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!akkX3mtT9HWbhzS3UC7bCNBSaKhbvQ937U5ePbM10Wdro75K-aTE0MZvfvGfyBfBwpQpD8YsK2NvRfeSbJXwow8$
>
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?
>
> 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.
> 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]
> <mailto:[email protected]>> on behalf of
> [email protected] <mailto:[email protected]>
> <[email protected] <mailto:[email protected]>>
> Sent: Friday, January 16, 2026 9:52 AM
> To: [email protected] <mailto:[email protected]>
> <[email protected] <mailto:[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] <mailto:[email protected]>>
> To: Michael Whitten <[email protected] <mailto:[email protected]>>
> Cc: "[email protected] <mailto:[email protected]>"
> <[email protected] <mailto:[email protected]>>
> Subject: Re: [petsc-users] petsc-users Digest, Vol 205, Issue 2
> Message-ID: <[email protected]
> <mailto:[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] <mailto:[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]>
> > <mailto:[email protected]>> on behalf of
> > [email protected]
> > <mailto:[email protected]><mailto:[email protected]>
> > <[email protected] <mailto:[email protected]>
> > <mailto:[email protected]>>
> > Sent: Saturday, January 3, 2026 1:00 PM
> > To: [email protected] <mailto:[email protected]>
> > <mailto:[email protected]> <[email protected]
> > <mailto:[email protected]> <mailto:[email protected]>>
> > Subject: petsc-users Digest, Vol 205, Issue 2
> >
> >
> > External Email - Use Caution
> >
> >
> >
> > Send petsc-users mailing list submissions to
> > [email protected] <mailto:[email protected]>
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> >
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> > >
> > or, via email, send a message with subject or body 'help' to
> > [email protected]
> > <mailto:[email protected]>
> > <mailto:[email protected]>
> >
> > You can reach the person managing the list at
> > [email protected]
> > <mailto:[email protected]>
> > <mailto:[email protected]>
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> > than "Re: Contents of petsc-users digest..."
> >
> >
> > 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]>
> > <mailto:[email protected]>>
> > To: Michael Whitten <[email protected] <mailto:[email protected]>
> > <mailto:[email protected]>>
> > Cc: "[email protected] <mailto:[email protected]>
> > <mailto:[email protected]>" <[email protected]
> > <mailto:[email protected]> <mailto:[email protected]>>
> > Subject: Re: [petsc-users] Having trouble with basic Fortran-PETSc
> > interoperability
> > Message-ID: <[email protected]
> > <mailto:[email protected]>
> > <mailto:[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]>
> > > <mailto:[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]>
> > <mailto:[email protected]>>
> > To: Michael Whitten <[email protected] <mailto:[email protected]>
> > <mailto:[email protected]>>
> > Cc: "[email protected] <mailto:[email protected]>
> > <mailto:[email protected]>" <[email protected]
> > <mailto:[email protected]> <mailto:[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><mailto: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]>
> > <mailto:[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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> >
> > <https://urldefense.us/v3/__https://nam02.safelinks.protection.outlook.com/?url=http*3A*2F*2Flists.mcs.anl.gov*2Fpipermail*2Fpetsc-users*2Fattachments*2F20260102*2Fe60566c6*2Fattachment-0001.html&data=05*7C02*7Cmwhitte6*40jhu.edu*7C24e75a940a1946f44a3608de550f304c*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639041720893598980*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=aq*2BqnjbbjP53P9Aa3gFyyJVzsaHVliHAF4*2FH5VaIkmo*3D&reserved=0__;JSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJQ!!G_uCfscf7eWS!akkX3mtT9HWbhzS3UC7bCNBSaKhbvQ937U5ePbM10Wdro75K-aTE0MZvfvGfyBfBwpQpD8YsK2NvRfeSxt_v3IA$
> >
> > <https://urldefense.us/v3/__https://nam02.safelinks.protection.outlook.com/?url=http*3A*2F*2Flists.mcs.anl.gov*2Fpipermail*2Fpetsc-users*2Fattachments*2F20260102*2Fe60566c6*2Fattachment-0001.html&data=05*7C02*7Cmwhitte6*40jhu.edu*7C24e75a940a1946f44a3608de550f304c*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639041720893598980*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=aq*2BqnjbbjP53P9Aa3gFyyJVzsaHVliHAF4*2FH5VaIkmo*3D&reserved=0__;JSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJQ!!G_uCfscf7eWS!bhoEoF0XYYKl-EOyDlgI2fLPdGcIPgvLKpHDlZJ8j2J8urOsoci3Ml3KBNAv13782FIpFxECA2a2yLsA4vaQHSDB$>
> > >>
> >
> > ------------------------------
> >
> > Subject: Digest Footer
> >
> > _______________________________________________
> > petsc-users mailing list
> > [email protected] <mailto:[email protected]>
> > <mailto:[email protected]>
> > https://urldefense.us/v3/__https://nam02.safelinks.protection.outlook.com/?url=https*3A*2F*2Furldefense.us*2Fv3*2F__https*3A*2F*2Fnam02.safelinks.protection.outlook.com*2F*3Furl*3Dhttps*3A*2F*2Flists.mcs.anl.gov*2Fmailman*2Flistinfo*2Fpetsc-users*26data*3D05*7C02*7Cmwhitte6*40jhu.edu*7C094639f3b5f5405cfc4e08de4af1fa67*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639030600350725058*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C*26sdata*3DUeb*2F8ra8XpzYQzLTm4qwWVo9lyMGs2P5*2FMD0nA3xsdg*3D*26reserved*3D0__*3BJSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJQ!!G_uCfscf7eWS!bIMcmpMdXbyTPg56scE6PGSFLgQ9D07NeoK_UCdRbCrnEdwa1m3aLzbnLed2KLUNJJyHJeWiICB-ePUpdaF8pOQ*24&data=05*7C02*7Cmwhitte6*40jhu.edu*7C24e75a940a1946f44a3608de550f304c*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639041720893633070*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=fmFnefcmhqrsOsKFsdq7Krc*2F0CDtDuB*2FCXbtDn6Rnr4*3D&reserved=0__;JSUlJSUlJSUlJSUqKioqKiolJSoqKioqKioqKioqKioqKiolJSoqKiUlJSUlJSUlJSUlJSUlJSUlJSUlJSUl!!G_uCfscf7eWS!akkX3mtT9HWbhzS3UC7bCNBSaKhbvQ937U5ePbM10Wdro75K-aTE0MZvfvGfyBfBwpQpD8YsK2NvRfeS9iTWLTQ$
> >
> > <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*7C094639f3b5f5405cfc4e08de4af1fa67*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639030600350725058*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=Ueb*2F8ra8XpzYQzLTm4qwWVo9lyMGs2P5*2FMD0nA3xsdg*3D&reserved=0__;JSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJQ!!G_uCfscf7eWS!bIMcmpMdXbyTPg56scE6PGSFLgQ9D07NeoK_UCdRbCrnEdwa1m3aLzbnLed2KLUNJJyHJeWiICB-ePUpdaF8pOQ$>
> >
> > <https://urldefense.us/v3/__https://nam02.safelinks.protection.outlook.com/?url=https*3A*2F*2Flists.mcs.anl.gov*2Fmailman*2Flistinfo*2Fpetsc-users&data=05*7C02*7Cmwhitte6*40jhu.edu*7C24e75a940a1946f44a3608de550f304c*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639041720893670294*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=o0XY48*2BZ5oVC0TxiObgNwusKIUub*2B3MPR6S30YvHQfI*3D&reserved=0__;JSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUlJQ!!G_uCfscf7eWS!akkX3mtT9HWbhzS3UC7bCNBSaKhbvQ937U5ePbM10Wdro75K-aTE0MZvfvGfyBfBwpQpD8YsK2NvRfeSfen92qo$
> >
> > <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$>
> > >
> >
> >
> > ------------------------------
> >
> > End of petsc-users Digest, Vol 205, Issue 2
> > *******************************************
> > <test2.F90>
>
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>
> ------------------------------
>
> Subject: Digest Footer
>
> _______________________________________________
> petsc-users mailing list
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> https://urldefense.us/v3/__https://nam02.safelinks.protection.outlook.com/?url=https*3A*2F*2Flists.mcs.anl.gov*2Fmailman*2Flistinfo*2Fpetsc-users&data=05*7C02*7Cmwhitte6*40jhu.edu*7C24e75a940a1946f44a3608de550f304c*7C9fa4f438b1e6473b803f86f8aedf0dec*7C0*7C0*7C639041720893746146*7CUnknown*7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ*3D*3D*7C0*7C*7C*7C&sdata=gNKZtz*2B5BHwQe2nCwMnYBo0dFDdYJ6lNPFaeElUrAOw*3D&reserved=0__;JSUlJSUlJSUlJSUlJSUlJSUlJSUlJSUl!!G_uCfscf7eWS!akkX3mtT9HWbhzS3UC7bCNBSaKhbvQ937U5ePbM10Wdro75K-aTE0MZvfvGfyBfBwpQpD8YsK2NvRfeSwXBSvPk$
> <https://lists.mcs.anl.gov/mailman/listinfo/petsc-users >
>
>
> ------------------------------
>
> End of petsc-users Digest, Vol 205, Issue 12
> ********************************************
