[petsc-users] Yet another inversion question

[Barry Smith]

On May 2, 2010, at 7:13 AM, Jed Brown wrote:

> On Sat, 1 May 2010 17:00:22 -0500, Barry Smith <bsmith at mcs.anl.gov>  
> wrote:
>>    This cannot be! Perhaps you used the PETSc default solver for my
>> suggestion but UMFPACK for your version? With a direct solve "all"  
>> the
>> work will be in the factorization and triangular solves so if you use
>> UMFPACK for both they both do the same amount of work.
>
> For what it's worth, extracting a column should cost a fair amount  
> less
> than doing a solve since the factored matrix will usually have a lot
> more entries.

    Perhaps. But the current get column code is terribly inefficient.

    So is the issue that MatMatSolve(A,B,X) requires TWO dense  
matrices B and X, instead of just one and that is too much memory?

    Perhaps we could modify MatMatSolve(A,B,X) to allow an in-place  
version (i.e. B == X) that uses only 1 or several work vectors then  
you no longer have this
memory issue? Should I add this?


    Barry

>
> Jed