VecNormAvailable#

Returns the vector norm if it is already known. That is, it has been previously computed and cached in the vector

Synopsis#

#include "petscvec.h"   
PetscErrorCode VecNormAvailable(Vec x, NormType type, PetscBool *available, PetscReal *val)

Not Collective

Input Parameters#

x - the vector
type - one of NORM_1 (sum_i |x[i]|), NORM_2 sqrt(sum_i (x[i])^2), NORM_INFINITY max_i |x[i]|. Also available NORM_1_AND_2, which computes both norms and stores them in a two element array.

Output Parameters#

available - PETSC_TRUE if the val returned is valid
val - the norm

Performance Issues#

    per-processor memory bandwidth
    interprocessor latency
    work load imbalance that causes certain processes to arrive much earlier than others

Developer Note#

PETSC_HAVE_SLOW_BLAS_NORM2 will cause a C (loop unrolled) version of the norm to be used, rather than the BLAS. This should probably only be used when one is using the FORTRAN BLAS routines (as opposed to vendor provided) because the FORTRAN BLAS NRM2() routine is very slow.