December 2023 - petsc-users - mailman3.cels.anl.gov

PETSc and MPI-3/RMA
by Maeder Alexander 09 Dec '23

09 Dec '23

I am a new user of PETSc and want to know more about the underlying implementation for matrix-vector multiplication (Ax=y). PETSc utilizes a 1D distribution and communicates only parts of the vector x utilized depending on the sparsity pattern of A. Is the communication of x done with MPI-3 RMA and utilizes cuda-aware mpi for RMA? Best regards, Alexander Maeder

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Scattering a vector to/from a subset of processors
by Sreeram R Venkat 06 Dec '23

06 Dec '23

Suppose I am running on 12 processors, and I have a vector "v" of size 36 partitioned over the first 4. v still uses the PETSC_COMM_WORLD, so it has a layout of (9, 9, 9, 9, 0, 0, ..., 0). Now, I would like to repartition it over all 12 processors, so that the layout becomes (3, 3, 3, ..., 3). I've been trying to use VecScatter to do this, but I'm not sure what IndexSets to use for the sender and receiver. The result I am trying to achieve is this: Assume the vector is v = <0, 1, 2, ..., 35> Start Finish Proc | Entries Proc | Entries 0 | 0,...,8 0 | 0, 1, 2 1 | 9,...,17 1 | 3, 4, 5 2 | 18,...,26 2 | 6, 7, 8 3 | 27,...,35 3 | 9, 10, 11 4 | None 4 | 12, 13, 14 5 | None 5 | 15, 16, 17 6 | None 6 | 18, 19, 20 7 | None 7 | 21, 22, 23 8 | None 8 | 24, 25, 26 9 | None 9 | 27, 28, 29 10 | None 10 | 30, 31, 32 11 | None 11 | 33, 34, 35 Appreciate any help you can provide on this. Thanks, Sreeram

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Help for MatNullSpaceCreateRigidBody
by Jordi Manyer Fuertes 05 Dec '23

05 Dec '23

Dear PETSc users/developpers, I am currently trying to use the method `MatNullSpaceCreateRigidBody` together with `PCGAMG` to efficiently precondition an elasticity solver in 2D/3D. I have managed to make it work in serial (or with 1 MPI rank) with h-independent number of iterations (which is great), but the solver diverges in parallel. I assume it has to do with the coordinate vector I am building the null-space with not being correctly setup. The documentation is not that clear on which nodes exactly have to be set in each partition. Does it require nodes corresponding to owned dofs, or all dofs in each partition (owned+ghost)? What ghost layout should the `Vec` have? Any other tips about what I might be doing wrong? Thanks, Jordi

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Pre-check before each line search function evaluation
by Alexander Lindsay 05 Dec '23

05 Dec '23

Hi I'm looking at the sources, and I believe the answer is no, but is there a dedicated callback that is akin to SNESLineSearchPrecheck but is called before *each* function evaluation in a line search method? I am using a Hybridized Discontinuous Galerkin method in which most of the degrees of freedom are eliminated from the global system. However, an accurate function evaluation requires that an update to the "global" dofs also trigger an update to the eliminated dofs. I can almost certainly do this manually but I believe it would be more prone to error than a dedicated callback.

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