## 8.2 Sparse Matrices     continued...

One of the key advantages of SPD matrices is that such a sparsity preserving ordering can be selected in advance of the numeric factorization, independent of the particular values of the nonzero entries: only the pattern of the nonzeros matters, not their numerical values. This would not be the case, in general, if we also had to take into account pivoting for numerical stability, which obviously would require knowledge of the nonzero values, and would introduce a potential conflict between preserving sparsity and preserving stability. For the SPD case, once the ordering is selected, the locations of all fill elements in L can be anticipated prior to the numeric factorization, and thus an efficient static data structure can be set up in advance to accommodate them (this process is usually called symbolic factorization). This feature also stands in contrast to general sparse linear systems, which usually require dynamic data structures to accommodate fill entries as they occur, since their locations depend on numerical information that becomes known only as the numeric factorization process unfolds. Thus, modern algorithms and software for solving sparse SPD systems include a symbolic preprocessing phase in which a sparsity-preserving ordering is computed and a static data structure is set up for storing the entries of L before any floating point computation takes place.

We introduce some concepts and notation that will be useful in our subsequent discussion of parallel sparse Cholesky factorization. An important tool in understanding the combinatorial aspects of sparse Cholesky factorization is the notion of the graph of a symmetric matrix A, which is an undirected graph having n vertices, with an edge between two vertices i and j if the corresponding entry of the matrix is nonzero. We denote the graph of A by . The structural effect of the factorization process can then be described by observing that the elimination of a variable adds fill edges to the corresponding graph so that the neighbors of the eliminated vertex become a clique (i.e. a fully connected subgraph). We also define the filled graph, denoted by , as having an edge between vertices i and j, with i > j, if in the Cholesky factor L (i.e. is simply with all fill edges added).