[petsc-users] Suggestions for code with dof >> 1 ?

[Christophe Ortiz]

On Wed, Oct 9, 2013 at 3:13 PM, Christophe Ortiz <christophe.ortiz at ciemat.es
> wrote:

>
> On Wed, Oct 9, 2013 at 2:56 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>
>> Christophe Ortiz <christophe.ortiz at ciemat.es> writes:
>>
>> > Hi,
>> >
>> > I am building my Jacobian (IJacobian) with dof > 1 and I am considering
>> to
>> > use either MatSetValuesLocal (what you did in ex10.c from
>> > advection-diffusion) or MatSetValuesBlocked.
>> >
>> > Which one do you recommend ?
>>
>> MatSetValuesBlocked or MatSetValuesBlockedLocal.
>>
>
> Ok.
>
>
>>
>> > For the moment I am trying to understand how MatSetValuesBlocked works
>> from
>> > the various examples given in ts tutorials, but there is a mistery for
>> > me...In these examples, the block size is not set. So when m and idxm[]
>> (n
>> > and idxn[]) are passed in, how does PETSc know the correspondance with
>> > global row and column indices in the matrix ?
>> >
>> > For instance, ex22.c
>> >
>> > for (i=info.xs; i<info.xs+info.xm; i++) {
>> > const PetscReal *k = user->k;
>> > PetscScalar     v[2][2];
>> >
>> > v[0][0] = a + k[0]; v[0][1] =  -k[1];
>> > v[1][0] =    -k[0]; v[1][1] = a+k[1];
>> > MatSetValuesBlocked(*Jpre,1,&i,1,&i,&v[0][0],INSERT_VALUES);
>> > }
>> >
>> > Here 4 terms due to chemical reactions are passed to the matrix at once.
>> > &i is passed for indice of row and column, which is supposed to be the
>> > indice of the block. But if the blocksize is not set, how PETSc knows to
>> > which row/colum it really corresponds in the matrix ?
>>
>> DMCreateMatrix() set the block size to match the "dof" argument to
>> DMDACreate1d.
>>
>
> I suspected it ! It's smart !
>
>
>>
>> > For instance, if we take the example from petsc (MatSetValuesBlocked
>> page),
>> > what should I do to pass the block of values 3, 4, 7 and 8 with
>> > MatSetValuesBlocked ?
>> >
>> >   1  2  | 3  4
>> >   5  6  | 7  8
>> >     - - - | - - -
>> >  9  10 | 11 12
>> > 13 14 | 15 16
>>
>> Block row 0, block column 1.  (Count from 0.)
>>
>
>
>
And if with the previous case I would like to pass the values 1, 2, 3, 4,
5, 6, 7 and 8 (the two first rows) what is the matrix val[ ] that I should
pass ? Should it be a matrix val[2][4] ?

If I understand correctly it would be 1 block row, the block row indice
being 0; 2 block columns, the column block indices being 0 and 1:

indices of block rows and block columns:
row = 0
col[0]=0; col[1]=1
MatSetValuesBlocked(*Jpre,1,&row,2,&col,&v[0][0],INSERT_VALUES);

Is it correct ? Is the matrix val correct ?

Christophe
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