[petsc-users] The multiplication of large matrices

[Matthew Knepley]

On Wed, May 29, 2013 at 6:43 AM, Joon hee Choi <choi240 at purdue.edu> wrote:

> Hello all,
>
> I am trying to compute the multiplication of large matrices using petsc.
> First,
>
> I=2.6*10^7, J=4.8*10^7.
> The matrix X is as follows: size of I*(I*J), block size of I, and
> non-zeros of 1.4*10^8.
> -> X=[X1 X2 ...]
> The matrix B is as follows: size of I*1, and dense.
> The matrix M is as follows: size of I*J. M is the multiplication of each
> block of the matrix X and the matrix B.
> -> M=[X1*B X2*B ...]
>
> I have to get the matrix M, given X and B. I successfully set up very
> large aij matrix X. However, I don't know what I have to do next. I tried
> two ways, but I failed.
>
> First, I tried to get each block X1, X2,... using ISCreate and
> MatGetLocalSubMatrix, compute X1*B, X2*B,... using MatMatMult, and then set
> up the matrix M using ISLocalToGlobalMappingCreate and
> MatSetLocalToGlobalMapping. However, I got "No support for this operation
> for this object type" error. Also, this code was very slow because of 48
> million loops.
>

Look, we have rules on this list. Without them, we cannot help you. You
MUST send the ENTIRE error output.
Without that, we are just guessing.



> Second, I tried to set up the new matrix BB from B. The matrix BB has the
> size of (I*J)*J and the block size of I*1. And every diagonal block of BB
> is B and all other blocks are 0 matrices. That is,
>
>      | B 0 0 .. 0 |
> BB = | 0 B 0 .. 0 |
>      | ...        |
>      | 0 0 0 .. B |
>
> This is not a block diagonal matrix because B is not square. Anyway, if I
> set up BB, I can get M easily because M=X*BB. However, I got "out of
> memory" error from MatCreateSeqAIJ. I think this is because BB has
> non-zeros of I*J=10^15.
>

If you get an Out Of Memory error (which we need to see), it means that you
are out of memory.

    Matt


> I never have any other ideas. If someone can fix my wrong ways correctly
> or has new ideas, then please please let me know. Thank you very much.
>
> Joon
>



-- 
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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