[petsc-users] Effect of -pc_gamg_threshold vs PETSc version

[Jeremy Theler]

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

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