strange convergency behavior with petsc (intialy produces good result, suddenly diverges)
Shengyong
lua.byhh at gmail.com
Mon Aug 18 00:35:22 CDT 2008
hi, Barry
Thanks for your kindly reply!
Here I report some experiments accoording to your hints.
-pc_type sor -pc_sor_local_symmetric _ksp_type cg failed.
icc(1) also failed.
When using -ksp_monitor_true _residual with icc(0), I have found the precond
residual is osillation around some value about 1.06e-004, the true residual
norm almost staying at 7.128 e-004. The ||Ae||/||Ax|| also stagnent at
6.3e-005.
After the maximum number of iterations (=2000) reached, the iteration fails.
Even when I set iteration number to 10000, it seems that petsc also fails.
Below is some resiual information near iteration number 2000 when with
ksp_monitor_true_residual.
1990 KSP preconditioned resid norm 1.064720311837e-004 true resid norm
7.1284721
18425e-004 ||Ae||/||Ax|| 6.302380221818e-005
1991 KSP preconditioned resid norm 1.062055494352e-004 true resid norm
7.1281202
17324e-004 ||Ae||/||Ax|| 6.302069101215e-005
1992 KSP preconditioned resid norm 1.061228895583e-004 true resid norm
7.1277740
15661e-004 ||Ae||/||Ax|| 6.301763019565e-005
1993 KSP preconditioned resid norm 1.062165148129e-004 true resid norm
7.1274335
10277e-004 ||Ae||/||Ax|| 6.301461974073e-005
1994 KSP preconditioned resid norm 1.064780917764e-004 true resid norm
7.1270986
90168e-004 ||Ae||/||Ax|| 6.301165955010e-005
1995 KSP preconditioned resid norm 1.068986431546e-004 true resid norm
7.1267695
37853e-004 ||Ae||/||Ax|| 6.300874946922e-005
1996 KSP preconditioned resid norm 1.074687043135e-004 true resid norm
7.1264460
30395e-004 ||Ae||/||Ax|| 6.300588929530e-005
1997 KSP preconditioned resid norm 1.081784773720e-004 true resid norm
7.1261281
40328e-004 ||Ae||/||Ax|| 6.300307878551e-005
1998 KSP preconditioned resid norm 1.090179775109e-004 true resid norm
7.1258158
36472e-004 ||Ae||/||Ax|| 6.300031766417e-005
1999 KSP preconditioned resid norm 1.099771672684e-004 true resid norm
7.1255090
84603e-004 ||Ae||/||Ax|| 6.299760562872e-005
2000 KSP preconditioned resid norm 1.110460758301e-004 true resid norm
7.1252078
48108e-004 ||Ae||/||Ax|| 6.299494235544e-005
On Sun, Aug 17, 2008 at 10:32 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
> Try using -pc_type sor -pc_sor_local_symmetric -ksp_type cg
>
> Also try running the original icc one with the additional option
> -ksp_monitor_true_residual, see if funky stuff starts to happen.
>
> You could also try adding -pc_factor_levels 1 to try ICC(1) instead of
> ICC(0).
>
> Your code looks fine, I don't see a problem there,
>
> Barry
>
>
> Here is my guess, as the simulation proceeds the variable coefficient
> problem changes enough so that the ICC produces
> a badly scaled preconditioner that messes up the iterative method. I see
> this on occasion and don't have a good fix, the shift
> positive definite helps sometimes but not always.
>
>
>
>
> On Aug 17, 2008, at 3:36 AM, Shengyong wrote:
>
> Hi,
>>
>> I am still struggling to use petsc to solve variable coefficient poisson
>> problems (which orinates from a multi-phase(liquid-gas two phase flow with
>> sharp interface method, the density ratio is 1000, and with surface tension)
>> flow problem) successively.
>>
>> Initially petsc produces good results with icc pc_type , cg iterative
>> method and with _pc_factor_shift_positive_define flag. I follow petsc's
>> null space method to constrain the singular property of coefficient matrix.
>> However, after several time steps good results of simulation, the KSP
>> Residual Norm suddenly reaches to a number greater than 1000. I guess that
>> petsc diverges. I have also tried to swith to other types of methods, e.g.
>> to gmeres, it just behaves similar to cg method. However, when I use my
>> previous self-coded SOR iterative solver, it produces nice results. And I
>> have tested the same petsc solver class for a single phase driven cavity
>> flow problem, it also produces nice results.
>>
>> It seems that I have missed something important setup procedure in my
>> solver class. could anyone to point me the problem ? I have attached large
>> part of the code below:
>>
>> //in Header
>> class CPETScPoissonSolver
>> {
>> typedef struct {
>> Vec x,b; //x, b
>> Mat A; //A
>> KSP ksp; //Krylov subspace preconditioner
>> PC pc;
>> MatNullSpace nullspace;
>> PetscInt l, m, n;//L, M, N
>> } UserContext;
>>
>> public:
>> CPETScPoissonSolver(int argc, char** argv);
>> ~CPETScPoissonSolver(void);
>>
>> //........
>>
>> private:
>>
>> //Yale Sparse Matrix format matrix
>> PetscScalar* A;
>> PetscInt * I;
>> PetscInt * J;
>>
>> // Number of Nonzero Element
>> PetscInt nnz;
>> //grid step
>> PetscInt L, M, N;
>> UserContext userctx;
>> private:
>> bool FirstTime;
>> };
>>
>> //in cpp
>> static char helpPetscPoisson[] = "PETSc class Solves a variable Poisson
>> problem with Null Space Method.\n\n";
>>
>> CPETScPoissonSolver::CPETScPoissonSolver(int argc, char** argv)
>> {
>> PetscInitialize(&argc, &argv, (char*)0, helpPetscPoisson);
>> FirstTime=true;
>> }
>> CPETScPoissonSolver::~CPETScPoissonSolver(void)
>> {
>> PetscFinalize();
>> }
>> ......
>> void CPETScPoissonSolver::SetAIJ(PetscScalar *a, PetscInt *i, PetscInt *j,
>> PetscInt Nnz)
>> {
>> A= a; I=i; J=j; nnz = Nnz;
>> }
>>
>> PetscErrorCode CPETScPoissonSolver::UserInitializeLinearSolver()
>> {
>> PetscErrorCode ierr = 0;
>> PetscInt Num = L*M*N;
>>
>> //Since we use successive solvers, so in the second time step we must
>> deallocate the original matrix then setup a new one
>> if(FirstTime==true)
>> {
>> ierr = MatCreateSeqAIJWithArrays(PETSC_COMM_SELF, Num, Num, I,
>> J, A, &userctx.A); CHKERRQ(ierr);
>> }
>> else
>> {
>> FirstTime = false;
>> ierr = MatDestroy(userctx.A);CHKERRQ(ierr);
>> ierr = MatCreateSeqAIJWithArrays(PETSC_COMM_SELF, Num, Num, I,
>> J, A, &userctx.A); CHKERRQ(ierr);
>> }
>>
>> if(FirstTime==true)
>> {
>>
>> ierr =
>> VecCreateSeqWithArray(PETSC_COMM_SELF,Num,PETSC_NULL,&userctx.b);CHKERRQ(ierr);
>> ierr = VecDuplicate(userctx.b,&userctx.x);CHKERRQ(ierr);
>> ierr = MatNullSpaceCreate(PETSC_COMM_SELF, PETSC_TRUE, 0,
>> PETSC_NULL, &userctx.nullspace); CHKERRQ(ierr);
>> ierr = KSPCreate(PETSC_COMM_SELF,&userctx.ksp);CHKERRQ(ierr);
>> /*Set Null Space for KSP*/
>> ierr = KSPSetNullSpace(userctx.ksp,
>> userctx.nullspace);CHKERRQ(ierr);
>> }
>> return 0;
>> }
>>
>>
>> PetscErrorCode CPETScPoissonSolver::UserSetBX(PetscScalar *x, PetscScalar
>> *b)
>> {
>> PetscErrorCode ierr ;
>> //below code we must set it every time step
>> ierr = VecPlaceArray(userctx.x,x);CHKERRQ(ierr);
>> ierr = VecPlaceArray(userctx.b,b);CHKERRQ(ierr);
>> ierr = MatNullSpaceRemove(userctx.nullspace,userctx.b,
>> PETSC_NULL);CHKERRQ(ierr);
>> return 0;
>> }
>>
>> PetscInt CPETScPoissonSolver::UserSolve()
>> {
>> PetscErrorCode ierr;
>> ierr =
>> KSPSetOperators(userctx.ksp,userctx.A,userctx.A,SAME_NONZERO_PATTERN);CHKERRQ(ierr);
>> ierr = KSPSetType(userctx.ksp, KSPCG);
>> ierr = KSPSetInitialGuessNonzero(userctx.ksp, PETSC_TRUE);
>> ierr = KSPGetPC(userctx.ksp,&userctx.pc);CHKERRQ(ierr);
>> ierr = PCSetType(userctx.pc,PCICC);CHKERRQ(ierr);
>> ierr = PCFactorSetShiftPd(userctx.pc, PETSC_TRUE);
>> ierr =
>> KSPSetTolerances(userctx.ksp,1.e-4,PETSC_DEFAULT,PETSC_DEFAULT,2000);
>> ierr = KSPSetFromOptions(userctx.ksp);CHKERRQ(ierr);
>> /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>> -
>> Solve the linear system
>> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>> - */
>> ierr = KSPSolve(userctx.ksp,userctx.b,userctx.x);CHKERRQ(ierr);
>>
>> ierr = VecResetArray(userctx.x);CHKERRQ(ierr);
>> ierr = VecResetArray(userctx.b);CHKERRQ(ierr);
>>
>> return 0;
>> }
>>
>> PetscErrorCode CPETScPoissonSolver::ReleaseMem()
>> {
>> PetscErrorCode ierr;
>> ierr = KSPDestroy(userctx.ksp);CHKERRQ(ierr);
>> ierr = VecDestroy(userctx.x); CHKERRQ(ierr);
>> ierr = VecDestroy(userctx.b); CHKERRQ(ierr);
>> ierr = MatDestroy(userctx.A); CHKERRQ(ierr);
>> ierr = MatNullSpaceDestroy(userctx.nullspace); CHKERRQ(ierr);
>> return 0;
>> }
>>
>> Thanks very much!
>>
>>
>> --
>> Pang Shengyong
>> Solidification Simulation Lab,
>> State Key Lab of Mould & Die Technology,
>> Huazhong Univ. of Sci. & Tech. China
>>
>
>
--
Pang Shengyong
Solidification Simulation Lab,
State Key Lab of Mould & Die Technology,
Huazhong Univ. of Sci. & Tech. China
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