On Sun, Apr 2, 2017 at 2:13 PM, Barry Smith <[email protected]> wrote:
> > > On Apr 2, 2017, at 9:25 AM, Justin Chang <[email protected]> wrote: > > > > Thanks guys, > > > > So I want to run SNES ex48 across 1032 processes on Edison, but I keep > getting segmentation violations. These are the parameters I am trying: > > > > srun -n 1032 -c 2 ./ex48 -M 80 -N 80 -P 9 -da_refine 1 -pc_type mg > -thi_mat_type baij -mg_coarse_pc_type gamg > > > > The above works perfectly fine if I used 96 processes. I also tried to > use a finer coarse mesh on 1032 but the error persists. > > > > Any ideas why this is happening? What are the ideal parameters to use if > I want to use 1k+ cores? > > > > Hmm, one should never get segmentation violations. You should only get > not completely useful error messages about incompatible sizes etc. Send an > example of the segmentation violations. (I sure hope you are checking the > error return codes for all functions?). He is just running SNES ex48. Matt > > Barry > > > Thanks, > > Justin > > > > On Fri, Mar 31, 2017 at 12:47 PM, Barry Smith <[email protected]> > wrote: > > > > > On Mar 31, 2017, at 10:00 AM, Jed Brown <[email protected]> wrote: > > > > > > Justin Chang <[email protected]> writes: > > > > > >> Yeah based on my experiments it seems setting pc_mg_levels to > $DAREFINE + 1 > > >> has decent performance. > > >> > > >> 1) is there ever a case where you'd want $MGLEVELS <= $DAREFINE? In > some of > > >> the PETSc tutorial slides (e.g., http://www.mcs.anl.gov/ > > >> petsc/documentation/tutorials/TutorialCEMRACS2016.pdf on slide > 203/227) > > >> they say to use $MGLEVELS = 4 and $DAREFINE = 5, but when I ran this, > it > > >> was almost twice as slow as if $MGLEVELS >= $DAREFINE > > > > > > Smaller coarse grids are generally more scalable -- when the problem > > > data is distributed, multigrid is a good solution algorithm. But if > > > multigrid stops being effective because it is not preserving sufficient > > > coarse grid accuracy (e.g., for transport-dominated problems in > > > complicated domains) then you might want to stop early and use a more > > > robust method (like direct solves). > > > > Basically for symmetric positive definite operators you can make the > coarse problem as small as you like (even 1 point) in theory. For > indefinite and non-symmetric problems the theory says the "coarse grid must > be sufficiently fine" (loosely speaking the coarse grid has to resolve the > eigenmodes for the eigenvalues to the left of the x = 0). > > > > https://www.jstor.org/stable/2158375?seq=1#page_scan_tab_contents > > > > > > > > -- 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
