On Thu, 22 Apr 2010 10:21:26 -0400, Boyce Griffith <[email protected]>
wrote:
>
>
> On 4/22/10 9:56 AM, Jed Brown wrote:
> > On Thu, 22 Apr 2010 09:32:12 -0400, Boyce Griffith<[email protected]>
> > wrote:
> >> How should one setup the index set? Is it just a mapping from Vec index
> >> to field index? E.g., each Vec entry is assigned to field 0 or 1 (or 2
> >> or ...)?
> >
> > If your variables are interlaced like
> >
> > u0,v0,w0,u1,v1,w1,...
>
> In the present context, the order of variables in memory is whatever is
> determined by libMesh --- and I haven't yet looked to see what that is.
> It's my impression that variables are not interlaced like this, but I
> could be mistaken.
Right, but Roy said this:
Block variable ordering with "--node_major_dofs" is perhaps the least
well-documented option in libMesh, which is saying a lot, but we're
already using it.
> > Then you can (a) set a block size for the matrix with MatSetBlockSize,
> > (b) use BAIJ in which case the block size is built in, or (c) use
> > -pc_fieldsplit_block_size. Then to define the splits, use
> >
> > -pc_fieldsplit_0_fields 0,3 -pc_fieldsplit_1_fields 1
> > -pc_fieldsplit_2_fields 2,4,5
> >
> > to split six fields into three splits of sizes 2,1,3.
>
> In this case, is field N the set of global Vec indices for which idx%6 == N?
Yes
> > split0: 0 3 6 9 | 12 15 18 21
> > split1: 1 7 | 13 19
> > split2: 2 4 5 8 10 11 | 14 16 17 20 22 23
> >
> > I hope this is clear.
>
> Sorry for being dense, but in this case, if I were to create the IS
> manually using ISCreateGeneral, on process 0, I think that I would use:
>
> idx[0] = 0
> idx[1] = 1
> idx[2] = 2
> idx[3] = 0
> ...
>
> and similarly on process 1:
>
> ...
> idx[4] = idx[16-12] = 2
> idx[5] = idx[17-12] = 2
> idx[6] = idx[18-12] = 0
> idx[7] = idx[19-12] = 1
> ...
To create the index set for split0:
/* This assumes communicator has size 2 */
PetscInt idx[2][] = {{0,3,6,9},{12,15,18,21}};
int rank;
IS is0;
MPI_Comm_rank(comm,rank);
ISCreateGeneral(comm,4,idx[rank],&is0);
This is a parallel index set containing the global indices of the dofs
in the desired split.
Jed
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