[petsc-dev] ASM request

[Barry Smith]

On Mar 16, 2010, at 1:12 PM, Matthew Knepley wrote:

> 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.

    For each application of PCApply() you reform the beasty and factor  
it?

    I've thought a little bit about the case of tiny overlapping  
blocks before. Then even all the IS's start to be large memory  
consumers.

    It may be that we want a COMPLETELY different implementation of  
ASM for tiny blocks; in the same way as for Jacobi/SOR we have  
separate implementation and don't just use block Jacobi (PCBJACOBI)  
and have tons of tiny blocks. Also, with tiny block ASM you probably  
want to do a multiplicative version instead of an additive version to  
SOR like instead of Jacobi like.  I don't see using a shell matrix to  
do this, just  a completely different PC.

   Barry
>
> 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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