[petsc-users] Suspicious long call to VecAXPY

[Matthew Knepley]

On Fri, Jan 6, 2017 at 4:31 PM, Łukasz Kasza <rpgwars at wp.pl> wrote:

>
>
> Dear PETSc Users,
>
>         Please consider the following 2 snippets which do exactly the same
> (calculate a sum of two vectors):
>         1.
>         VecAXPY(amg_level_x[level],1.0,amg_level_residuals[level]);
>
>         2.
>         VecGetArray(amg_level_residuals[level], &values);
>         VecSetValues(amg_level_x[level],size,indices,values,ADD_VALUES);
>         VecRestoreArray(amg_level_residuals[level], &values);
>         VecAssemblyBegin(amg_level_x[level]);
>         VecAssemblyEnd(amg_level_x[level]);
>
> In my program I have both of the snippets executed in a loop. The problem
> with the first one is that the longer the program goes the longer it takes
> to execute it. At the same time the execution time of the second snippet is
> more or less constant. I don't know why but after a few hundreds of
> iterations VecAXPY takes more than MatMult on the matrix and vector of the
> same size and after that it still grows! Always returning a correct value
> though. I am using 4.5.3 version, the vectors are
> sequential. VecAXPY in such case is just a wrapper for blas, do you have
> any idea why the execution time of this function constantly grows?
>

2 should be MUCH slower than 1.

Version 4.5.3 of what?

I cannot understand what would make this happen. Can you send the output of
-log_view for two different run lengths?

   Matt


> Best regards.
>



-- 
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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