[petsc-users] Fieldsplit with LSC for constrained elasticity/poroelasticity?

[Tabrez Ali]

Hello

I am using the following options (below) for solving linear 
elasticity/poroelasticity problems involving slip between two surfaces 
involving non-trivial geometries, i.e., elements with high aspect 
ratios, large contrasts in material properties etc. The constraints are 
imposed using Lagrange Multipliers.

A picture (shows displacement magnitude) is attached. The boundary 
nodes, i.e., the base and the four side are pinned.

The following options appear to work well for the saddle point problem:

-pc_type fieldsplit -pc_fieldsplit_type schur 
-pc_fieldsplit_detect_saddle_point -fieldsplit_0_pc_type gamg 
-fieldsplit_0_ksp_type preonly -fieldsplit_1_pc_type lsc 
-fieldsplit_1_ksp_type preonly -pc_fieldsplit_schur_fact_type lower 
-ksp_monitor

However, the number of iterations keep on increasing with the problems 
size (see attached plot), e.g.,

120K Tets *507* Iterations (KSP Residual norm 8.827362494659e-05)in  17 
secs on   3 cores
1 Million Tets *1374* Iterations (KSP Residual norm 
7.164704416296e-05)in 117 secs on  20 cores
8 Million Tets *2495* Iterations (KSP Residual norm 9.101247550026e-05) 
in 225 secs on 160 cores

So what other options should I try to improve solver performance? Any 
tips/insights would be appreciated as preconditioning is black magic to me.

Thanks in advance.

Tabrez
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