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.
> 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?
> For something more general, use ISCreate() and PCFieldSplitSetIS(). The
> index sets should be created on the same communicator as the matrix and
> contain the global indices corresponding to the desired field. So if we
> had two processors with two nodes each (24 dofs total), the splits above
> would be given by the following index sets (where '|' separates processors):
>
> 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
...
Is this the basic idea?
Thanks,
-- Boyce
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