[petsc-users] GAMG memory consumption

[Dave May]

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 <knepley at gmail.com> wrote:

> On Wed, Nov 24, 2021 at 12:26 PM Karthikeyan Chockalingam - STFC UKRI <
> karthikeyan.chockalingam at 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.
>>
>>
>>
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>
> --
> 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/>
>
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