[petsc-users] MG Preconditioning
Barry Smith
bsmith at mcs.anl.gov
Sat Sep 1 17:33:11 CDT 2012
On Sep 1, 2012, at 5:16 PM, Gaetan Kenway <kenway at utias.utoronto.ca> wrote:
> Hi Jed
>
> After probing why the following code snippit isn't run:
>
> if (mg->galerkin == 1) {
> Mat B;
> /* currently only handle case where mat and pmat are the same on coarser levels */
> dB = B;
> }
> }
>
> It is because after I use PCMGSetGalerkin(), mg->galerkin has a value of -1 not 1. From what I can tell the value of PETSC_TRUE is architecture/implementation dependant. I can't see where/how the value of mg->galerkin obtains a value of 1 and runs the code above.
>
> As for the other issue, after converting my baij matrix to aij, running MatRART() is ok.
>
> So we just have to figure out why calling
>
>
> call PCMGSetGalerkin(pc, PETSC_TRUE, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> sets mg->galerkin to a value of -1 and the code above is expecting a value of 1.
This is a mistake with our Fortran interface. Some Fortran compilers (for what crazy reason I don't know) use -1 as bool for true. Normally this doesn't matter because we have if (boolvalue) { checks and so negative one and one work the same way;
But the code mg->galerkin = (PetscInt)use; in PCMGSetGalekin() fails because we then check the numerical value of the result, instead of just if it is zero or nonzero.
I can change this conversion to mg->galerkin = use ? 1 : 0 so the the Fortran use of -1 gets converted to one and this should resolve the problem.
I have pushed this fix into the petsc-3.3 and petsc-dev repositories. If you are using 3.3 I've attached a new src/ksp/pc/impls/mg/mg.c that you can drop into that directory and run make in that directory to update your library. If you are using petsc-dev then do an hg pull -u and rerun make in that directory.[see attached file: mg.c]
Thanks for determining the cause of this rather nasty bug.
Barry
>
> Gaetan
>
>
>
>
> On Sat, Sep 1, 2012 at 5:34 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> On Sat, Sep 1, 2012 at 4:20 PM, Gaetan Kenway <kenway at utias.utoronto.ca> wrote:
> Sorry about the last email...I hit sent it too early.
>
> The full calling sequence is:
>
>
> call KSPCreate(SUMB_COMM_WORLD, ksp, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> useAD = .False.
> useTranspose = .True.
> usePC = .True.
> call setupStateResidualMatrix(drdwpret,useAD,usePC,useTranspose)
>
> ! dRdwT has already been created
> call KSPSetOperators(ksp, dRdWT, dRdwpreT, &
> DIFFERENT_NONZERO_PATTERN, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> call KSPSetFromOptions(ksp, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> call KSPSetType(ksp, KSPFGMRES, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> call KSPGMRESSetRestart(ksp, adjRestart, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> call KSPGMRESSetCGSRefinementType(ksp, &
> KSP_GMRES_CGS_REFINE_IFNEEDED, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> call kspgetpc(ksp, pc, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> call pcsettype(pc, PCMG, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> call PetscOptionsSetValue('-pc_mg_levels','2',PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> ! Set command line options
> call PCSetFromOptions(pc, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> call PCMGSetGalerkin(pc, PETSC_TRUE, PETScIerr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> ! Create the restriction operator between finest and one below:
> call createRestrictOperator(RL1, 1)
>
> call PCMGSetRestriction(pc, 1, RL1, PETScierr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> call PCSetup(pc, PETScierr)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> The error occurs in PCSetup() which calls PCSetup_MG() and the issue is in KSPGetVecs() on line 644 in mg.c. Just to clarify, this isn't a memory problem/segfault.
>
> The output below is a SEGV.
>
> PETSc returns control cleanly and I my code exits when the error is detected. Therefore a backtrace or valgrind doesn't yield any additional information. ( I Ran both).
>
> You can still set a breakpoint in PetscError (-start_in_debugger / -on_error_attach_debugger does this automatically). Then you can examine the stack.
>
> The terminal output is:
>
> [0]PETSC ERROR: ------------------------------------------------------------------------
> [0]PETSC ERROR: Caught signal number 11 SEGV: Segmentation Violation, probably memory access out of range
> [0]PETSC ERROR: Try option -start_in_debugger or -on_error_attach_debugger
> [0]PETSC ERROR: or see http://www.mcs.anl.gov/petsc/documentation/faq.html#valgrind[0]PETSC ERROR: or try http://valgrind.org on GNU/linux and Apple Mac OS X to find memory corruption errors
> [0]PETSC ERROR: likely location of problem given in stack below
> [0]PETSC ERROR: --------------------- Stack Frames ------------------------------------
> =================================================================
> PETSc or MPI Error. Error Code 73. Detected on Proc 0
> Error at line: 130 in file: setupPETScKsp.F90
> =================================================================
> [0]PETSC ERROR: Note: The EXACT line numbers in the stack are not available,
> [0]PETSC ERROR: INSTEAD the line number of the start of the function
> [0]PETSC ERROR: is given.
> [0]PETSC ERROR: [0] MatGetVecs line 8142 /nfs/mica/home/kenway/Downloads/petsc-3.3/src/mat/interface/matrix.c
> [0]PETSC ERROR: [0] KSPGetVecs line 774 /nfs/mica/home/kenway/Downloads/petsc-3.3/src/ksp/ksp/interface/iterativ.c
> [0]PETSC ERROR: [0] PCSetUp_MG line 508 /nfs/mica/home/kenway/Downloads/petsc-3.3/src/ksp/pc/impls/mg/mg.c
> [0]PETSC ERROR: [0] PCSetUp line 810 /nfs/mica/home/kenway/Downloads/petsc-3.3/src/ksp/pc/interface/precon.c
>
> I had a look at ex42.c but it uses a DM which I am not doing.
>
> It uses the DM for convenience to build the operators, but it does *not* tell the KSP about the DM. That is, it does not call KSPSetDM() or PCSetDM(). So the code in mg.c does not know about the DM.
>
> From the example there is the following:
>
> ierr = PCMGSetLevels(pc,nlevels,PETSC_NULL);CHKERRQ(ierr);
> ierr = PCMGSetType(pc,PC_MG_MULTIPLICATIVE);CHKERRQ(ierr);
> ierr = PCMGSetGalerkin(pc,PETSC_TRUE);CHKERRQ(ierr);
>
> for (k=1; k<nlevels; k++){
> ierr = DMCreateInterpolation(da_list[k-1],da_list[k],&R,PETSC_NULL);CHKERRQ(ierr);
> ierr = PCMGSetInterpolation(pc,k,R);CHKERRQ(ierr);
> ierr = MatDestroy(&R);CHKERRQ(ierr);
> }
>
> Is there anything special about "R" in this case?
>
> Nothing special, it is just an MAIJ matrix that performs the interpolation.
>
> As I mentioned before, I know what the issue is, the coarse grid operators are not set at all. From what I can tell, they should be set in the routine PCSetup_MG in mg.c, but the code is never run unless pc->dm is True.
>
> What code is not run unless pc->dm?
>
> I guess I'll just create the operators myself. Even if I modify the code to make it enter the if statement at line 577
>
> You called PCMGSetGalerkin() so the code below should run, constructing the coarse levels. You could try setting a breakpoint in there to see why it's not running.
>
>
> if (mg->galerkin == 1) {
> Mat B;
> /* currently only handle case where mat and pmat are the same on coarser levels */
> ierr = KSPGetOperators(mglevels[n-1]->smoothd,&dA,&dB,&uflag);CHKERRQ(ierr);
> if (!pc->setupcalled) {
> for (i=n-2; i>-1; i--) {
> ierr = MatPtAP(dB,mglevels[i+1]->interpolate,MAT_INITIAL_MATRIX,1.0,&B);CHKERRQ(ierr);
> ierr = KSPSetOperators(mglevels[i]->smoothd,B,B,uflag);CHKERRQ(ierr);
> if (i != n-2) {ierr = PetscObjectDereference((PetscObject)dB);CHKERRQ(ierr);}
> dB = B;
> }
> if (n > 1) {ierr = PetscObjectDereference((PetscObject)dB);CHKERRQ(ierr);}
> } else {
> for (i=n-2; i>-1; i--) {
> ierr = KSPGetOperators(mglevels[i]->smoothd,PETSC_NULL,&B,PETSC_NULL);CHKERRQ(ierr);
> ierr = MatPtAP(dB,mglevels[i+1]->interpolate,MAT_REUSE_MATRIX,1.0,&B);CHKERRQ(ierr);
> ierr = KSPSetOperators(mglevels[i]->smoothd,B,B,uflag);CHKERRQ(ierr);
> dB = B;
> }
> }
>
> which seems like where it should be done....the MatPTAP will not run.
>
> What do you mean "will not run"?
>
> I'm using a mpibaij matrix for 'dRdwPreT' and my restriction matrix 'RL1' is a mpiaij matrix. When I try to run MatRART myself with the code:
>
> call MatRART(dRdwT, RL1, MAT_INITIAL_MATRIX, 1.0, newMat, PETScIERR)
> call EChk(PETScIerr,__FILE__,__LINE__)
>
> I receive error code 56: /* no support for requested operation */ (this is a clean exit from PETSc)
>
> MatRARt (or MatPtAP, for that matter) is not implemented for BAIJ with an AIJ projection. The AIJ projection will lose the block structure anyway, so just use AIJ for both.
>
>
> In conclusion, I have two issues: 1) The coarse grid operators are not being set automatically in mg.c. 2) Using MatRART() with a mpibaij matrix and a mpiaij matrix is unsupported.
>
> Is there a PCMG example that doesn't use a DM to generate the restriction/prolongation/matrix information I can have a look at?
>
>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: mg.c
Type: application/octet-stream
Size: 41758 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120901/3f090e13/attachment-0001.obj>
More information about the petsc-users
mailing list