[petsc-users] PCMG behaviour on various levels

[Filippo Leonardi]

Hi,

Quick PCMG question.

Default behaviour of PCMG (I'm using 3D DMDAs) is to use all processes for 
every level and redundantly solve the coarse level locally (to my knowledge).

However, if I want to keep the level numbers to be log_c N for (c=2^d, 
d=dimension, N = dof of finest mesh) so that the coarse level is alway the same 
size (say N_0) I'll occur in scaling problems. At some point, if I increase N 
and the number of processors (I do N \sim #proc), I'll obtain too many 
processes solving the coarsest levels (to the point where each processor has 
less than 1 dof).

Is PETSc doing this by default (and therefore, I am missing something?)?

PCMGLevels is the right choice for this case. My idea is to subdivide 
PETSC_COMM_WORLD with Goup_Inlc groups. Have anyone tried/have an example for 
this (all I've got are bad failures)?

My plan is solving a 3D ploblem scaling as folloiwng:
32^3	1 proc		4-6 levels
64^3	8 proc		5-7 levels
128^3	64 proc       	6-8 levels
....

Best,
Filippo
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