[petsc-users] Bad memory scaling with PETSc 3.10

[Mark Adams]

In looking at this larger scale run ...

* Your eigen estimates are much lower than your tiny test problem.  But
this is Stokes apparently and it should not work anyway. Maybe you have a
small time step that adds a lot of mass that brings the eigen estimates
down. And your min eigenvalue (not used) is positive. I would expect
negative for Stokes ...

* You seem to be setting a threshold value of 0.1 -- that is very high

* v3.6 says "using nonzero initial guess" but this is not in v3.10. Maybe
we just stopped printing that.

* There were some changes to coasening parameters in going from v3.6 but it
does not look like your problem was effected. (The coarsening algo is
non-deterministic by default and you can see small difference on different
runs)

* We may have also added a "noisy" RHS for eigen estimates by default from
v3.6.

* And for non-symetric problems you can try -pc_gamg_agg_nsmooths 0, but
again GAMG is not built for Stokes anyway.


On Tue, Mar 5, 2019 at 11:53 AM Myriam Peyrounette <
myriam.peyrounette at idris.fr> wrote:

> I used PCView to display the size of the linear system in each level of
> the MG. You'll find the outputs attached to this mail (zip file) for both
> the default threshold value and a value of 0.1, and for both 3.6 and 3.10
> PETSc versions.
>
> For convenience, I summarized the information in a graph, also attached
> (png file).
>
> As you can see, there are slight differences between the two versions but
> none is critical, in my opinion. Do you see anything suspicious in the
> outputs?
>
> + I can't find the default threshold value. Do you know where I can find
> it?
>
> Thanks for the follow-up
>
> Myriam
>
> Le 03/05/19 à 14:06, Matthew Knepley a écrit :
>
> On Tue, Mar 5, 2019 at 7:14 AM Myriam Peyrounette <
> myriam.peyrounette at idris.fr> wrote:
>
>> 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?
>>
> Mark, what is the option she can give to output all the GAMG data?
>
> Also, run using -ksp_view. GAMG will report all the sizes of its grids, so
> it should be easy to see
> if the coarse grid sizes are increasing, and also what the effect of the
> threshold value is.
>
>   Thanks,
>
>      Matt
>
>> 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 <
>> petsc-users at mcs.anl.gov> 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
>> --
>>
>>
>
> --
> 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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