  In this article, we prove Talenti's comparison theorem for Poisson equation
on complete noncompact Riemannian manifold with nonnegative Ricci curvature.
Furthermore, we obtain the Faber-Krahn inequality for the first eigenvalue of
Dirichlet Laplacian, $L^1$- and $L^\infty$-moment spectrum, especially
Saint-Venant theorem for torsional rigidity and a reverse H\"older inequality
for eigenfunctions of Dirichlet Laplacian.
