[petsc-users] Floating point exception, only occurs when assembling "large" matrices

[Karl Rupp]

Hi Justin,

did you run your code through Valgrind? I suspect that this is caused by 
some memory corruption, particularly as you claim that smaller matrix 
sizes are fine. Also, do you check all the error codes returned by PETSc 
routines?

Also, are you sure about the nonzeropattern of your matrix? From your 
description it sounds like you are solving 8192 decoupled problems of 
size 3x3, while for e.g. typical finite element applications you get 3x3 
local mass matrices per cell, with the total number of degrees of 
freedom given by the number of vertices.

If the above doesn't help: Any chance of sending us the relevant source 
code?

Best regards,
Karli


On 11/27/2013 09:12 AM, Justin Dong (Me) wrote:
> I am assembling a global mass matrix on a mesh consisting of 8192
> elements and 3 basis functions per element (so the global dimension is
> 24,576. For some reason, when assembling this matrix I get tons of
> floating point exceptions everywhere:
>
> [0]PETSC ERROR: --------------------- Error Message
> ------------------------------------
> [0]PETSC ERROR: Floating point exception!
> [0]PETSC ERROR: Inserting nan at matrix entry (0,0)!
>
> I get this error for every 3rd diagonal entry of the matrix, but the
> problem is bigger than that I suspect. In my computation of the local
> 3x3 element mass matrices, printing out the values gives all zeros
> (which is what the entries are initialized to be in my code).
>
> I’m at a loss for how to debug this problem. My first thought was that
> there is an error in the mesh, but I’m certain now that this is not the
> case since the mesh is generated using the exact same routine that
> generates all of my coarser meshes before this one that fails. For
> example, the mesh that is one refinement level below this one has 2048
> elements and works completely fine. This is how I am creating the global
> matrix:
>
> MatCreateSeqAIJ(PETSC_COMM_SELF, NLoc*nElems, NLoc*nElems, NLoc,
>     PETSC_NULL, &Mglobal);
>
> where I allocate NLoc = 3 non-zero entries per row in this case.