[petsc-users] Need help improving working PETSC code

[Ganesh Vijayakumar]

Hello,

 First of all..thanks to the PETSC developers and everyone else
contributing supporting material on the web.

 I need to solve a system of equations Ax =b, where A is symmetric, sparse,
unstructured and in parallel as a part of a finite volume solver for CFD.
Decomposition is already done. I initialize the matrix as MPIAIJ even
though it's symmetric as that's the only thing I could get to work.

MatCreate(PETSC_COMM_WORLD,&A);

MatSetType(A,MATMPIAIJ);
MatSetSizes(A,nCellsCurProc,nCellsCurProc,nTotalCells,nTotalCells);

MatMPIAIJSetPreallocation(A,10,PETSC_NULL,5,PETSC_NULL);

MatSetUp(A) ;
// Assemble matrix using MatSetValue.. assemble full matrix even though
it's symmetric.
MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY) ;
MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY) ;
//Create solver
KSPCreate(PETSC_COMM_WORLD,&ksp);
KSPSetType(ksp,KSPFGMRES;)

KSPGetPC(ksp, &pc);
PCFactorSetShiftType(pc, MAT_SHIFT_POSITIVE_DEFINITE);

//PCSetType(pc, PCML); //Trilinos - Multilevel

//PCSetType(pc, PCBJACOBI) ; //Incomplete LU
//PetscOptionsSetValue("-sub_pc_type", "ilu") ;
KSPSetFromOptions(ksp);


// PCSetType(pc, PCASM); //Additive Schwarz Methods
// PCASMSetOverlap(pc,overlap);

PCSetType(pc, PCHYPRE); //HYPRE
PCHYPRESetType(pc, "boomeramg");

//PCHYPRESetType(pc, "parasails");

KSPSetPC(ksp, pc);
KSPSetTolerances(ksp, 1e-13,  1e-13,  PETSC_DEFAULT,  PETSC_DEFAULT) ;
KSPSetInitialGuessNonzero(ksp, PETSC_TRUE);
KSPSetOperators(ksp, A, A);
KSPMonitorDefault(ksp, 1, 1, pVoid) ;

KSPSolve(ksp,b,x);
VecGetArray(x, &getResultArray) ;
KSPGetResidualNorm(ksp, &endNorm) ;
KSPGetIterationNumber(ksp, &nIterations);

KSPDestroy(&ksp);


 I know that this works and gives me the correct result. However, boomerAMG
is proving to be too slow.. atleast the way I use it at 256 processors with
around 12 million unknowns. I need your advice on the preconditioner. Am I
using it right? Is there anything else I could I be doing better?

ganesh
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