[petsc-users] Flops pf MatMult is less than expected

[Matthew Knepley]

On Fri, Dec 30, 2011 at 4:30 PM, Junchao Zhang <junchao.zhang at gmail.com>wrote:

> Hello,
>   I have a sparse matrix A. Its size is 494 x 494. It has 1666 nonzeros.
>   I call MatMult(A, x, y).  From the profiling result (got by
> -log_summary), I find MatMult's flops is 2838.
>   I think the flops should be nnz x 2 = 1666 x 2 = 3332.  I don't know how
> PETSc got this number (2838).
>   I also observed the phenomena with other matrices. It seems PETSc always
> give smaller flops.  Why?
>

There is 1 fewer adds than multiplies in every row:

  3332-494 = 2838

    Matt


>
>  Thank you!
> -- Junchao Zhang
>



-- 
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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