[petsc-users] using real and complex together

[Matthew Knepley]

On Mon, Sep 28, 2020 at 3:43 PM Sam Guo <sam.guo at cd-adapco.com> wrote:

> Hi Stefano and PETSc dev team,
>    I want to try your suggestion to always load complex version of PETSc
> but if my input matrix A is real, I want to create shell matrix to
> matrix-vector and factorization using real only.
>

I do not think that will work as you expect. I will try to explain below.


>    I still need to understand how MatRealPart works. Does it just zero out
> the image numerical values or does it delete the image memory?
>

When we have complex values, we use the "complex" type to allocate and
store them. Thus you cannot talk about just the memory to store imaginary
parts.
MatRealPart sets the imaginary parts of all the matrix elements to zero.


> If my input matrix A is real, how do I create a shell matrix to matrix
> -vector multiplication y=A*x where A is real, PestcScalar = complex, x and
> y are Vec? I notice there is a VecRealPart but it seems it just zeros the
> image numerical values. It seems I still have to create a PetscReal
> pointer  to copy the real part of PetacScalar pointers like following. Can
> you comment on it?
>

What you suggest would mean rewriting the matrix multiplication algorithm
by hand after extracting the values. I am not sure if this
is really what you want to do. Is the matrix memory really your limiting
factor? Even if you tried to do this with templates, the memory
from temporaries would be very hard to control.

  Thanks,

     Matt


> Thanks,
> Sam
>
> PetscScalar *px = nullptr;
> VecGetArrayRead(x, &px);
> PetscScalar *py = nullptr;
> VecGetArray(y, &py);
> int localSize = 0;
> VecGetLocalSize(x, &localSize);
> std::vector<PetasReal> realX(localSize); // I am using c++ to call PETSc
>
> //retrieve real part
> for(int i = 0; i < localSize; i++)  realX[i] = PetscRealPart(px[i]);
>
> // do real matrix-vector multiplication
> // realY=A*realX
> // here where realY is std::vector<PetscReal>
>
> //put real part back to py
>   for(int i = 0; i < localSize; i++)  pv[i] = realY[i];
> VecRestoreArray(y,&py);
>
> On Tue, May 26, 2020 at 1:49 PM Sam Guo <sam.guo at cd-adapco.com> wrote:
>
>> Thanks
>>
>> On Tuesday, May 26, 2020, Stefano Zampini <stefano.zampini at gmail.com>
>> wrote:
>>
>>> All the solvers/matrices/vectors works for PetscScalar types (i.e. in
>>> your case complex)
>>> If you need to solve for the real part only, you can duplicate the
>>> matrix and call MatRealPart to zero out the imaginary part. But the solve
>>> will always run in the complex space
>>> You should not be worried about doubling the memory for a matrix (i.e.
>>> real and imaginary part)
>>>
>>>
>>> On May 26, 2020, at 11:28 PM, Sam Guo <sam.guo at cd-adapco.com> wrote:
>>>
>>> complex version is needed since matrix sometimes is real and sometimes
>>> is complex. I want to solve real matrix without allocating memory for
>>> imaginary part((except eigen pairs).
>>>
>>> On Tuesday, May 26, 2020, Zhang, Hong <hzhang at mcs.anl.gov> wrote:
>>>
>>>> You can build PETSc with complex version, and declare some variables
>>>> as 'PETSC_REAL'.
>>>> Hong
>>>>
>>>> ------------------------------
>>>> *From:* petsc-users <petsc-users-bounces at mcs.anl.gov> on behalf of Sam
>>>> Guo <sam.guo at cd-adapco.com>
>>>> *Sent:* Tuesday, May 26, 2020 1:00 PM
>>>> *To:* PETSc <petsc-users at mcs.anl.gov>
>>>> *Subject:* [petsc-users] using real and complex together
>>>>
>>>> Dear PETSc dev team,
>>>>    Can I use both real and complex versions together?
>>>>
>>>> Thanks,
>>>> Sam
>>>>
>>>
>>>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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