[petsc-users] MatSetSizes for MATMPIBAIJ

[Barry Smith]

On Feb 5, 2010, at 6:46 PM, NovA wrote:

> 2010/2/6 Matthew Knepley <knepley at gmail.com>:
>> On Fri, Feb 5, 2010 at 6:17 PM, NovA <av.nova at gmail.com> wrote:
>>>
>>> 2010/2/6 Barry Smith <bsmith at mcs.anl.gov>:
>>>>
>>>> On Feb 5, 2010, at 5:28 PM, NovA wrote:
>>>>
>>>>> Thanks for the quick response!
>>>>>
>>>>> 2010/2/6 Matthew Knepley <knepley at gmail.com>:
>>>>>>
>>>>>> 1) It will distribute block rows, so you will get 8 and 4 as  
>>>>>> you want
>>>>>
>>>>> How can it distribute in such a way, if it doesn't know block  
>>>>> size yet?
>>>>> Let's continue the example:
>>>>>  MatCreate(comm, &A);
>>>>>  MatSetSizes(A, PETSC_DECIDE,12, 12,12);
>>>>
>>>>                                                                 
>>>> ^^^^^^
>>>>                            This is wrong! Look at the manual page  
>>>> for
>>>> MatCreateMPIBAIJ() for information on the meaning of this.
>>>
>>> Sorry, but I can't find any additional info on this at
>>>
>>> http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/Mat/MatCreateMPIBAIJ.html
>>> and around. I could just expect that those sizes mean number of  
>>> blocks
>>> not individual values, but can't find any confirmation...
>>
>> From the page: It says m,n,M,N are "rows" and the values in  
>> d_nz,o_nz are
>> "blocks".
> Yep, sure. And how is this concerns the line
>   MatSetSizes(A, /*m*/PETSC_DECIDE,/*n*/12, /*M*/12, /*N*/12) ?

    n is the number of "owned" columns it is NOT the same as the total  
number of columns. If you read the manual page for MatCreateMPIBAIJ  
you will see the example that discusses what n is.

n
- number of local columns (or PETSC_DECIDE to have calculated if N is  
given) This value should be the same as the local size used in  
creating the x vector for the matrix-vector product y = Ax.

Barry

>
> Here we need not specify noznzero pattern. Am I wrong?
>
> Thanks!
>  Andrey
>
>>>
>>>>>  MatSetType(A, MATMPIBAIJ);
>>>>>  MatGetOwnershipRange(A, &rowS, &rowE);
>>>>>  Then loop local rows from rowS/4 to rowE/4 to preallocate storage
>>>>> using MatMPIBAIJSetPreallocation(A,4,...)
>>>>
>>>>   You cannot do it this way. You cannot call MatGetOwnershipRange()
>>>> before
>>>> setting the preallocation.
>>>
>>> Oh, ok. This applies to BAIJ, right? For AIJ this works, so I  
>>> thought
>>> it should for BAIJ either.
>>
>> This is not correct for AIJ either.
>>
>>    Matt
>>
>>>
>>>>
>>>>   You can either,
>>>>
>>>> 1) figure out yourself the local sizes you want to use, then just  
>>>> call
>>>> MatCreateMPIBAIJ() with all the information (or call MatCreate(),
>>>> MatSetType(), MatSetSizes(), MatMPIBAIJSetPreallocation()) or
>>>> 2) Use PetscMap (see manual page for PetscMapInitialize) to see how
>>>> PETSc
>>>> would decompose the rows and then use that information to do 1).
>>>
>>> Thanks for the tip. I'll try to sort it out.
>>>
>>> Best wishes,
>>>    Andrey
>>
>> --
>> What most experimenters take for granted before they begin their  
>> experiments
>> is infinitely more interesting than any results to which their  
>> experiments
>> lead.
>> -- Norbert Wiener
>>