[petsc-users] Calculating the PETSc index of a DMComposite vector

[Jed Brown]

Anush Krishnan <anush at bu.edu> writes:
>> C * C^T is the influence of particles on each other?  Why can't you
>> compute that analytically (without referencing the grid directly)?
>>
>
> I haven't thought about that. The interpolation coefficients would depend
> upon the relative positions of the body points with respect to the grid,
> and the product matrix will depend on their positions with respect to each
> other. I don't know if this can be done without referencing the grid.

Are you on a structured grid?  It seems like you would use fmod to
transplant the body point into a region of a reference patch, but I
don't see why you need global indices of those grid points.

> I also have a couple of related questions:
>
> [1] Suppose my body moves, and the positions of the body points change,
> will I have to completely deallocate C^T and reallocate the new rows on
> each process? It's possible that the number of non-zeros on each process
> can change. What would you suggest as the best strategy to do this?

Just create a new matrix.

> [2] For a two dimensional staggered grid, I would need to interpolate both
> the x- and y- velocity components. So my system [C][u]=[b] will be:
>
> /Cx 0 \ /u\ = /bx\
> \0  Cy/ \v/   \by/
>
> Currently, I set up the right hand side as a single vector of size
> 2*numBodyPoints using VecCreate, and use the global index of the rows when
> I want to assemble the matrix C. Suppose I made it a DMComposite of bx and
> by (which I can create as 1-D DAs), how can I reference the correct row of
> the vector?

I would have interlaced the bx and by (dof=2) so you can index it as one
vector.  Splitting components at collocated points is confusing and bad
for performance.

> Also, is it possible to create a composite vector of two vectors that are
> not distributed arrays?

You can do anything with VecScatter.
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