[petsc-users] A qustion of PETSc

[w_ang_temp]

Besides,I test several situations.
When process=1(mpiexec -n 1),the results are right,compared with the true results.
When process=2 or 5,both of the results are same.And from the tenth loop, it begins the strange error Ax!=b.
When process=3 or 4,both of the results are same.And from the first loop,it begins the error.
Thanks.




>At 2012-05-26 15:01:28,w_ang_temp <w_ang_temp at 163.com> wrote:

>Hello, Matt
>   First,thanks for your help, and I have a new recognition of MPI. As you said(only part of a Vec is stored on each MPI process),I use >MPI_Send,MPI_Recv and MPI_Bcast to deal with it and get some right results.
>   However,I have not yet fully solved this problem.The number of loops of the subroutine named PETSCSOLVE to solve Ax=b with PETSc is 20 >times.I print the results of each loop and find that the front 9 times are right. In the 10 time,there is a strange error: plug the result x into the >equation I find that Ax!=b.
>   The code and the A,x,b are shown below.So can you give me some hints? Thank you very much!
>                                                        Jim
>   (1)code
>      call MatView(A,PETSC_VIEWER_STDOUT_WORLD,ierr)
>      call VecView(b,PETSC_VIEWER_STDOUT_WORLD,ierr)
>      call KSPSolve(ksp,b,x,ierr)
>      call VecView(x,PETSC_VIEWER_STDOUT_WORLD,ierr)
>   (2)Ax=b
>  A:
>  type: mpiaij
> row 0: (0, 1)  (1, 0)  (2, 0)  (3, 0)  (4, 0)  (5, 0)  (6, 0)  (7, 0)  (8, 0)
> row 1: (0, 0)  (1, 1)  (2, 0)  (3, 0)  (4, 0)  (5, 0)  (6, 0)  (7, 0)  (8, 0)
> row 2: (0, 0)  (1, 0)  (2, -2e-06)  (3, -50)  (4, 0)  (5, 0)  (6, 0)  (7, -50)  (8, 0)
> row 3: (0, 0)  (1, 0)  (2, -50)  (3, 1.36265e+07)  (4, 0)  (5, 0)  (6, 0)  (7, 1.23877e+06)  (8, 0)
> row 4: (0, 0)  (1, 0)  (2, 0)  (3, 0)  (4, 1)  (5, 0)  (6, 0)  (7, 0)  (8, 0)
> row 5: (0, 0)  (1, 0)  (2, 0)  (3, 0)  (4, 0)  (5, 1)  (6, 0)  (7, 0)  (8, 0)
> row 6: (0, 0)  (1, 0)  (2, 0)  (3, 0)  (4, 0)  (5, 0)  (6, 1)  (7, 0)  (8, 0)
> row 7: (0, 0)  (1, 0)  (2, -50)  (3, 1.23877e+06)  (4, 0)  (5, 0)  (6, 0)  (7, 1.36265e+07)  (8, 0)
> row 8: (0, 0)  (1, 0)  (2, 0)  (3, 0)  (4, 0)  (5, 0)  (6, 0)  (7, 0)  (8, 1)
>  b:
>  type: mpi
> Process [0]
> 0
> 0
> -0.75
> 96949.5
> 0
> Process [1]
> 0
> -0.015
> 96949.5
> -0.015
 > x:
  >type: mpi
 >Process [0]
> 0
> 0
> 1167.84
> 0.00819383
 >0
 >Process [1]
 >0
> -0.00824055
> 0.00390865
> -0.00824055





>在 2012-05-22 19:33:10，"Matthew Knepley" <knepley at gmail.com> 写道：
>On Tue, May 22, 2012 at 5:17 AM, w_ang_temp <w_ang_temp at 163.com> wrote:

>Hello
 >    I am a beginner of PETSc.I find the petsc-users lists is very useful, however I do not know how to ask a question in it. So can you tell me? >And below is what I want to ask.
 >   I am solving Ax=b with PETSc. And I only use it in a fortran subroutine to get x ,then the main function of my program use x to do other >things. The question is that: when the process is 1(mpiexec -n 1),the result of my program is ok, but when the process is more than 1 the > result is wrong.



> "Wrong" is not an acceptable description. I suspect that you do not understand that only part of a Vec
> is stored on each MPI process. First, get the book Using MPI, and then read the PETSc manual
> section on vectors.


  > Thanks,


     > Matt
 
>     So where could be wrong? Can you give me some hints on how I can debug? Thanks.
>                                   Jim
> --
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their > experiments lead.
> -- Norbert Wiener


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