In this paper we try to achieve h-independent convergence with preconditioned GMRES ([Y. Saad and M. H. Schultz, SIAM J. Sci. Comput., 7 (1986), pp. 856-869]) and BiCGSTAB ([H. A. Van der Vorst, SIAM J. Sci. Comput., 13 (1992), pp. 63-644]) for two-dimensional (2D) singularly perturbed equations. Three recently developed multigrid methods are adopted as a preconditioner. They are also used as solution methods in order to compare the performance of the methods as solvers and as preconditioners.Two of the multigrid methods differ only in the transfer operators. One uses standard matrixdependent prolongation operators from []. Both employ the Galerkin coarse grid approximation and an alternating zebra line Gauss-Seidel smoother. The third method is based on the block LU decomposition of a matrix and on an approximate Schur complement. This multigrid variant is presented in [A. Reusken, A Multigrid Method Based on Incomplete Gaussian Elimination, University of Eindhoven, the Netherlands, 1995]. All three multigrid algorithms are algebraic methods.The eigenvalue spectra of the three multigrid iteration matrices are analyzed for the equations solved in order to understand the convergence of the three algorithms. Furthermore, the construction of the search directions for the multigrid preconditioned GMRES solvers is investigated by the calculation and solution of the minimal residual polynomials.For Poisson and convection-diffusion problems all solution methods are investigated and evaluated for finite volume discretizations on fine grids. The methods have been parallelized with a grid partitioning technique and are compared on an MIMD machine. ).
C. W. OOSTERLEE AND T. WASHIOwith jumping coefficients, can be solved efficiently. The first algorithm includes wellknown matrix-dependent operators from the literature ([3], [6]). In [3] the operators have been designed so that problems on grids with arbitrary mesh sizes, not just powers of two (+1), can be solved with similar efficiency. Although in [3] these operators are mainly used for symmetric interface problems, we will also consider them here for unsymmetric problems.The second algorithm uses the prolongation operators introduced by de Zeeuw ([25]), which are specially designed for solving unsymmetric problems. Both algorithms employ Galerkin coarsening ([6], [24]) for building the matrices on coarser grids. A robust smoother is a necessary requirement in standard multigrid for achieving grid-size-independent convergence for many problems. The alternating zebra line Gauss-Seidel relaxation method was found to be a robust smoother for 2D model equations ([15], [24]). This smoother is used since it is possible to parallelize a line solver efficiently on a parallel machine ([23]). Constructing robust parallel solvers is an important purpose of our work.The third multigrid algorithm in our comparison is based on the block Gaussian elimination of a matrix and an approximate Schur complement. This method has been recently introduced by Reusken ([12]). An interesting aspect ...