Hi Matt, I plotted the memory scalings using different threshold values. The two scalings are slightly translated (from -22 to -88 mB) but this gain is neglectable. The 3.6-scaling keeps being robust while the 3.10-scaling deteriorates.
Do you have any other suggestion? Thanks Myriam Le 03/02/19 à 02:27, Matthew Knepley a écrit : > On Fri, Mar 1, 2019 at 10:53 AM Myriam Peyrounette via petsc-users > <[email protected] <mailto:[email protected]>> wrote: > > Hi, > > I used to run my code with PETSc 3.6. Since I upgraded the PETSc > version > to 3.10, this code has a bad memory scaling. > > To report this issue, I took the PETSc script ex42.c and slightly > modified it so that the KSP and PC configurations are the same as > in my > code. In particular, I use a "personnalised" multi-grid method. The > modifications are indicated by the keyword "TopBridge" in the attached > scripts. > > To plot the memory (weak) scaling, I ran four calculations for each > script with increasing problem sizes and computations cores: > > 1. 100,000 elts on 4 cores > 2. 1 million elts on 40 cores > 3. 10 millions elts on 400 cores > 4. 100 millions elts on 4,000 cores > > The resulting graph is also attached. The scaling using PETSc 3.10 > clearly deteriorates for large cases, while the one using PETSc 3.6 is > robust. > > After a few tests, I found that the scaling is mostly sensitive to the > use of the AMG method for the coarse grid (line 1780 in > main_ex42_petsc36.cc). In particular, the performance strongly > deteriorates when commenting lines 1777 to 1790 (in > main_ex42_petsc36.cc). > > Do you have any idea of what changed between version 3.6 and version > 3.10 that may imply such degradation? > > > I believe the default values for PCGAMG changed between versions. It > sounds like the coarsening rate > is not great enough, so that these grids are too large. This can be > set using: > > > https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCGAMGSetThreshold.html > > There is some explanation of this effect on that page. Let us know if > setting this does not correct the situation. > > Thanks, > > Matt > > > Let me know if you need further information. > > Best, > > Myriam Peyrounette > > > -- > Myriam Peyrounette > CNRS/IDRIS - HLST > -- > > > > -- > 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/%7Eknepley/> -- Myriam Peyrounette CNRS/IDRIS - HLST --
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