On Thu, Jul 26, 2018 at 11:04 AM Pierre Jolivet <pierre.joli...@enseeiht.fr> wrote:
> > > > On 26 Jul 2018, at 4:24 PM, Karl Rupp <r...@iue.tuwien.ac.at> wrote: > > > > Hi Pierre, > > > >> I’m using GAMG on a shifted Laplacian with these options: > >> -st_fieldsplit_pressure_ksp_type preonly > >> -st_fieldsplit_pressure_pc_composite_type additive > >> -st_fieldsplit_pressure_pc_type composite > >> -st_fieldsplit_pressure_sub_0_ksp_pc_type jacobi > >> -st_fieldsplit_pressure_sub_0_pc_type ksp > >> -st_fieldsplit_pressure_sub_1_ksp_pc_gamg_square_graph 10 > >> -st_fieldsplit_pressure_sub_1_ksp_pc_type gamg > >> -st_fieldsplit_pressure_sub_1_pc_type ksp > >> and I end up with the following logs on 512 (top) and 2048 (bottom) > processes: > >> MatMult 1577790 1.0 3.1967e+03 1.2 4.48e+12 1.6 7.6e+09 > 5.6e+03 0.0e+00 7 71 75 63 0 7 71 75 63 0 650501 > >> MatMultAdd 204786 1.0 1.3412e+02 5.5 1.50e+10 1.7 5.5e+08 > 2.7e+02 0.0e+00 0 0 5 0 0 0 0 5 0 0 50762 > >> MatMultTranspose 204786 1.0 4.6790e+01 4.3 1.50e+10 1.7 5.5e+08 > 2.7e+02 0.0e+00 0 0 5 0 0 0 0 5 0 0 145505 > >> [..] > >> KSPSolve_FS_3 7286 1.0 7.5506e+02 1.0 9.14e+11 1.8 7.3e+09 > 1.5e+03 2.6e+05 2 14 71 16 34 2 14 71 16 34 539009 > >> MatMult 1778795 1.0 3.5511e+03 4.1 1.46e+12 1.9 4.0e+10 > 2.4e+03 0.0e+00 7 66 75 61 0 7 66 75 61 0 728371 > >> MatMultAdd 222360 1.0 2.5904e+0348.0 4.31e+09 1.9 2.4e+09 > 1.3e+02 0.0e+00 14 0 4 0 0 14 0 4 0 0 2872 > >> MatMultTranspose 222360 1.0 1.8736e+03421.8 4.31e+09 1.9 2.4e+09 > 1.3e+02 0.0e+00 0 0 4 0 0 0 0 4 0 0 3970 > >> [..] > >> KSPSolve_FS_3 7412 1.0 2.8939e+03 1.0 2.66e+11 2.1 3.5e+10 > 6.1e+02 2.7e+05 17 11 67 14 28 17 11 67 14 28 148175 > >> MatMultAdd and MatMultTranspose (performed by GAMG) somehow ruin the > scalability of the overall solver. The pressure space “only” has 3M > unknowns so I’m guessing that’s why GAMG is having a hard time strong > scaling. > > > > 3M unknowns divided by 512 processes implies less than 10k unknowns per > process. It is not unusual to see strong scaling roll off at this size. > Also note that the time per call(!) for "MatMult" is the same for both > cases, indicating that your run into a latency-limited regime. > > > > Also, have a look at the time ratios: With 2048 processes, MatMultAdd > and MatMultTranspose show a time ratio of 48 and 421, respectively. Maybe > one of your MPI ranks is getting a huge workload? > > Maybe inside GAMG itself (how could I check this?), but since the timing > and ratio of the MatMult look OK and the distribution of the pressure space > is the same as the other three fields, I’m guessing this does not come from > my global Mat, but I may be wrong. > > >> For the other fields, the matrix is somehow distributed nicely, i.e., I > don’t want to change the overall distribution of the matrix. > >> Do you have any suggestion to improve the performance of GAMG in that > scenario? I had two ideas in mind but please correct me if I’m wrong or if > this is not doable: > >> 1) before setting up GAMG, first use a PCTELESCOPE to avoid having too > many processes work on this small problem > Do you mean the coarse grids? GAMG reduces active processors (and repartitions the coarse grids if you ask it to) like telescope. > >> 2) have the sub_0_ and the sub_1_ work on two different nonoverlapping > communicators of size PETSC_COMM_WORLD/2, do the solve concurrently, and > then sum the solutions (only worth doing because of -pc_composite_type > additive). I have no idea if this easily doable with PETSc command line > arguments > > > > 1) is the more flexible approach, as you have better control over the > system sizes after 'telescoping’. > > Right, but the advantage of 2) is that I wouldn't have one half or more of > processes idling and I could overlap the solves of both subpc in the > PCCOMPOSITE. > > I’m attaching the -log_view for both runs (I trimmed some options). > > Thanks for your help, > Pierre > > > > Best regards, > > Karli > >
