[petsc-users] using real and complex together

Sam Guo sam.guo at cd-adapco.com
Mon Sep 28 19:20:21 CDT 2020 

Hi Randy and Matt,
   Thanks a lot. I’ll look into it.

BR,
Sam

On Monday, September 28, 2020, Matthew Knepley <knepley at gmail.com> wrote:

> On Mon, Sep 28, 2020 at 8:15 PM Randall Mackie <rlmackie862 at gmail.com>
> wrote:
>
>> Sam, you can solve a complex matrix using a real version of PETSc by
>> doubling the size of your matrix and spitting out real/imaginary parts.
>>
>> See this paper:
>>
>> https://epubs.siam.org/doi/abs/10.1137/S1064827500372262?mobileUi=0
>>
>
> Thanks Randy. Yes, I meant that one.
>
>    Matt
>
>
>> Randy M.
>>
>>
>> On Sep 28, 2020, at 5:12 PM, Sam Guo <sam.guo at cd-adapco.com> wrote:
>>
>> All I want is to solve both real and complex matrix using either real or
>> complex version of PETSc. If I load complex version of PETSc, I waste
>> memory solving real matrix. If I can solve complex matrix using real
>> version of PETSc, I will accept it.
>>
>> On Monday, September 28, 2020, Sam Guo <sam.guo at cd-adapco.com> wrote:
>>
>>> If I load complex version of PETSc, how can Vec be real?
>>>
>>> On Monday, September 28, 2020, Matthew Knepley <knepley at gmail.com>
>>> wrote:
>>>
>>>> On Mon, Sep 28, 2020 at 7:44 PM Sam Guo <sam.guo at cd-adapco.com> wrote:
>>>>
>>>>> I want to make sure I understand you.  I load real version of PETSc.
>>>>> If my input matrix is complex,
>>>>
>>>>
>>>> You said that your matrix was real.
>>>>
>>>>
>>>>> just get real and imagine parts of PETSc Vec
>>>>
>>>>
>>>> No, the PETSc Vec would be real. You would have two vectors.
>>>>
>>>>   Thanks,
>>>>
>>>>     Matt
>>>>
>>>>
>>>>> and do the matrix vector multiplication. Right?
>>>>>
>>>>> On Monday, September 28, 2020, Matthew Knepley <knepley at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> On Mon, Sep 28, 2020 at 6:29 PM Sam Guo <sam.guo at cd-adapco.com>
>>>>>> wrote:
>>>>>>
>>>>>>> “ I think it would be much easier to just decompose your complex
>>>>>>> work into real and imaginary parts and use PETSc with real scalars to
>>>>>>> compute them separately.
>>>>>>> Since you know your matrices have 0 imaginary part, this becomes
>>>>>>> very straightforward.”
>>>>>>>
>>>>>>> I think this is what I am trying to do. But since I have to provide
>>>>>>> matrix-vector operator(since I am using shell matrix), the Vec I receive is
>>>>>>> complex. I need to convert complex vec to real one and then convert it
>>>>>>> back(that’s the code I shown before).
>>>>>>>
>>>>>>
>>>>>> No, this is not what I am advocating. Keep your vectors real, just
>>>>>> keep one for the real part and one for the imaginary part. Then you can
>>>>>> just call MatMult() twice with your matrix.
>>>>>>
>>>>>>   Thanks,
>>>>>>
>>>>>>      Matt
>>>>>>
>>>>>>
>>>>>>> On Monday, September 28, 2020, Matthew Knepley <knepley at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> On Mon, Sep 28, 2020 at 5:01 PM Sam Guo <sam.guo at cd-adapco.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> Hi Matt,
>>>>>>>>>    Since I use MUMPS as preconditioner, complex uses too much
>>>>>>>>> memory if my input matrix is real. Ideally if I can compile real and
>>>>>>>>> complex into different symbols (like MUMPS) , I can load both version
>>>>>>>>> without conflict.
>>>>>>>>>
>>>>>>>>
>>>>>>>> What I mean to say is that it would be great if it were as simple
>>>>>>>> as using two different symbols, but unfortunately the problem is more
>>>>>>>> difficult. I was trying to use
>>>>>>>> the example of templates. This would be a very intrusive change no
>>>>>>>> matter what technology you are using.
>>>>>>>>
>>>>>>>> So your main memory usage is from the MUMPS factorization, and you
>>>>>>>> cannot afford to double that usage?
>>>>>>>>
>>>>>>>> You could consider writing a version of AIJ that stores real
>>>>>>>> entries, but allows complex vector values. It would promote to complex for
>>>>>>>> the row dot product.
>>>>>>>> However, you would also have to do the same work for all the solves
>>>>>>>> you do with MUMPS.
>>>>>>>>
>>>>>>>> I think it would be much easier to just decompose your complex work
>>>>>>>> into real and imaginary parts and use PETSc with real scalars to compute
>>>>>>>> them separately.
>>>>>>>> Since you know your matrices have 0 imaginary part, this becomes
>>>>>>>> very straightforward.
>>>>>>>>
>>>>>>>>   Thanks,
>>>>>>>>
>>>>>>>>       Matt
>>>>>>>>
>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> Sam
>>>>>>>>>
>>>>>>>>> On Mon, Sep 28, 2020 at 12:52 PM Matthew Knepley <
>>>>>>>>> knepley at gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> 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/>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>> 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/>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>> --
>>>>>> 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/>
>>>>>>
>>>>>
>>>>
>>>> --
>>>> 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/>
>>>>
>>>
>>
>
> --
> 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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