[petsc-users] dmmg_grid_sequence

[Xuefeng Li]

On Mon, 2 Apr 2012, Matthew Knepley wrote:

> On Mon, Apr 2, 2012 at 10:11 AM, Xuefeng Li <li at loyno.edu> wrote:
>
>> Hi, all.
>>
>> I am using multigrid with DMMG (5 levels) in my petsc program.
>> Turning on the dmmg_grid_sequence option seems
>> to slow down the program dramatically. For instance,
>> the program converges in 3 iterations in 6.270e+01 sec
>> when dmmg_grid_sequence is OFF; it also converges
>> in 3 iterations but in 1.699e+04 sec when dmmg_grid_sequence
>> is ON.
>>
>> Is this typical of petsc (I'm using petsc-3.1-p8)?
>> Are there ways to improve the performance when
>> dmmg_grid_sequence is ON?
>>
>
> Its impossible to know what is happening from this description. There is no
> "generic" performance of grid sequencing. The best thing I can say to this
> is,
> it looks like the interpolated solution to your coarse problem is not a
> great
> solution to your fine problem.
>
That is possible.



> However, you could be oversolving the coarse
> problems, blowing up the work, or interpolating badly for your problem.
>
What do you mean by "oversolving the coarse problems"?
I am using the same stopping conditions in the two runs.
The only difference is dmmg_grid_sequence ON and OFF.

I also observed that in other situations, when dmmg_grid_sequence
is ON, the execution will abort due to "Out of memory" error,
whereas when OFF, the program converges without error.
How much more memory does grid-sequence employ? Twice as much?

Regards,

--Xuefeng Li, (504)865-3340(phone)
   Like floating clouds, the heart rests easy
   Like flowing water, the spirit stays free
        http://www.loyno.edu/~li/home
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