KSPCGUseSingleReduction#

Merge the two inner products needed in KSPCG into a single MPI_Allreduce() call.

Synopsis#

#include "petscksp.h" 
PetscErrorCode KSPCGUseSingleReduction(KSP ksp, PetscBool flg)

Logically Collective

Input Parameters#

  • ksp - the iterative context

  • flg - turn on or off the single reduction

Options Database Key#

  • -ksp_cg_single_reduction - Merge inner products into single MPI_Allreduce()

Notes#

The algorithm used in this case is described as Method 1 in [1]. V. Eijkhout credits the algorithm initially to Chronopoulos and Gear.

It requires two extra work vectors than the conventional implementation in PETSc.

See also KSPPIPECG, KSPPIPECR, and KSPGROPPCG that use non-blocking reductions. Pipelined Krylov Methods,

References#

  • [1] - Lapack Working Note 56, “Conjugate Gradient Algorithms with Reduced Synchronization Overhead Distributed Memory Multiprocessors”, by E. F. D’Azevedo, V. L. Eijkhout, and C. H. Romine, December 3, 1999.

See Also#

KSP: Linear System Solvers, Pipelined Krylov Methods, KSP, KSPCG, KSPGMRES, KSPPIPECG, KSPPIPECR, and KSPGROPPCG

Level#

intermediate

Location#

src/ksp/ksp/impls/cg/cgtype.c

Implementations#

KSPCGUseSingleReduction_CG in src/ksp/ksp/impls/cg/cg.c

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