What is the practical difference between setLeftPrec and setRightPrec in Belos?

[jjramsey]

I have a decent idea of what left and right preconditioning mean mathematically, but I'm not sure what it means in practice for Belos, especially when used with Ifpack2 or other packages.

A lot of the examples I see for Belos use setRightPrec(), but the Ifpack2 guide uses setLeftPrec(), and it's not clear to me if it's okay to use one or the other as a default, or if the correct choice depends on the preconditioning scheme, for (a possibly wrong) example, if I use the "DIAGONAL" preconditioner from Ifpack2, I need to use setLeftPrec() while using "CHEBYCHEV".

Or does Belos figure out how to do the "right thing", somehow, with performance being the main difference between the choice of setLeftPrec() and setRightPrec()?

@trilinos/belos

[cgcgcg]

Yes, it means the same for Belos as what you are thinking. Unless I am forgetting something, it should be ok to use left or right preconditioning for every possible preconditioner. I would recommend using right preconditioning because it should give you a consistent norm for convergence checks that does not depend on the choice of preconditioner.