[petsc-users] Better understanding VecScatter

[Phil Tooley]

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