[petsc-users] GAMG scalability for serendipity 20 nodes hexahedra

[TARDIEU Nicolas]

Dear PETSc team,


I have run a simple weak scalability test based on canonical 3D elasticity problem : a cube, meshed with 8 nodes hexaedra, clamped on one of its face and submited to a pressure load on the opposite face.

I am using the FGMRES ksp with GAMG as preconditioner. I have set the rigid body modes using MatNullSpaceCreateRigidBody and its works like a charm. The solver exhibit a perfect scalability until 800 cores (I haven't tested with more cores). The ksp always converges in 11 or 12 iterations. Let me emphasize that I use GAMG default options.


Nevertheless, if I switch to a quadratic mesh with 20 nodes serendipity hexaedra, the weak scalability deteriorates. For instance the number of iteration for the ksp increases from 20 iterations for the smallest problem to 30 for the biggest.

Here is my question : I wonder what is the right tuning for GAMG to recover the same weak scalability as in the linear case? I apologize if this is a stupid question...

I look forward to reading you,
Nicolas




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