[petsc-users] Block unstructured grid

[Mathieu Dutour]

Dear all,

I would like to work with a special kind of linear system that ought to be
very common but I am not sure that it is possible in PETSC.

What we have is an unstructured grid with say 3.10^5 nodes in it.
At each node, we have a number of frequency/direction and together
this makes about 1000 values at the node. So, in total the linear system
has say 3.10^8 values.

We managed to implement this system with Petsc but the performance
was unsatisfactory. We think that Petsc is not exploiting the special
structure of the matrix and we wonder if this structure can be implemented
in Petsc.

By special structure we mean the following. An entry in the linear system
is of the form (i, j) with 1<=i<=1000 and 1<=j<=N   with N = 3.10^5.
The node (i , j) is adjacent to all the nodes (i' , j) and thus they make a
block
diagonal entry. But the node (i , j) is also adjacent to some nodes (i , j')
[About 6 such nodes, but it varies].

Would there be a way to exploit this special structure in Petsc? I think
this should be fairly common and significant speedup could be obtained.

Best,

  Mathieu
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