def _get_voi_key(self, voi, grp):
        """Return the voi name, which allows for parallel derivative calculations
        (currently only works with LinearGaussSeidel), or None for those
        solvers that can only do a single linear solve at a time.
        """
        if (voi in self._driver_vois and
                isinstance(self.root.ln_solver, LinearGaussSeidel)):
            if (len(grp) > 1 or
                    self.root.ln_solver.options['single_voi_relevance_reduction']):
                return voi

        return None
dataset/ETHPy150Open OpenMDAO/OpenMDAO/openmdao/core/problem.py/Problem._get_voi_key/original