[petsc-users] Get wrong answer when use multi-process

[Barry Smith]

  1) make sure the matrix is the same with 1 and 2 processes. Once the matrix is built you can call MatView(mat,NULL) to display it.

  2) once the matrices are the same make sure the eigensolver converges in both cases.

   Barry


On May 13, 2014, at 2:28 AM, 张国熙 <altriaex86 at gmail.com> wrote:

> Hi, all
> 
> I am confused about my code, for it could return right answer when I use 1 process, but return totally wrong answer when more than 1 process.
> 
> This is how I feed data to it. 
> 
> I have a CSR matrix, represented by Ap(pointer),Ai(index),and temp(data).
> 
> First determine local matrix for each process. Then feed data to them.
>   
>   int temprank,localsize,line_pos;
>     line_pos = 0;
>     if(rank == 0)
>     {    
>         localsize   = size/pro + ((size % pro) > rank);
>     }    
>     else
>     {
>         for (temprank = 0;temprank <rank;temprank++)
>         {     
>             localsize   = size/pro + ((size % pro) > temprank);
>             line_pos += localsize;
>         }
>     }
> 
>     Lin_index = new int [localsize+1];
>     for(i=0;i<localsize+1;i++)
>     {
>         Lin_index [i] = Ap[line_pos+i]-Ap[line_pos];
>     }
>     std::cerr<<"line_pos    "<<line_pos<<"\n";
>     MatMPIAIJSetPreallocationCSR(A,Lin_index,Ai+line_pos,temp+line_pos);
> 
> I use spectral transform with MATSOLVERMUMPS to calculate eigenvalue.
> 
> 
> The strange thing is, when I run it with one process, the eigenvalue is what I want, typically,  
>  (8.39485e+13,5.3263)  (3.93842e+13,-82.6948) first two.
> But for 2 process:
> eigenvalue    (2.76523e+13,7.62222e+12)
> eigenvalue    (2.76523e+13,-7.62222e+12)
> 
> 3 process:
> eigenvalue    (6.81292e+13,-3071.82)
> eigenvalue    (3.49533e+13,2.48858e+13)
> 
> 4
> eigenvalue    (9.7562e+13,5012.4)
> eigenvalue    (7.2019e+13,8.28561e+13)
> 
> However, it could pass simple test like
>     int n = 12;
>     int nz = 12;
>     int Ap[13] = {0,1,2,3,4,5,6,7,8,9,10,11,12};
>     int Ai[12] = { 0,1,2,3,4,5,6,7,8,9,10,11};
>     double Ax[12] = {-1,-2,-3,-4,-5,6,7,8,9,10,11};
>     double Az[12] = {0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0,0,0,0};
> 
> 
> Do you have any idea about it?
> 
> Thanks a lot!!
> 
>