[petsc-users] Better understanding VecScatter

[Mark Adams]

So if you have three processors you would want the first processor to have
all x gradients, etc.?

You could use ISCreateGeneral:
https://www.mcs.anl.gov/petsc/petsc-current/src/dm/examples/tutorials/ex6.c.html

You need to create an integer array with the global indices that you want
to bring to your new vector locally.

On Thu, Oct 4, 2018 at 8:56 AM Phil Tooley <phil.tooley at sheffield.ac.uk>
wrote:

> Hi all,
>
> I am trying to understand how to create a custom scatter from petsc
> ordering in a local vector to natural ordering in a global vector.
>
> I have a 3D DMDA and local vector containing field data and am
> calculating the x y and z gradients into their own local vectors.  I
> then need to scatter these gradients to different parts of a single
> vector so they are arranged [x_1 .. x_n y_1 .. y_n z_1 .. z_n] with
> natural ordering.  (This is for use in a custom numerical scheme that I
> have been asked to parallelise)
>
> I can of course do this using multiple scatters but I need to perform
> this operation regularly and would like to express it as a single
> scatter operation if possible.
>
> I know I need to get the relevant AO from my DMDA and use it to
> construct appropriate ISs to build the scatter context but I am unsure
> how exactly to go about this.
>
> Can someone point me in the right direction please?
>
> Many Thanks
>
> Phil
>
> --
> Phil Tooley
> Research Software Engineering
> University of Sheffield
>
>
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