PCASM#

Use the (restricted) additive Schwarz method, each block is (approximately) solved with its own KSP object.

Options Database Keys#

-pc_asm_blocks - Sets total blocks. Defaults to one block per MPI rank.
-pc_asm_overlap - Sets overlap
-pc_asm_type [basic,restrict,interpolate,none] - Sets PCASMType, default is restrict. See PCASMSetType()
-pc_asm_local_type [additive, multiplicative] - Sets PCCompositeType, default is additive. See PCASMSetLocalType()

Notes#

If you run with, for example, 3 blocks on 1 processor or 3 blocks on 3 processors you will get a different convergence rate due to the default option of -pc_asm_type restrict. Use -pc_asm_type basic to get the same convergence behavior

Each processor can have one or more blocks, but a block cannot be shared by more than one processor. Use PCGASM for subdomains shared by multiple processes.

To set options on the solvers for each block append -sub_ to all the KSP, and PC options database keys. For example, -sub_pc_type ilu -sub_pc_factor_levels 1 -sub_ksp_type preonly

To set the options on the solvers separate for each block call PCASMGetSubKSP() and set the options directly on the resulting KSP object (you can access its PC with KSPGetPC())

References#

**** -*** M Dryja, OB Widlund, An additive variant of the Schwarz alternating method for the case of many subregions Courant Institute, New York University Technical report
**** -*** Barry Smith, Petter Bjorstad, and William Gropp, Domain Decompositions: Parallel Multilevel Methods for Elliptic Partial Differential Equations, Cambridge University Press.