# Multi-level Schwarz Preconditioners
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Multi-level Schwarz Preconditioners

The Multilevel preconditioners implemented in MLD2P4 are obtained by combining AS preconditioners with coarse-space corrections; therefore we first provide a sketch of the AS preconditioners.

Given the linear system , where is a nonsingular sparse matrix with a symmetric nonzero pattern, let be the adjacency graph of , where and are the vertex set and the edge set of , respectively. Two vertices are called adjacent if there is an edge connecting them. For any integer , a -overlap partition of can be defined recursively as follows. Given a 0-overlap (or non-overlapping) partition of , i.e. a set of disjoint nonempty sets such that , a -overlap partition of is obtained by considering the sets obtained by including the vertices that are adjacent to any vertex in .

Let be the size of and the restriction operator that maps a vector onto the vector containing the components of corresponding to the vertices in . The transpose of is a prolongation operator from to . The matrix can be considered as a restriction of corresponding to the set .

The classical one-level AS preconditioner is defined by

where is assumed to be nonsingular. Its application to a vector within a Krylov solver requires the following three steps:

1. restriction of as , ;
2. solution of the linear systems , ;
3. prolongation and sum of the 's, i.e. .

Note that the linear systems at step 2 are usually solved approximately, e.g. using incomplete LU factorizations such as ILU(), MILU() and ILU() [20, Chapter 10].

A variant of the classical AS preconditioner that outperforms it in terms of convergence rate and of computation and communication time on parallel distributed-memory computers is the so-called Restricted AS (RAS) preconditioner [5,14]. It is obtained by zeroing the components of corresponding to the overlapping vertices when applying the prolongation. Therefore, RAS differs from classical AS by the prolongation operators, which are substituted by , where is obtained by zeroing the rows of corresponding to the vertices in :

Analogously, the AS variant called AS with Harmonic extension (ASH) is defined by

We note that for the three variants of the AS preconditioner are all equal to the block-Jacobi preconditioner.

As already observed, the convergence rate of the one-level Schwarz preconditioned iterative solvers deteriorates as the number of partitions of increases [7,21]. To reduce the dependency of the number of iterations on the degree of parallelism we may introduce a global coupling among the overlapping partitions by defining a coarse-space approximation of the matrix . In a pure algebraic setting, is usually built with the Galerkin approach. Given a set of coarse vertices, with size , and a suitable restriction operator , is defined as

and the coarse-level correction matrix to be combined with a generic one-level AS preconditioner is obtained as

where is assumed to be nonsingular. The application of to a vector corresponds to a restriction, a solution and a prolongation step; the solution step, involving the matrix , may be carried out also approximately.

The combination of and may be performed in either an additive or a multiplicative framework. In the former case, the two-level additive Schwarz preconditioner is obtained:

Applying to a vector within a Krylov solver corresponds to applying and to independently and then summing up the results.

In the multiplicative case, the combination can be performed by first applying the smoother and then the coarse-level correction operator :

this corresponds to the following two-level hybrid pre-smoothed Schwarz preconditioner:

On the other hand, by applying the smoother after the coarse-level correction, i.e. by computing

the two-level hybrid post-smoothed Schwarz preconditioner is obtained:

One more variant of two-level hybrid preconditioner is obtained by applying the smoother before and after the coarse-level correction. In this case, the preconditioner is symmetric if , and are symmetric.

As previously noted, on parallel computers the number of submatrices usually matches the number of available processors. When the size of the system to be preconditioned is very large, the use of many processors, i.e. of many small submatrices, often leads to a large coarse-level system, whose solution may be computationally expensive. On the other hand, the use of few processors often leads to local sumatrices that are too expensive to be processed on single processors, because of memory and/or computing requirements. Therefore, it seems natural to use a recursive approach, in which the coarse-level correction is re-applied starting from the current coarse-level system. The corresponding preconditioners, called multi-level preconditioners, can significantly reduce the computational cost of preconditioning with respect to the two-level case (see [21, Chapter 3]). Additive and hybrid multilevel preconditioners are obtained as direct extensions of the two-level counterparts. For a detailed descrition of them, the reader is referred to [21, Chapter 3]. The algorithm for the application of a multi-level hybrid post-smoothed preconditioner to a vector , i.e. for the computation of , is reported, for example, in Figure 1. Here the number of levels is denoted by and the levels are numbered in increasing order starting from the finest one, i.e. the finest level is level 1; the coarse matrix and the corresponding basic preconditioner at each level are denoted by and , respectively, with , while the related restriction operator is denoted by .

Figure 1: Application of the multi-level hybrid post-smoothed preconditioner.

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