[petsc-users] Number of levels of multigrid : 2-3 is sufficient ??

Wed Oct 14 22:53:29 CDT 2015

OK,

Richardson is 30-70% faster for these tests, but other than this I don't
see any change.

Timothee

2015-10-15 12:37 GMT+09:00 Barry Smith <bsmith at mcs.anl.gov>:

>
>   Timothee,
>
>      Thank you for reporting this issue, it is indeed disturbing and could
> be due to a performance regression we may have introduced by being too
> clever for our own good. Could you please rerun with the additional option
> -mg_levels_ksp_type richardson and send the same output?
>
>    Thanks
>
>   Barry
>
> > On Oct 14, 2015, at 9:32 PM, Timothée Nicolas <
> timothee.nicolas at gmail.com> wrote:
> >
> > Thank you Barry for pointing this out. Indeed on a system with no
> debugging the Jacobian evaluations no longer dominate the time (less than
> 10%). However the rest is similar, except the improvement from 2 to 3
> levels is much better. Still it saturates after levels=3. I understand it
> in terms of CPU time thanks to Matthew's explanations, however what
> surprises me more is that KSP iterations are not more efficient. At the
> least, even if it takes more time to have more levels because of memory
> issues, I would expect KSP iterations to converge more rapidly with more
> levels, but it is not the case as you can see. Probably there is also a
> rationale behind this but I cannot see easily.
> >
> > I send the new outputs
> >
> > Best
> >
> > Timothee
> >
> > 2015-10-15 3:02 GMT+09:00 Barry Smith <bsmith at mcs.anl.gov>:
> > 1) Your timings are meaningless! You cannot compare timings when built
> with all debugging on, PERIOD!
> >
> >   ##########################################################
> >       #                                                        #
> >       #                          WARNING!!!                    #
> >       #                                                        #
> >       #   This code was compiled with a debugging option,      #
> >       #   To get timing results run ./configure                #
> >       #   using --with-debugging=no, the performance will      #
> >       #   be generally two or three times faster.              #
> >       #                                                        #
> >       ##########################################################
> >
> > 2) Please run with -snes_view .
> >
> > 3) Note that with 7 levels
> >
> > SNESJacobianEval      21 1.0 2.4364e+01 1.0 0.00e+00 0.0 0.0e+00 0.0e+00
> 0.0e+00 54  0  0  0  0  54  0  0  0  0     0
> >
> > with 2 levels
> >
> > SNESJacobianEval       6 1.0 2.2441e+01 1.0 0.00e+00 0.0 0.0e+00 0.0e+00
> 0.0e+00 34  0  0  0  0  34  0  0  0  0     0
> >
> >
> > The Jacobian evaluation is dominating the time! Likely if you fix the
> debugging this will be less the case
> >
> >   Barry
> >
> > > On Oct 13, 2015, at 9:23 PM, Timothée Nicolas <
> timothee.nicolas at gmail.com> wrote:
> > >
> > > Dear all,
> > >
> > > I have been playing around with multigrid recently, namely with
> /ksp/ksp/examples/tutorials/ex42.c, with /snes/examples/tutorial/ex5.c and
> with my own implementation of a laplacian type problem. In all cases, I
> have noted no improvement whatsoever in the performance, whether in CPU
> time or KSP iteration, by varying the number of levels of the multigrid
> solver. As an example, I have attached the log_summary for ex5.c with
> nlevels = 2 to 7, launched by
> > >
> > > mpiexec -n 1 ./ex5 -da_grid_x 21 -da_grid_y 21 -ksp_rtol 1.0e-9
> -da_refine 6 -pc_type mg -pc_mg_levels # -snes_monitor -ksp_monitor
> -log_summary
> > >
> > > where -pc_mg_levels is set to a number between 2 and 7.
> > >
> > > So there is a noticeable CPU time improvement from 2 levels to 3
> levels (30%), and then no improvement whatsoever. I am surprised because
> with 6 levels of refinement of the DMDA the fine grid has more than 1200
> points so with 3 levels the coarse grid still has more than 300 points
> which is still pretty large (I assume the ratio between grids is 2). I am
> wondering how the coarse solver efficiently solves the problem on the
> coarse grid with such a large number of points ? Given the principle of
> multigrid which is to erase the smooth part of the error with relaxation
> methods, which are usually efficient only for high frequency, I would
> expect optimal performance when the coarse grid is basically just a few
> points in each direction. Does anyone know why the performance saturates at
> low number of levels ? Basically what happens internally seems to be quite
> different from what I would expect...
> > >
> > > Best
> > >
> > > Timothee
> > >
> <ex5_2_levels_of_multigrid.log><ex5_3_levels_of_multigrid.log><ex5_4_levels_of_multigrid.log><ex5_5_levels_of_multigrid.log><ex5_6_levels_of_multigrid.log><ex5_7_levels_of_multigrid.log>
> >
> >
> >
> <ex5_2_multigrid_levels.log><ex5_3_multigrid_levels.log><ex5_4_multigrid_levels.log><ex5_5_multigrid_levels.log><ex5_6_multigrid_levels.log><ex5_7_multigrid_levels.log>
>
>
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