[petsc-users] Suspicious long call to VecAXPY

[Łukasz Kasza]

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? 

Best regards.