[petsc-users] ISGlobalToLocalMappingApplyBlock

[Dave May]

On Wed, 15 Nov 2017 at 05:55, Adrian Croucher <a.croucher at auckland.ac.nz>
wrote:

> hi
>
> I'm trying to use ISGlobalToLocalMappingApplyBlock() and am a bit
> puzzled about the results it's giving.
>
> I've attached a small test to illustrate. It just sets up a
> local-to-global mapping with 10 elements. Running on two processes the
> first has global indices 0 - 4 and the the second has 5 - 9. I then try
> to find the local index corresponding to global index 8.
>
> If I set the blocksize parameter to 1, it correctly gives the results -1
> on rank 0 and 3 on rank 1.
>
> But if I set the blocksize to 2 (or more), the results are -253701943 on
> rank 0 and -1 on rank 1. Neither of these are what I expected- I thought
> they should be the same as in the blocksize 1 case.


The man page says to use "block global  numbering"


>
> I'm presuming the global indices I pass in to
> ISGlobalToLocalMappingApplyBlock() should be global block indices (i.e.
> not scaled up by blocksize).


Yes, the indices should relate to the blocks

If I do scale them up it doesn't give the
> answers I expect either.
>
> Or am I wrong to expect this to give the same results regardless of
> blocksize?


Yep.

However the large negative number being printed looks an uninitialized
variable. This seems odd as with mode = MASK nout should equal N and any
requested block indices not in the IS should result in -1 being inserted in
your local_indices array.

What's the value of nout?

Thanks,
  Dave


>
> Cheers, Adrian
>
> --
> Dr Adrian Croucher
> Senior Research Fellow
> Department of Engineering Science
> University of Auckland, New Zealand
> email: a.croucher at auckland.ac.nz
> tel: +64 (0)9 923 4611
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20171115/d8c17868/attachment.html>