[petsc-dev] ASM request

[Matthew Knepley]

On Tue, Mar 16, 2010 at 1:51 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Mar 16, 2010, at 11:28 AM, Matthew Knepley wrote:
>
>  There is a request to make a more memory efficient version of ASM for
>> running
>> very large system of equations. They are using LU on small diagonal
>> blocks, and
>> have asked that the blocks be factored, applied, and discarded, rather
>> than being
>> saved for all iterations. Does anyone think this is easy in the current
>> ASM code?
>> Have an alternate proposal?
>>
>
>   I need a better understanding. Is it that after the KSPSolve() they want
> more memory available for their own use that they will allocate after the
> KSPSolve and free before the next PCSetUp()?  If this is what they want then
> I think it is trivial to implement, but we will need an API so that they can
> say when they are done with the solves and want us to free up the memory.
>
>  Normally ASM is memory intense because
> 1) it makes copies of the matrix subblocks
> 2) it factors those subblocks into additional memory.
>
> If you use ILU(0) and the inplace option then it only needs one copy of the
> blocks instead of the two.


Yes, told them that so 1/2 off memory. I guess the easiest way to give them
this might be to put a shell matrix in the KSP
that sucks out the submatrix, factors it in place, applies it, and discards
it.

Here is the motivation:

   You have a preconditioner which is ASM, but with a LOT of small blocks.
You do not save very much by discarding one
big block, but if you only ever form each small block, you get a lot of
savings.

    Matt


>
>   Barry
>
>
>>   Matt
>>
>> --
>> 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
>>
>
>


-- 
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
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