I would first run with -ksp_monitor_true_residual -ksp_converged_reason to make sure that those "very fast" cases are actually converging in those runs also use -ksp_view to see what the GMAG parameters are. Also use the -info option to have it print details on the solution process.
Barry > On Nov 29, 2019, at 4:14 PM, Felipe Giacomelli <fe.wall...@gmail.com> wrote: > > Hello, > > I'm trying to solve Biot's poroelasticity (Cryer's sphere problem) through a > fully coupled scheme. Thus, the solution of a single linear system yields > both displacement and pressure fields, > > |K L | | u | = |b_u|. > |Q (A + H) | | p | = |b_p| > > The linear system is asymmetric, given that the discrete equations were > obtained through the Element based Finite Volume Method (EbFVM). An > unstructured tetrahedral grid is utilised, it has about 10000 nodal points > (not coarse, nor too refined). Therefore, GMRES and GAMG are employed to > solve it. > > Furthermore, the program was parallelised through a Domain Decomposition > Method. Thus, each processor works in its subdomain only. > > So far, so good. For a given set of poroelastic properties (which are > constant throughout time and space), the speedup increases as more processors > are utilised: > > coupling intensity: 7.51e-01 > > proc solve time [s] > 1 314.23 > 2 171.65 > 3 143.21 > 4 149.26 (> 143.21, but ok) > > However, after making the problem MORE coupled (different poroelastic > properties), a strange behavior is observed: > > coupling intensity: 2.29e+01 > > proc solve time [s] > 1 28909.35 > 2 192.39 > 3 181.29 > 4 14463.63 > > Recalling that GMRES and GAMG are used, KSP takes about 4300 iterations to > converge when 1 processor is employed. On the other hand, for 2 processors, > KSP takes around 30 iterations to reach convergence. Hence, explaining the > difference between the solution times. > > Increasing the coupling even MORE, everything goes as expected: > > coupling intensity: 4.63e+01 > > proc solve time [s] > 1 229.26 > 2 146.04 > 3 121.49 > 4 107.80 > > Because of this, I ask: > > * What may be the source of this behavior? Can it be predicted? > * How can I remedy this situation? > > At last, are there better solver-pc choices for coupled poroelasticity? > > Thank you, > Felipe
