I think your run with -pc_type mg is defining a multigrid hierarchy with a only single level. (A single level mg PC would also explain the 100+ iterations required to converge.) The gamg configuration is definitely coarsening your problem and has a deeper hierarchy. A single level hierarchy will require less memory than a multilevel hierarchy.
Cheers, Dave On Wed 24. Nov 2021 at 19:03, Matthew Knepley <knep...@gmail.com> wrote: > On Wed, Nov 24, 2021 at 12:26 PM Karthikeyan Chockalingam - STFC UKRI < > karthikeyan.chockalin...@stfc.ac.uk> wrote: > >> Hello, >> >> >> >> I would like to understand why more memory is consumed by -pc_type gamg >> compared to -pc_type mg for the same problem size >> >> >> >> ksp/ksp/tutorial: ./ex45 -da_grid_x 368 -da_grid_x 368 -da_grid_x 368 >> -ksp_type cg >> >> >> >> -pc_type mg >> >> >> >> Maximum (over computational time) process memory: total 1.9399e+10 >> max 9.7000e+09 min 9.6992e+09 >> >> >> >> -pc_type gamg >> >> >> >> Maximum (over computational time) process memory: total 4.9671e+10 >> max 2.4836e+10 min 2.4835e+10 >> >> >> >> >> Am I right in understanding that the memory limiting factor is ‘max >> 2.4836e+10’ as it is the maximum memory used at any given time? >> > > Yes, I believe so. > > GAMG is using A_C = P^T A P, where P is the prolongation from coarse to > fine, in order to compute the coarse operator A_C, rather than > rediscretization, since it does not have any notion of discretization or > coarse meshes. This takes more memory. > > Thanks, > > Matt > > >> I have attached the -log_view output of both the preconditioners. >> >> >> >> Best regards, >> >> Karthik. >> >> >> >> This email and any attachments are intended solely for the use of the >> named recipients. If you are not the intended recipient you must not use, >> disclose, copy or distribute this email or any of its attachments and >> should notify the sender immediately and delete this email from your >> system. UK Research and Innovation (UKRI) has taken every reasonable >> precaution to minimise risk of this email or any attachments containing >> viruses or malware but the recipient should carry out its own virus and >> malware checks before opening the attachments. UKRI does not accept any >> liability for any losses or damages which the recipient may sustain due to >> presence of any viruses. >> > > > -- > What most experimenters take for granted before they begin their > experiments is infinitely more interesting than any results to which their > experiments lead. > -- Norbert Wiener > > https://www.cse.buffalo.edu/~knepley/ > <http://www.cse.buffalo.edu/~knepley/> >