On Fri, Jan 16, 2026 at 12:18 PM Michael Whitten via petsc-users <
[email protected]> wrote:
> I realized the subject line was incorrect so I fixed it.
>
> I want use SNES for sure and possibly TS and Tao. I would also like to be
> able to use the matrix-vector multiplication in a similar way. Is there
> similar guidance for SNES (TS and Tao) as there is for KSP somewhere?
>
It should work exactly the same as it does KSP. If you have a more specific
question, please mail back.
Thanks,
Matt
> I had started reading the PETSc user manual while simultaneously
> experimenting with some code and hadn't made it to that section. It's a
> large ecosystem and I am trying to get oriented. Your help is much
> appreciated.
>
> Michael
>
> ------------------------------
> *From:* Barry Smith <[email protected]>
> *Sent:* Friday, January 16, 2026 11:38 AM
> *To:* Michael Whitten <[email protected]>
> *Cc:* [email protected] <[email protected]>
> *Subject:* Re: [petsc-users] petsc-users Digest, Vol 205, Issue 12
>
>
>
> * External Email - Use Caution *
>
>
>
>
> Michael,
>
> Yes, I misunderstood the question.
>
> Are you specifically wanting to use PETSc's KSP linear solver in
> parallel while your code runs sequentially (and you do not plan to use SNES
> or TS or Tao)? If so then we have code just for that purpose, explained in
> the section
>
> Using PETSc's MPI parallel linear solvers from a non-MPI program
>
> in the users manual available at petsc.org
> <https://urldefense.us/v3/__http://petsc.org/__;!!G_uCfscf7eWS!fz7gu-ydCmGQL5TnY6D1UYsCtPN06PH7qvMAmLG7pfOYZPc2OyKY_uIne_Yau1PHu2cOo9BZ_hNO2c3-h6blFkFi$>
>
>
> The PETSc code manages all the parallelism and spreading out of the matrix
> and vectors to the multiple processes. You just write a sequential program
> that uses KSP and calls KSPSolve(), make sure it works correctly and then
> add the options listed in the users manual to run the linear solve in
> parallel.
>
> If you want to use SNES or TS it will require you to manage moving the
> vectors and matrices in your code from sequential to parallel using PETSc
> calls. But again you would just get your code running correctly with PETSc
> sequentially and then add some extra boilerplate to use the PETSc parallel
> solvers with it.
>
> Barry
>
>
>
>
>
> On Jan 16, 2026, at 10:03 AM, Michael Whitten via petsc-users <
> [email protected]> wrote:
>
> Both of your points elude to my other question so I am going to ask it in
> this thread.
>
> Is it possible/recommended/reasonable to run my current code sequentially,
> call a function that interfaces with the PETSc solver which runs parallel,
> and return a sequential vector to my code?
>
>
> 1. I understand your point about the additional memory usage. However,
> I am imagining v1 as my Fortran vector passed into a function. It is
> possible that my current plan is quite inefficient as I am becoming more
> aware of how much of a novice I am in parallel computing.
> 2. I don't need PETSc's entire parallel vector on each process. This
> was not my understanding of that part of the code. I need the entire vector
> returned to my sequential code.
>
>
> I think I am trying to do something intermediate between completely
> sequential and completely parallel.
>
> Michael
> ------------------------------
> *From:* petsc-users <[email protected]> on behalf of
> [email protected] <[email protected]>
> *Sent:* Friday, January 16, 2026 9:52 AM
> *To:* [email protected] <[email protected]>
> *Subject:* petsc-users Digest, Vol 205, Issue 12
>
>
> External Email - Use Caution
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> 1. Re: petsc-users Digest, Vol 205, Issue 2 (Barry Smith)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 16 Jan 2026 08:51:29 -0600
> From: Barry Smith <[email protected]>
> To: Michael Whitten <[email protected]>
> Cc: "[email protected]" <[email protected]>
> Subject: Re: [petsc-users] petsc-users Digest, Vol 205, Issue 2
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset="utf-8"
>
>
>
> 1) For the code
> v1 = 1.0d0
>
> ....
> PetscCallA(VecCreate(PETSC_COMM_WORLD,v1p,ierr))
> ...
> PetscCallA(VecGetArray(v1p,v1ptr,ierr))
> v1ptr = v1
> PetscCallA(VecRestoreArray(v1p,v1ptr,ierr))
>
> This produces two copies of the array plus time to copy values over.
> Instead drop the v1 completely and just use
> PetscCallA(VecGetArray(v1p,v1ptr,ierr))
> v1ptr = 1.0d0 ! or fill it up with a loop etc
> PetscCallA(VecRestoreArray(v1p,v1ptr,ierr))
>
> 2) If you want/need PETSc's entire parallel vector on each process then
> you have no choice but to use the code you wrote with VecScatter. Normally
> when writing a new MPI code from scratch one designs it so the entire
> vector is never needed together on a single process (because that is not
> scalable) but if you have a current code you need to work with this is an
> ok way to start.
>
> Barry
>
>
>
> > On Jan 16, 2026, at 8:40?AM, Michael Whitten via petsc-users <
> [email protected]> wrote:
> >
> > I hope that I am replying in the correct manner to respond to the
> "Having trouble with basic Fortran-PETSc interoperability" thread I
> previously started. If not, please correct me.
> >
> > Thank you everyone for your replies, they were very helpful.
> >
> > I have what appears to be a working example code using the suggested
> updates, specifically the VecGetArray() and VecRestoreArray() and getting
> the sequential vector back from PETSc using the information from the FAQ.
> <<Example code attached>>
> >
> > I have a question about this example code to make sure I am writing
> reasonably efficient code. It seems like I have to create an additional
> PETSc vector 'out_seq' which will essentially be a copy of the PETSc vector
> 'v1p' which is not the most efficient use of memory. It also seems to me
> like there isn't a way around this additional 'out_seq' vector because
> there needs a place to aggregate the data from the various processes. Is
> this a reasonable use of PETSc or is there a more efficient way? Note, I am
> trying to interface my existing code base with PETSc to use the solvers and
> this may be the performance trade-off for not developing my program fully
> within the PETSc ecosystem.
> >
> > I have another question only tangentially related to this topic. Should
> I ask it as part of this thread or create a new topic?
> >
> > Michael
> > From: petsc-users <[email protected] <
> mailto:[email protected] <[email protected]>>>
> on behalf of [email protected]<
> mailto:[email protected] <[email protected]>>
> <[email protected] <mailto:[email protected]
> <[email protected]>>>
> > Sent: Saturday, January 3, 2026 1:00 PM
> > To: [email protected] <mailto:[email protected]
> <[email protected]>> <[email protected] <
> mailto:[email protected] <[email protected]>>>
> > Subject: petsc-users Digest, Vol 205, Issue 2
> >
> >
> > External Email - Use Caution
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> > When replying, please edit your Subject line so it is more specific
> > than "Re: Contents of petsc-users digest..."
> >
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> > Today's Topics:
> >
> > 1. Re: Having trouble with basic Fortran-PETSc interoperability
> > (Barry Smith)
> > 2. Re: Having trouble with basic Fortran-PETSc interoperability
> > (Matthew Knepley)
> >
> >
> > ----------------------------------------------------------------------
> >
> > Message: 1
> > Date: Fri, 2 Jan 2026 16:33:27 -0500
> > From: Barry Smith <[email protected] <mailto:[email protected]
> <[email protected]>>>
> > To: Michael Whitten <[email protected] <mailto:[email protected]
> <[email protected]>>>
> > Cc: "[email protected] <mailto:[email protected]
> <[email protected]>>" <[email protected] <
> mailto:[email protected] <[email protected]>>>
> > Subject: Re: [petsc-users] Having trouble with basic Fortran-PETSc
> > interoperability
> > Message-ID: <[email protected] <
> mailto:[email protected]
> <[email protected]>>>
> > Content-Type: text/plain; charset="utf-8"
> >
> >
> > VecGetValues() uses 0 based indexing in both Fortran and C.
> >
> > You don't want to use VecGetValues() and VecSetValues() usually since
> they result in two copies of the arrays and copying entire arrays back and
> forth.
> >
> > You can avoid copying between PETSc vectors and your arrays by using
> VecGetArray(), VecGetArrayWrite(), and VecGetArrayRead(). You can also use
> VecCreateMPIWithArray() to create a PETSc vector using your array; for
> example for input to the right hand side of a KSP. These arrays start their
> indexing with the Fortran default of 1.
> >
> >
> > Barry
> >
> >
> >
> > > On Jan 2, 2026, at 2:42?PM, Michael Whitten via petsc-users <
> [email protected] <mailto:[email protected]
> <[email protected]>>> wrote:
> > >
> > > Hi PETSc mailing list users,
> > >
> > > I have managed to install PETSc and run some PETSc examples and little
> test codes of my own in Fortran. I am now trying to make PETSc work with my
> existing Fortran code. I have tried to build little test examples of the
> functionality that I can then incorporate into my larger code base.
> However, I am having trouble just passing vectors back and forth between
> PETSc and Fortran.
> > >
> > > I have attached a minimum semi-working example that can be compiled
> with the standard 'Makefile.user'. It throws an error when I try to copy
> the PETSc vector back to a Fortran vector using VecGetValues(). I get that
> it can only access values of the array on the local process but how do I
> fix this? Is this even the right approach?
> > >
> > > In the final implementation I want to be able to assemble my matrix
> and vector, convert them to PETSc data structures, use PETSc to solve, and
> then convert the solution vector back to Fortran and return. I want to be
> able to do this with both the linear and nonlinear solvers. It seems like
> this is what PETSc is, in part, built to do. Is this a reasonable
> expectation to achieve? Is this a reasonable use case for PETSc?
> > >
> > > Thanks in advance for any help you can offer.
> > >
> > > best,
> > > Michael
> > > <test.F90>
> >
> > -------------- next part --------------
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> >>
> >
> > ------------------------------
> >
> > Message: 2
> > Date: Fri, 2 Jan 2026 17:49:30 -0500
> > From: Matthew Knepley <[email protected] <mailto:[email protected]
> <[email protected]>>>
> > To: Michael Whitten <[email protected] <mailto:[email protected]
> <[email protected]>>>
> > Cc: "[email protected] <mailto:[email protected]
> <[email protected]>>" <[email protected] <
> mailto:[email protected] <[email protected]>>>
> > Subject: Re: [petsc-users] Having trouble with basic Fortran-PETSc
> > interoperability
> > Message-ID:
> > <
> CAMYG4GmKRM1=cok6jwbo6itwvpkksepeoh1q1cllkgr2qhk...@mail.gmail.com<
> mailto:CAMYG4GmKRM1=cok6jwbo6itwvpkksepeoh1q1cllkgr2qhk...@mail.gmail.com
> <CAMYG4GmKRM1=cok6jwbo6itwvpkksepeoh1q1cllkgr2qhk...@mail.gmail.com>>>
> > Content-Type: text/plain; charset="utf-8"
> >
> > On Fri, Jan 2, 2026 at 2:48?PM Michael Whitten via petsc-users <
> > [email protected] <mailto:[email protected]
> <[email protected]>>> wrote:
> >
> > > Hi PETSc mailing list users,
> > >
> > > I have managed to install PETSc and run some PETSc examples and little
> > > test codes of my own in Fortran. I am now trying to make PETSc work
> with my
> > > existing Fortran code. I have tried to build little test examples of
> the
> > > functionality that I can then incorporate into my larger code base.
> > > However, I am having trouble just passing vectors back and forth
> between
> > > PETSc and Fortran.
> > >
> > > I have attached a minimum semi-working example that can be compiled
> with
> > > the standard 'Makefile.user'. It throws an error when I try to copy the
> > > PETSc vector back to a Fortran vector using VecGetValues(). I get that
> it
> > > can only access values of the array on the local process but how do I
> fix
> > > this? Is this even the right approach?
> > >
> >
> > No. You should just call VecGetArray(), to get back an F90 pointer to the
> > values. This is much more convenient.
> >
> >
> > > In the final implementation I want to be able to assemble my matrix and
> > > vector, convert them to PETSc data structures, use PETSc to solve, and
> then
> > > convert the solution vector back to Fortran and return. I want to be
> able
> > > to do this with both the linear and nonlinear solvers. It seems like
> this
> > > is what PETSc is, in part, built to do. Is this a reasonable
> expectation to
> > > achieve? Is this a reasonable use case for PETSc?
> > >
> >
> > Yes, that should work getting the array directly.
> >
> > Thanks,
> >
> > Matt
> >
> >
> > > Thanks in advance for any help you can offer.
> > >
> > > best,
> > > Michael
> > >
> >
> >
> > --
> > 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
> >
> >
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> <
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>
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> >>
> >
> > ------------------------------
> >
> > Subject: Digest Footer
> >
> > _______________________________________________
> > petsc-users mailing list
> > [email protected] <mailto:[email protected]
> <[email protected]>>
> >
> https://urldefense.us/v3/__https://nam02.safelinks.protection.outlook.com/?url=https*3A*2F*2Furldefense.us*2Fv3*2F__https*3A*2F*2Fnam02.safelinks.protection.outlook.com*2F*3Furl*3Dhttps*3A*2F*2Flists.mcs.anl.gov*2Fmailman*2Flistinfo*2Fpetsc-users*26data*3D05*7C02*7Cmwhitte6*40jhu.edu*7C094639f3b5f5405cfc4e08de4af1fa67*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639030600350725058*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C*26sdata*3DUeb*2F8ra8XpzYQzLTm4qwWVo9lyMGs2P5*2FMD0nA3xsdg*3D*26reserved*3D0__*3BJSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJQ!!G_uCfscf7eWS!bIMcmpMdXbyTPg56scE6PGSFLgQ9D07NeoK_UCdRbCrnEdwa1m3aLzbnLed2KLUNJJyHJeWiICB-ePUpdaF8pOQ*24&data=05*7C02*7Cmwhitte6*40jhu.edu*7C24e75a940a1946f44a3608de550f304c*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639041720893633070*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=fmFnefcmhqrsOsKFsdq7Krc*2F0CDtDuB*2FCXbtDn6Rnr4*3D&reserved=0__;JSUlJSUlJSUlJSUqKioqKiolJSoqKioqKioqKioqKioqKiolJSoqKiUlJSUlJSUlJSUlJSUlJSUlJSUlJSUl!!G_uCfscf7eWS!fCGSZ-K1lcfkVVEE95E8-TrDnok3eRJazqaX6N_DdWheUklbAIZiBVeslnlaotQT6iOErmawK4OEv-4OOChV$
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> <
> https://urldefense.us/v3/__https://nam02.safelinks.protection.outlook.com/?url=https*3A*2F*2Flists.mcs.anl.gov*2Fmailman*2Flistinfo*2Fpetsc-users&data=05*7C02*7Cmwhitte6*40jhu.edu*7C24e75a940a1946f44a3608de550f304c*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639041720893670294*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=o0XY48*2BZ5oVC0TxiObgNwusKIUub*2B3MPR6S30YvHQfI*3D&reserved=0__;JSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJQ!!G_uCfscf7eWS!fCGSZ-K1lcfkVVEE95E8-TrDnok3eRJazqaX6N_DdWheUklbAIZiBVeslnlaotQT6iOErmawK4OEv7yMEE_u$
>
> <https://urldefense.us/v3/__https://nam02.safelinks.protection.outlook.com/?url=https*3A*2F*2Flists.mcs.anl.gov*2Fmailman*2Flistinfo*2Fpetsc-users&data=05*7C02*7Cmwhitte6*40jhu.edu*7C24e75a940a1946f44a3608de550f304c*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639041720893670294*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=o0XY48*2BZ5oVC0TxiObgNwusKIUub*2B3MPR6S30YvHQfI*3D&reserved=0__;JSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJQ!!G_uCfscf7eWS!bhoEoF0XYYKl-EOyDlgI2fLPdGcIPgvLKpHDlZJ8j2J8urOsoci3Ml3KBNAv13782FIpFxECA2a2yLsA4l248JZj$>
> >
> >
> >
> > ------------------------------
> >
> > End of petsc-users Digest, Vol 205, Issue 2
> > *******************************************
> > <test2.F90>
>
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>
> ------------------------------
>
> Subject: Digest Footer
>
> _______________________________________________
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> <https://lists.mcs.anl.gov/mailman/listinfo/petsc-users >
>
>
> ------------------------------
>
> End of petsc-users Digest, Vol 205, Issue 12
> ********************************************
>
>
>
--
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
https://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!fCGSZ-K1lcfkVVEE95E8-TrDnok3eRJazqaX6N_DdWheUklbAIZiBVeslnlaotQT6iOErmawK4OEvw-qbTXN$
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>
