On Sat, Jun 9, 2012 at 6:43 PM, Mark F. Adams <mark.adams@columbia.edu> wrote:

1) I need a G-S kernel that takes an IS of indices to process and a flag to process them in forward or reverse order. How should I proceed to do this. Should I just clone sor?

You are going to have several of these index sets? You could have a PCSORSetIS(). Probably need to add a MatOp for MatSORIS(). Barry might have other ideas.

2) I don't want to use Richardson iterations for G-S. Should I make a G-S KPS method? I don't want to take a residual in the iterator (KSP) and if symmetric G-S is requested then it should drive this I think.

Look at PCApplyRichardson_SOR().

SOR does two sweeps in each application; I'm not wild about that because a good way to run G-S in a V(1,1) cycle is to do a forward sweep in pre smoothing and a backward sweep in post smoothing.

Well, MatSOR() has this flag MatSORType that can specify forward and reverse. You have one PC for the down-smoother and another for the up-smoother, then configure one to be a forward sweep and the other to be reverse.