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Research into the design of Krylov subspace methods for solving nonsymmetric linear systems is an active field of research and new methods are still emerging. In this book, we have included only the best known and most popular methods, and in particular those for which extensive computational experience has been gathered. In this section, we shall briefly highlight some of the recent developments and other methods not treated here. A survey of methods up to about 1991 can be found in Freund, Golub and Nachtigal . Two more recent reports by Meier-Yang  and Tong  have extensive numerical comparisons among various methods, including several more recent ones that have not been discussed in detail in this book.
Several suggestions have been made to reduce the increase in memory and computational costs in GMRES. An obvious one is to restart (this one is included in §): GMRES(m). Another approach is to restrict the GMRES search to a suitable subspace of some higher-dimensional Krylov subspace. Methods based on this idea can be viewed as preconditioned GMRES methods. The simplest ones exploit a fixed polynomial preconditioner (see Johnson, Micchelli and Paul , Saad , and Nachtigal, Reichel and Trefethen ). In more sophisticated approaches, the polynomial preconditioner is adapted to the iterations (Saad ), or the preconditioner may even be some other (iterative) method of choice (Van der Vorst and Vuik , Axelsson and Vassilevski ). Stagnation is prevented in the GMRESR method (Van der Vorst and Vuik ) by including LSQR steps in some phases of the process. In Desturler and Fokkema , part of the optimality of GMRES is maintained in the hybrid method GCRO, in which the iterations of the preconditioning method are kept orthogonal to the iterations of the underlying GCR method. All these approaches have advantages for some problems, but it is far from clear a priori which strategy is preferable in any given case.
Recent work has focused on endowing the BiCG method with several desirable properties: (1) avoiding breakdown; (2) avoiding use of the transpose; (3) efficient use of matrix-vector products; (4) smooth convergence; and (5) exploiting the work expended in forming the Krylov space with for further reduction of the residual.
As discussed before, the BiCG method can have two kinds of breakdown: Lanczos breakdown (the underlying Lanczos process breaks down), and pivot breakdown (the tridiagonal matrix implicitly generated in the underlying Lanczos process encounters a zero pivot when Gaussian elimination without pivoting is used to factor it). Although such exact breakdowns are very rare in practice, near breakdowns can cause severe numerical stability problems.
The pivot breakdown is the easier one to overcome and there have been several approaches proposed in the literature. It should be noted that for symmetric matrices, Lanczos breakdown cannot occur and the only possible breakdown is pivot breakdown. The SYMMLQ and QMR methods discussed in this book circumvent pivot breakdown by solving least squares systems. Other methods tackling this problem can be found in Fletcher , Saad , Gutknecht , and Bank and Chan .
Lanczos breakdown is much more difficult to eliminate. Recently, considerable attention has been given to analyzing the nature of the Lanczos breakdown (see Parlett , and Gutknecht ), as well as various look-ahead techniques for remedying it (see Brezinski and Sadok , Brezinski, Zaglia and Sadok , Freund and Nachtigal , Parlett , Nachtigal , Freund, Gutknecht and Nachtigal , Joubert , Freund, Golub and Nachtigal , and Gutknecht ). However, the resulting algorithms are usually too complicated to give in template form (some codes of Freund and Nachtigal are available on netlib.) Moreover, it is still not possible to eliminate breakdowns that require look-ahead steps of arbitrary size (incurable breakdowns). So far, these methods have not yet received much practical use but some form of look-ahead may prove to be a crucial component in future methods.
In the BiCG method, the need for matrix-vector multiplies with can be inconvenient as well as doubling the number of matrix-vector multiplies compared with CG for each increase in the degree of the underlying Krylov subspace. Several recent methods have been proposed to overcome this drawback. The most notable of these is the ingenious CGS method by Sonneveld  discussed earlier, which computes the square of the BiCG polynomial without requiring -- thus obviating the need for . When BiCG converges, CGS is often an attractive, faster converging alternative. However, CGS also inherits (and often magnifies) the breakdown conditions and the irregular convergence of BiCG (see Van der Vorst ).
CGS also generated interest in the possibility of product methods, which generate iterates corresponding to a product of the BiCG polynomial with another polynomial of the same degree, chosen to have certain desirable properties but computable without recourse to . The Bi-CGSTAB method of Van der Vorst  is such an example, in which the auxiliary polynomial is defined by a local minimization chosen to smooth the convergence behavior. Gutknecht  noted that Bi-CGSTAB could be viewed as a product of BiCG and GMRES(1), and he suggested combining BiCG with GMRES(2) for the even numbered iteration steps. This was anticipated to lead to better convergence for the case where the eigenvalues of are complex. A more efficient and more robust variant of this approach has been suggested by Sleijpen and Fokkema in , where they describe how to easily combine BiCG with any GMRES(m), for modest m.
Many other basic methods can also be squared. For example, by squaring the Lanczos procedure, Chan, de Pillis and Van der Vorst  obtained transpose-free implementations of BiCG and QMR. By squaring the QMR method, Freund and Szeto  derived a transpose-free QMR squared method which is quite competitive with CGS but with much smoother convergence. Unfortunately, these methods require an extra matrix-vector product per step (three instead of two) which makes them less efficient.
In addition to Bi-CGSTAB, several recent product methods have been designed to smooth the convergence of CGS. One idea is to use the quasi-minimal residual (QMR) principle to obtain smoothed iterates from the Krylov subspace generated by other product methods. Freund  proposed such a QMR version of CGS, which he called TFQMR. Numerical experiments show that TFQMR in most cases retains the desirable convergence features of CGS while correcting its erratic behavior. The transpose free nature of TFQMR, its low computational cost and its smooth convergence behavior make it an attractive alternative to CGS. On the other hand, since the BiCG polynomial is still used, TFQMR breaks down whenever CGS does. One possible remedy would be to combine TFQMR with a look-ahead Lanczos technique but this appears to be quite complicated and no methods of this kind have yet appeared in the literature. Recently, Chan et. al.  derived a similar QMR version of Van der Vorst's Bi-CGSTAB method, which is called QMRCGSTAB. These methods offer smoother convergence over CGS and Bi-CGSTAB with little additional cost.
There is no clear best Krylov subspace method at this time, and there will never be a best overall Krylov subspace method. Each of the methods is a winner in a specific problem class, and the main problem is to identify these classes and to construct new methods for uncovered classes. The paper by Nachtigal, Reddy and Trefethen  shows that for any of a group of methods (CG, BiCG, GMRES, CGNE, and CGS), there is a class of problems for which a given method is the winner and another one is the loser. This shows clearly that there will be no ultimate method. The best we can hope for is some expert system that guides the user in his/her choice. Hence, iterative methods will never reach the robustness of direct methods, nor will they beat direct methods for all problems. For some problems iterative schemes and for others direct methods (or multigrid) will be most attractive. We hope to find suitable methods (and preconditioners) for classes of very large problems that we are yet unable to solve by any known method, because of CPU-restrictions, memory, convergence problems, ill-conditioning, et cetera.