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# KSPPIPEGCR

Implements a Pipelined Generalized Conjugate Residual method.

### Options Database Keys

| -ksp_pipegcr_mmax <N> | - the max number of Krylov directions to orthogonalize against
| -ksp_pipegcr_unroll_w | - unroll w at the storage cost of a maximum of (mmax+1) extra vectors with the benefit of better pipelining (default: PETSC_TRUE)
| -ksp_pipegcr_nprealloc <N> | - the number of vectors to preallocated for storing Krylov directions. Once exhausted new directions are allocated blockwise (default: 5)
| -ksp_pipegcr_truncation_type <standard,notay> | - which previous search directions to orthogonalize against

### Notes

The PIPEGCR Krylov method supports non-symmetric matrices and permits the use of a preconditioner which may vary from one iteration to the next. Users can can define a method to vary the preconditioner between iterates via KSPPIPEGCRSetModifyPC(). Restarts are solves with x0 not equal to zero. When a restart occurs, the initial starting solution is given by the current estimate for x which was obtained by the last restart iterations of the PIPEGCR algorithm. The method implemented requires at most the storage of 4 x mmax + 5 vectors, roughly twice as much as GCR.

Only supports left preconditioning.

The natural "norm" for this method is (u,Au), where u is the preconditioned residual. This norm is available at no additional computational cost, as with standard CG. Choosing preconditioned or unpreconditioned norm types involves a blocking reduction which prevents any benefit from pipelining.

### Reference

P. Sanan, S.M. Schnepp, and D.A. May, "Pipelined, Flexible Krylov Subspace Methods," SIAM Journal on Scientific Computing 2016 38:5, C441-C470, DOI: 10.1137/15M1049130

### See Also

KSPCreate(), KSPSetType(), KSPType (for list of available types), KSP,
KSPPIPEFGMRES, KSPPIPECG, KSPPIPECR, KSPPIPEFCG,KSPPIPEGCRSetTruncationType(),KSPPIPEGCRSetNprealloc(),KSPPIPEGCRSetUnrollW(),KSPPIPEGCRSetMmax()

### Level

intermediate

### Location

src/ksp/ksp/impls/gcr/pipegcr/pipegcr.c
Index of all KSP routines
Table of Contents for all manual pages
Index of all manual pages