[petsc-users] two-level Schwarz: PCMG or PCComposite

[Hui Zhang]

I want to implement the two-level Schwarz method.  Let us denote P as the one-level
Schwarz projection and P_0 as the coarse projection.  If I want the hybrid form of 
preconditioned system as follows:

     (I-P_0) P (I-P_0) 

which is suitable:  PCMG or PCComposite?  It seems to me that PCComposite is good.
But PCMG is not suitable because it can only implement the following form:

     (I-P) P_0 (I-P)

Any opinions?  Thanks!