[petsc-users] Pre-conditioners in PETSc

[Gaurish Telang]

I am quite confused on using pre-conditioners in PETSc

(1)
When we use KSPSetOperators(KSP ksp,Mat Amat,Mat Pmat,MatStructure
flag), why does the manual page say that Pmat is usually the same as
Amat?
Is Pmat the preconditioning matrix itself, or is Pmat a matrix to
which preconditioning techniques must be applied via "-pc_type
<option_name>" ?

(2)
Also suppose the succeeding statement of KSPSetOperators is
KSPSetFromOptions(ksp_context) and I pass "-pc_type none" at the
terminal, would this mean that Pmat is not at all needed
in PETSc's calculations??

Thank you,

Gaurish
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