[petsc-users] Trying to apply FieldSplitPC by reading bloc matrix

Matthew Knepley knepley at gmail.com
Fri Dec 19 19:29:09 CST 2014 

On Dec 19, 2014 6:14 AM, "Gilles Steiner" <gilles.steiner at epfl.ch> wrote:
>
> Hello Petsc Users,
>
> I have an issue trying to use FiledSplitPC in parallel.
>
> My goal : I want to get a linear system from petsc binary files and solve
this in parallel with the FieldSplitPC.
>
> The problem I want to solve is an FE approximation of the Stokes
equations.
>
> Skipping the details, my code looks like :
>
> // Reading the four blocs UU, UP, PU and PP
> for(int i=0; i < 4; ++i)
> {
>       string name = matrix + to_string(i) + ".petscbin";
>       PetscViewer    PETSC_matreader;
>       PetscViewerBinaryOpen(PETSC_COMM_WORLD, name.c_str(),
FILE_MODE_READ, &PETSC_matreader);
>       MatCreate(PETSC_COMM_WORLD,&PETSC_subA[i]);
>       MatLoad(PETSC_subA[i],PETSC_matreader);
>       PetscViewerDestroy(&PETSC_matreader);
> }
>
> // Reading the RHS vector and duplicating it to create the solution vector
> PetscViewerBinaryOpen(PETSC_COMM_WORLD, rhs.c_str(), FILE_MODE_READ,
&PETSC_vecreader);
> VecCreate(PETSC_COMM_WORLD,&PETSC_rhs);
> VecLoad(PETSC_rhs,PETSC_vecreader);
> PetscViewerDestroy(&PETSC_vecreader);
> VecDuplicate(PETSC_rhs,&PETSC_sol);
>
> // Create global matrixwith MatCreateNest
> MatCreateNest(PETSC_COMM_WORLD, 2, NULL, 2, NULL, PETSC_subA, &PETSC_A);
> MatNestGetISs(PETSC_A, PETSC_isg, NULL);
>
> // Setting up the ksp and precond
> KSPCreate(PETSC_COMM_WORLD,&PETSC_ksp);
> KSPSetOperators(PETSC_ksp,PETSC_A,PETSC_A);
> KSPSetFromOptions(PETSC_ksp);
>
> KSPGetPC(PETSC_ksp, &PETSC_pc);
> PCSetType(PETSC_pc, PCFIELDSPLIT);
> PCFieldSplitSetIS(PETSC_pc, "0", PETSC_isg[0]);
> PCFieldSplitSetIS(PETSC_pc, "1", PETSC_isg[1]);
> PCSetFromOptions(PETSC_pc);
>
> // Solving the system and writing back the solution in rhs file
> KSPSolve(PETSC_ksp,PETSC_rhs,PETSC_sol);
>
> PetscViewer    PETSC_vecwriter;
> PetscViewerBinaryOpen(PETSC_COMM_WORLD, rhs.c_str(), FILE_MODE_WRITE,
&PETSC_vecwriter);
> VecView(PETSC_sol,PETSC_vecwriter);
> PetscViewerDestroy(&PETSC_vecwriter);
>
> When I run it with 1 proc, everything works fine and I get the correct
solution :
>

When you get this behavior with true residuals, it very often arises from
failure to account for a pressure null space. Do you have one?

   Matt

  0 KSP preconditioned resid norm 1.271697253018e+03 true resid norm
2.400000000000e+01 ||r(i)||/||b|| 1.000000000000e+00
>   1 KSP preconditioned resid norm 5.009545069728e+01 true resid norm
9.166803391041e-02 ||r(i)||/||b|| 3.819501412934e-03
>   2 KSP preconditioned resid norm 6.460631387766e+00 true resid norm
4.995542253831e-02 ||r(i)||/||b|| 2.081475939096e-03
>   3 KSP preconditioned resid norm 1.155895209298e+00 true resid norm
1.515734830704e-02 ||r(i)||/||b|| 6.315561794600e-04
>   4 KSP preconditioned resid norm 7.407384739634e-02 true resid norm
9.992802256200e-03 ||r(i)||/||b|| 4.163667606750e-04
>   5 KSP preconditioned resid norm 1.574456882990e-02 true resid norm
9.994876664681e-03 ||r(i)||/||b|| 4.164531943617e-04
>   6 KSP preconditioned resid norm 2.383022349902e-03 true resid norm
9.990760645581e-03 ||r(i)||/||b|| 4.162816935659e-04
>   7 KSP preconditioned resid norm 6.175379834254e-04 true resid norm
9.990821066459e-03 ||r(i)||/||b|| 4.162842111025e-04
>   8 KSP preconditioned resid norm 6.867982689960e-05 true resid norm
9.990532094790e-03 ||r(i)||/||b|| 4.162721706163e-04
>   9 KSP preconditioned resid norm 1.041091257246e-05 true resid norm
9.990558069113e-03 ||r(i)||/||b|| 4.162732528797e-04
>  10 KSP preconditioned resid norm 1.447793722489e-06 true resid norm
9.990557786778e-03 ||r(i)||/||b|| 4.162732411158e-04
>  11 KSP preconditioned resid norm 2.139317335854e-07 true resid norm
9.990557262754e-03 ||r(i)||/||b|| 4.162732192814e-04
>  12 KSP preconditioned resid norm 4.383129810322e-08 true resid norm
9.990557306920e-03 ||r(i)||/||b|| 4.162732211217e-04
>  13 KSP preconditioned resid norm 3.351461304399e-09 true resid norm
9.990557311707e-03 ||r(i)||/||b|| 4.162732213211e-04
>  14 KSP preconditioned resid norm 5.169032607321e-10 true resid norm
9.990557312817e-03 ||r(i)||/||b|| 4.162732213674e-04
>
> [14:49:10::INFO   ] System Solved. Final tolerance reached is 5.16903e-10
in 14 iterations.
>
> But if I do it with 2 procs, the resolution seems fine but the solution
is wrong :
>   0 KSP preconditioned resid norm 1.247694088756e+03 true resid norm
2.400000000000e+01 ||r(i)||/||b|| 1.000000000000e+00
>   1 KSP preconditioned resid norm 4.481954484303e+01 true resid norm
5.277507840772e-01 ||r(i)||/||b|| 2.198961600321e-02
>   2 KSP preconditioned resid norm 1.110647693456e+01 true resid norm
4.005558168981e-02 ||r(i)||/||b|| 1.668982570409e-03
>   3 KSP preconditioned resid norm 1.220368027409e+00 true resid norm
1.877650834971e-02 ||r(i)||/||b|| 7.823545145714e-04
>   4 KSP preconditioned resid norm 2.834261749922e-01 true resid norm
1.613967205264e-02 ||r(i)||/||b|| 6.724863355265e-04
>   5 KSP preconditioned resid norm 4.215090288154e-02 true resid norm
1.562561614611e-02 ||r(i)||/||b|| 6.510673394212e-04
>   6 KSP preconditioned resid norm 1.209476134754e-02 true resid norm
1.563808960492e-02 ||r(i)||/||b|| 6.515870668718e-04
>   7 KSP preconditioned resid norm 2.038835108629e-03 true resid norm
1.564163643064e-02 ||r(i)||/||b|| 6.517348512765e-04
>   8 KSP preconditioned resid norm 1.928844666836e-04 true resid norm
1.564072761376e-02 ||r(i)||/||b|| 6.516969839065e-04
>   9 KSP preconditioned resid norm 3.138911950605e-05 true resid norm
1.564047323377e-02 ||r(i)||/||b|| 6.516863847403e-04
>  10 KSP preconditioned resid norm 4.950062975470e-06 true resid norm
1.564048216528e-02 ||r(i)||/||b|| 6.516867568865e-04
>  11 KSP preconditioned resid norm 7.677242244159e-07 true resid norm
1.564049253364e-02 ||r(i)||/||b|| 6.516871889019e-04
>  12 KSP preconditioned resid norm 1.870521888617e-07 true resid norm
1.564049269566e-02 ||r(i)||/||b|| 6.516871956526e-04
>  13 KSP preconditioned resid norm 3.077235724319e-08 true resid norm
1.564049264800e-02 ||r(i)||/||b|| 6.516871936666e-04
>  14 KSP preconditioned resid norm 6.584409191524e-09 true resid norm
1.564049264183e-02 ||r(i)||/||b|| 6.516871934095e-04
>  15 KSP preconditioned resid norm 1.091619359913e-09 true resid norm
1.564049263170e-02 ||r(i)||/||b|| 6.516871929874e-04
>
> [15:10:58::INFO   ] System Solved. Final tolerance reached is 1.09162e-09
in 15 iterations.
>
> Any idea of what is wrong with this ? Is it the code or the base concept ?
>
> Thank you.
> Gilles
>
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