[petsc-users] Nullspaces for schur complement PCs

[Lawrence Mitchell]

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On 11/11/14 15:18, Jed Brown wrote:

...


> Matt and I have a long-running disagreement about nullspace
> handling in PETSc.  He added the code that hangs it on an IS
> despite it not logically belonging on IS, simply because the
> implementation in PETSc happens to pass the IS along to the place
> he needed a nullspace.  He's also making a bunch of assumptions
> about nullspaces agreeing in cases where there is no such guarantee
> (demonstrably, and in problems that we care about, but not on the
> problem he was solving at that time).
> 
> Matt would be happy if someone fixed the code to handle nullspaces
> in a logically consistent way, but we can't just tear out his
> inconsistent code without having a replacement.  Sadly, all the
> replacements I can think of are onerous for the user and
> potentially hard to implement/maintain.  So I mostly stay quiet and
> work on other things, but it pains me every time a user grows a
> dependency on the abuse of ISs and perverse assumptions about
> nullspaces agreeing.  I'd love to hear suggestions about how to get
> nullspace information into inner solvers without violating the
> object model.

So is the problem that the nullspace belongs to the appropriate
operator, but at the point you tell the solver, you don't necessarily
have all the operators to hand?  Moreover, depending on the PC choice,
the nullspaces of the operator blocks may or may not be relevant?

>> I guess my best option is to user a "user" PC for the schur 
>> complement, because then I can set everything up by hand.
> 
> That isn't a bad choice.

I'll go with this, who knows, it may cause me to unearth some better
ideas.

Cheers,

Lawrence


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