[petsc-users] Fwd: nonzero prescribed boundary condition

[amrit poudel]

After running my simulation multiple times on a multiprocessor computer I've just verified that using iterative solver (default gmres) in PETSc  to solve a linear system of equations ( Cx=b)  with more than 2 processors setting ALWAYS lead to erroneous result. Running identical code with identical setting except for the number of processors ( set this to 2) ALWAYS gives me correct result .

I am really not sure what is the point behind including iterative solvers if they result into erroneous result on a multiprocessor computer. The result I get from multiprocessor computer is a complete garbage, so I am really not talking about small percentage of error here.  Also, if somebody could enlighten why the iterative solvers are error prone on multiprocessors that will be highly appreciated. 

I am very hopeful that there is a way around to this problem, because PETSc is such a powerful and useful library that I really do not want to give up on this and start something else from scratch. 


Would you think that a DIRECT SOLVER would circumvent this problem? My problem is that I have a very large system of equations and the size of a sparse coefficient matrix is huge ( > 1e+8). I assemble this matrix in MATLAB, write to a binary file, and read it in PETSc. So I really need to be able to solve this system of equations in a cluster of computers (which inherently has multiprocessors and distributed memory setting). Does this mean I am completely out of luck with PETSc's iterative solver package and the only hope for me is the direct solver? I do have MUMPS downloaded and compiled with PETSc, so I will give that a try and see what results I obtain, but I am really surprised that iterative solvers are no good in a large multiprocessor settings.

Any insights, suggestions/advice will be highly appreciated.

Thanks.

PS (I can attach my entire code, plots that compare the results obtained by solving Cx=b in 2 processors vs 12 or 6 processors if any body wants to take a look at it. I get garbage if I run iterative solver on 12 processors)
 		 	   		  
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