Of course, I'll try to explain. I am solving a parabolic equation with space-time FEM and I want an efficient solver/preconditioner for the resulting system. The corresponding matrix, call it X, has an e.g. block bi-diagonal structure, if the cG(1)-dG(0) method is used (i.e. implicit Euler solved in batch). Every block-row of X corresponds to a time instant. I want to introduce parallelism in time by subdividing X into overlapping submatrices of e.g 2x2 or 3x3 blocks, along the block diagonal. For instance, call X_i the individual blocks. The submatrices would be, for various i, (X_{i-1,i-1},X_{i-1,i};X_{i,i-1},X_{i,i}). I'd like each submatrix to be solved in parallel, to combine the various results together in an ASM like fashion. Every submatrix has thus a predecessor and a successor, and it overlaps with both, so that as far as I could understand, GASM has to be used in place of ASM. Hope this helps. Best, Leonardo Il giorno gio 4 mag 2023 alle ore 18:05 Matthew Knepley <[email protected]> ha scritto:
On Thu, May 4, 2023 at 11:24 AM LEONARDO MUTTI < [email protected]> wrote:
Thank you for the help. Adding to my example:
* call PCGASMSetSubdomains(pc,NSub, subdomains_IS, inflated_IS,ierr) call PCGASMDestroySubdomains(NSub,subdomains_IS,inflated_IS,ierr)* results in:
* Error LNK2019 unresolved external symbol PCGASMDESTROYSUBDOMAINS referenced in function ... *
* Error LNK2019 unresolved external symbol PCGASMSETSUBDOMAINS referenced in function ... * I'm not sure if the interfaces are missing or if I have a compilation problem.
I just want to make sure you really want GASM. It sounded like you might able to do what you want just with ASM. Can you tell me again what you want to do overall?
Thanks,
Matt
Thank you again. Best, Leonardo
Il giorno sab 29 apr 2023 alle ore 20:30 Barry Smith <[email protected]> ha scritto:
Thank you for the test code. I have a fix in the branch barry/2023-04-29/fix-pcasmcreatesubdomains2d <https://gitlab.com/petsc/petsc/-/commits/barry/2023-04-29/fix-pcasmcreatesubdomains2d> with merge request https://gitlab.com/petsc/petsc/-/merge_requests/6394
The functions did not have proper Fortran stubs and interfaces so I had to provide them manually in the new branch.
Use
git fetch git checkout barry/2023-04-29/fix-pcasmcreatesubdomains2d <https://gitlab.com/petsc/petsc/-/commits/barry/2023-04-29/fix-pcasmcreatesubdomains2d> ./configure etc
Your now working test code is in src/ksp/ksp/tests/ex71f.F90 I had to change things slightly and I updated the error handling for the latest version.
Please let us know if you have any later questions.
Barry
On Apr 28, 2023, at 12:07 PM, LEONARDO MUTTI < [email protected]> wrote:
Hello. I am having a hard time understanding the index sets to feed PCGASMSetSubdomains, and I am working in Fortran (as a PETSc novice). To get more intuition on how the IS objects behave I tried the following minimal (non) working example, which should tile a 16x16 matrix into 16 square, non-overlapping submatrices:
#include <petsc/finclude/petscmat.h> #include <petsc/finclude/petscksp.h> #include <petsc/finclude/petscpc.h> USE petscmat USE petscksp USE petscpc
Mat :: A PetscInt :: M, NSubx, dof, overlap, NSub INTEGER :: I,J PetscErrorCode :: ierr PetscScalar :: v KSP :: ksp PC :: pc IS :: subdomains_IS, inflated_IS
call PetscInitialize(PETSC_NULL_CHARACTER , ierr)
!-----Create a dummy matrix M = 16 call MatCreateAIJ(MPI_COMM_WORLD, PETSC_DECIDE, PETSC_DECIDE, & M, M, & PETSC_DEFAULT_INTEGER, PETSC_NULL_INTEGER, & PETSC_DEFAULT_INTEGER, PETSC_NULL_INTEGER, & A, ierr)
DO I=1,M DO J=1,M v = I*J CALL MatSetValue (A,I-1,J-1,v, & INSERT_VALUES , ierr) END DO END DO
call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY , ierr) call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY , ierr)
!-----Create KSP and PC call KSPCreate(PETSC_COMM_WORLD,ksp, ierr) call KSPSetOperators(ksp,A,A, ierr) call KSPSetType(ksp,"bcgs",ierr) call KSPGetPC(ksp,pc,ierr) call KSPSetUp(ksp, ierr) call PCSetType(pc,PCGASM, ierr) call PCSetUp(pc , ierr)
!-----GASM setup NSubx = 4 dof = 1 overlap = 0
call PCGASMCreateSubdomains2D(pc, & M, M, & NSubx, NSubx, & dof, overlap, & NSub, subdomains_IS, inflated_IS, ierr)
call ISView(subdomains_IS, PETSC_VIEWER_STDOUT_WORLD, ierr)
call KSPDestroy(ksp, ierr) call PetscFinalize(ierr)
Running this on one processor, I get NSub = 4. If PCASM and PCASMCreateSubdomains2D are used instead, I get NSub = 16 as expected. Moreover, I get in the end "forrtl: severe (157): Program Exception - access violation". So: 1) why do I get two different results with ASM, and GASM? 2) why do I get access violation and how can I solve this? In fact, in C, subdomains_IS, inflated_IS should pointers to IS objects. As I see on the Fortran interface, the arguments to PCGASMCreateSubdomains2D are IS objects:
subroutine PCGASMCreateSubdomains2D(a,b,c,d,e,f,g,h,i,j,z) import tPC,tIS PC a ! PC PetscInt b ! PetscInt PetscInt c ! PetscInt PetscInt d ! PetscInt PetscInt e ! PetscInt PetscInt f ! PetscInt PetscInt g ! PetscInt PetscInt h ! PetscInt IS i ! IS IS j ! IS PetscErrorCode z end subroutine PCGASMCreateSubdomains2D Thus: 3) what should be inside e.g., subdomains_IS? I expect it to contain, for every created subdomain, the list of rows and columns defining the subblock in the matrix, am I right?
Context: I have a block-tridiagonal system arising from space-time finite elements, and I want to solve it with GMRES+PCGASM preconditioner, where each overlapping submatrix is on the diagonal and of size 3x3 blocks (and spanning multiple processes). This is PETSc 3.17.1 on Windows.
Thanks in advance, Leonardo
-- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener
https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>