double compute_residual_norm2(fei::LinearSystem& fei_ls, fei::Vector& r)
{
fei::SharedPtr<fei::Matrix> A = fei_ls.getMatrix();
fei::SharedPtr<fei::Vector> x = fei_ls.getSolutionVector();
fei::SharedPtr<fei::Vector> b = fei_ls.getRHS();
//form r = A*x
A->multiply(x.get(), &r);
//form r = b - r (i.e., r = b - A*x)
r.update(1, b.get(), -1);
//!!!!!! fix this !!!!!!!!!
//terrible data copy. fei::Vector should provide a norm operation instead
//of making me roll my own here...
std::vector<int> indices;
r.getVectorSpace()->getIndices_Owned(indices);
std::vector<double> coefs(indices.size());
r.copyOut(indices.size(), &indices[0], &coefs[0]);
double local_sum = 0;
for(size_t i=0; i<indices.size(); ++i) {
local_sum += coefs[i]*coefs[i];
}
#ifdef HAVE_MPI
MPI_Comm comm = r.getVectorSpace()->getCommunicator();
double global_sum = 0;
int num_doubles = 1;
MPI_Allreduce(&local_sum, &global_sum, num_doubles, MPI_DOUBLE, MPI_SUM, comm);
#else
double global_sum = local_sum;
#endif
return std::sqrt(global_sum);
}
