I don't know if we are ready for this dramatic change. I wouldn't do it until 
there was a clear need (not just potential future usage) that would be 
cumbersome to do without this generality. I am a little scared of this without 
demonstrated general need.

  I would start by having an abstract inner product object (which includes a 
norm function) that has the standard l2 implementation, then another 
implementation that is based on passing in a matrix, maybe one based on passing 
in a vector.  Then each solver would have a XXXSetInnerProduct() while 
defaulting to l2.

  Barry


> On Apr 12, 2018, at 12:21 PM, Munson, Todd <tmun...@mcs.anl.gov> wrote:
> 
> 
> There is a bit of code in TAO that allows the user to change the norm to 
> a matrix norm.  This was introduced to get some mesh independent 
> behavior in one example (tao/examples/tutorials/ex3.c).  That 
> norm, however, does not propagate down into the KSP methods
> and is only used for testing convergence of the nonlinear
> problem.
> 
> A few questions then:  Is similar functionality needed in SNES?  Are 
> TAO and SNES even the right place for this functionality?  Should 
> it belong to the Vector class so that you can change the inner 
> products and have all the KSP methods (hopefully) work 
> correctly?
> 
> Note: that this discussion brings us to the brink of supporting an 
> optimize-then-discretize approach.  I am not convinced we should 
> go down that rabbit hole.
> 
> Thanks, Todd.
> 

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