one. I do not think that very is a bug.
Sounds like a bug to me, most likely in your code.
> Do you think that these small differences are normal?
It depends on what small means. Floating point operations in a
computer are generally not commutative, so parallelization may in deed
lead to different results.
> Is there any way to get the same results? is some align problem?
Impossible to say without knowing your code.
Cheers
-Andreas
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