Hi Mark. So glad you answered. > 0) what is your test problem? eg, 3D Lapacian with Q1 finite > elements.
I said in my first email it was linear elasticty (and I gave a link where you can see the geometry, BCs, etc.) but I did not specifty further details. It is linear elasticity with displacement-based FEM formulation using unstructured curved 10-noded tetrahedra. The matrix is marked as SPD with MatSetOption() and the solver is indeed CG and not the default GMRES. > First, you can get GAMG diagnostics by running with '-info :pc' and > grep on GAMG. Great advice. Now I have a lot more of information but I'm not sure how to analyze it. Find attached for each combination of threshold and PETSc version the output of -info :pc -ksp_monitor -ksp_view In general it looks like 3.18 and 3.19 have less KSP iterations than 3.17 but the overall time is larger. > Anyway, sorry for the changes. > I hate changing GAMG for this reason and I hate AMG for this reason! No need to apologize, I just want to better understand how to better exploit your code! Thanks -- jeremy > > Thanks, > Mark > > > > On Thu, Apr 13, 2023 at 8:17 AM Jeremy Theler <jer...@seamplex.com> > wrote: > > When using GAMG+cg for linear elasticity and providing the near > > nullspace computed by MatNullSpaceCreateRigidBody(), I used to find > > "experimentally" that a small value of -pc_gamg_threshold in the > > order > > of 0.0001 would slightly decrease the solve time. > > > > Starting with 3.18, I started seeing that any positive value for > > the > > treshold would increase the solve time. I did a quick parametric > > (serial) run solving an elastic problem with a matrix size of > > approx > > 570k x 570k for different values of GAMG threshold and different > > PETSc > > versions (compiled with the same compiler, options and flags). > > > > I noted that > > > > 1. starting from 3.18, a threshold of 0.0001 that used to improve > > the > > speed now worsens it. > > 2. PETSc 3.17 looks like a "sweet spot" of speed > > > > I would like to hear any comments you might have. > > > > The wall time shown includes the time needed to read the mesh and > > assemble the stiffness matrix. It is a refined version of the > > NAFEMS > > LE10 benchmark described here: > > https://seamplex.com/feenox/examples/mechanical.html#nafems-le10- > > thick-plate-pressure-benchmark > > > > If you want, I could dump the matrix, rhs and near nullspace > > vectors > > and share them. > > > > -- > > jeremy theler > >
log.tar.gz
Description: application/compressed-tar
