Re: [petsc-users] Loading only upper + MatSetOption(A,MAT_SYMMETRIC,PETSC_TRUE);

[Smith, Barry F. via petsc-users]

> On Mar 4, 2019, at 7:03 AM, Klaus Burkart via petsc-users 
> <petsc-users@mcs.anl.gov> wrote:
> 
> Hello,
> 
> I want to solve many symmetric linear systems one after another in parallel 
> using boomerAMG + KSPCG  and need to make the matrix transfer more efficient. 
> Matrices are symmetric in structure and values. boomerAMG + KSPCG work fine.
> 
> So far I have been loading the entire matrices but I read in a paper, that 
> it's sufficient to load the upper part only and tell petsc that the matrix is 
> symmetric using MatSetOption(A,MAT_SYMMETRIC,PETSC_TRUE); Unfortunately all 
> computations fail if I load only the upper values and use 
> MatSetOption(A,MAT_SYMMETRIC,PETSC_TRUE);

   Unfortunately it doesn't work that way. The MatSetOption() is just a marker 
that indicates that the user states the matrix is symmetric. It doesn't change 
the format or values.

    You could write a custom MatLoad() that reads in just the symmetric part 
and builds the entire MPIAIJ (since BoomerAMG requires the full MPIAIJ) but 
that is good amount of work because you have to do the careful communication 
needed to "fill in" the "transpose" part of the matrix. Basically read in the 
matrix and do a parallel transpose on the fly (also requires care in getting 
the matrix preallocation correct).

   Barry


    
> 
> The idea is:
> 
>     if (matrix_.symmetric())
>     {
>         MatSetOption(A,MAT_SYMMETRIC,PETSC_TRUE);
>        //load only upper part of the matrix MatSetValues(...)
> 
>    }else //asymmetric matrix
>            {
>                //load the entire matrix MatSetValues(...)
>            }
> 
> Is it possible at all? 
> 
> Klaus