# use of VecPlaceArray in parallel with fortran

Wienand Drenth w.drenth at gmail.com
Mon Nov 9 10:30:16 CST 2009

```Hello Barry,

Thank you for that.

Just another question. As I wrote in my first email, in the current
code, we utilize a local non-PetSc arrays and using VecPlaceArray we
"give" this array to PetSc vectors to do the KSPSolve. Afterwards, we
can just continue with our local non-PetSc arrays. If I understand you
correctly, and for my knowledge, this approach will not be possible in
a parallel setting?

When I do, with for example two processors, and with local array being
blocal = 1, 2, .... , 10
then for the zeroth processor I have also values 1, 2, ... , 10 and
not just half (i.e., 1,2,3,4,5,0,0,0,0,0).
for the first processor I have only part of the values, but they start
with the first entry of my array, and not half-way:
0,0,0,0,0, 1,2,3,4,5 instead of 0,0,0,0,0, 6,7,8,9,10

Regards,

Wienand

On Fri, Nov 6, 2009 at 7:48 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>  VecPlaceArray() gives to the vector its local (on process) part  of the
> array, not the whole array (and requires no communication). If you want the
> entire array of the vector on one or all processes you can use
> VecScatterCreateToAll() or VecScatterCreateToZero() and then use the
> VecScatter created to move the values to where you want them.
>
>   Barry
>
> On Nov 6, 2009, at 12:00 PM, Wienand Drenth wrote:
>
>> Hello all,
>>
>> In my research code I solve a linear system of equations, and (of
>> course) I use PetSc routines for that. However, in the code we have
>> our own data arrays for the right handside vector B, and solution
>> vector X. Only just prior to the call to KSPSolve, we use the routine
>> VecPlaceArray to synchronize the Fortran array B and X with their
>> PetSc counterparts (M_B and M_X, for example, respectively).
>>
>> I was wondering if this would work in parallel as well? I have adapted
>> one of the tutorial examples  (ex2f from the ksp tutorials) to utilize
>> the VecPlaceArray mechanism. I encountered no problems, except when I
>> want to run the program in parallel.
>>
>> When I do that, and print my own vector X afterwards, different
>> processors show different parts of the solution. For example, for a
>> vector of length 10, and with two processors, processor one will have
>> values for the first five elements (remainder is zero), and processor
>> two will have values for the last five elements in the array.
>>
>>> From the same ksp tutorials, I have tried ex13 as well, the c program.
>>
>> Here I do not get partial outputs for different processors.
>>
>> I wonder whether one cannot use VecPlaceArray in a parralel setting in
>> Fortran, except by doing extra bookkeeping? I hope someone can
>> enlighten me, and indicate where I missed something in my programming
>> or otherwise.
>>
>>
>> Wienand Drenth
>>
>>
>>
>> --
>> Wienand Drenth PhD
>> Eindhoven, the Netherlands
>
>

--
Wienand Drenth PhD
Eindhoven, the Netherlands
```