MatScatter: scatter insert mode

[Jed Brown]

Dmitry Karpeev wrote:
> Currently MatScatter wraps a VecScatter and presents the scatter application
> as the action of a linear operator (which I like a lot).
> However, the underlying scatter is invoked in the ADD_VALUES mode.
> Shouldn't we allow INSERT_VALUES as well?  Perhaps with a
> MatScatterSetInsertMode call?

No.

We had a discussion about this a few months ago.  Without a number of
constraints on what function the scatter represents, you can easily end
up with an operation that is not linear.  If these conditions are
satisfied, then INSERT_VALUES is equivalent to the ADD_VALUES
implementation and we would just be optimizing for streaming.

Jed

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