Dear Diego, I will suggest you read the following two. It will give you some good understanding as to what is happening:
https://en.wikipedia.org/wiki/Butterfly_effect http://www.amazon.com/The-End-Error-Computing-Computational/dp/1482239868 --Bibrak On Wed, Oct 28, 2015 at 6:58 PM, Diego Avesani <diego.aves...@gmail.com> wrote: > dear Damin, > I wrote the solver by myself. I have not understood your answer. > > Diego > > > On 28 October 2015 at 23:09, Damien <dam...@khubla.com> wrote: > >> 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> 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 >>> >>> >>> -- >>> ========================================================== >>> Andreas Schäfer >>> HPC and Grid Computing >>> Department of Computer Science 3 >>> Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany >>> +49 9131 85-27910 >>> PGP/GPG key via keyserver >>> http://www.libgeodecomp.org >>> ========================================================== >>> >>> (\___/) >>> (+'.'+) >>> (")_(") >>> This is Bunny. Copy and paste Bunny into your >>> signature to help him gain world domination! >>> >>> _______________________________________________ >>> users mailing list >>> us...@open-mpi.org >>> Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users >>> Link to this post: >>> http://www.open-mpi.org/community/lists/users/2015/10/27933.php >>> >> >> >> >> _______________________________________________ >> users mailing listus...@open-mpi.org >> Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users >> >> Link to this post: >> http://www.open-mpi.org/community/lists/users/2015/10/27934.php >> >> >> >> _______________________________________________ >> users mailing list >> us...@open-mpi.org >> Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users >> Link to this post: >> http://www.open-mpi.org/community/lists/users/2015/10/27935.php >> > > > _______________________________________________ > users mailing list > us...@open-mpi.org > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > Link to this post: > http://www.open-mpi.org/community/lists/users/2015/10/27936.php >
