A.multTranspose(x, y) produces y = A^t * x If it is not the case, you are doing something wrong I minimal working example will help us debug your approach.
You can take a look at the test folder https://urldefense.us/v3/__https://gitlab.com/petsc/petsc/-/tree/main/src/binding/petsc4py/test__;!!G_uCfscf7eWS!dhEH-iztM8EuysrQ-SA3_P74GVXmb3CXNESdTF-hHBHpAa-fQbeM0Lf27pHtwK5mW_VFNYIFPHdQbhUpPxZdxitcNaZSyTY$ , the demo folder https://urldefense.us/v3/__https://gitlab.com/petsc/petsc/-/tree/main/src/binding/petsc4py/demo__;!!G_uCfscf7eWS!dhEH-iztM8EuysrQ-SA3_P74GVXmb3CXNESdTF-hHBHpAa-fQbeM0Lf27pHtwK5mW_VFNYIFPHdQbhUpPxZdxitcupJqo4Q$ There are also Python examples here https://urldefense.us/v3/__https://gitlab.com/stefanozampini/petscexamples__;!!G_uCfscf7eWS!dhEH-iztM8EuysrQ-SA3_P74GVXmb3CXNESdTF-hHBHpAa-fQbeM0Lf27pHtwK5mW_VFNYIFPHdQbhUpPxZdxitcntk_xfM$ Il giorno mar 15 lug 2025 alle ore 14:49 SCOTTO Alexandre via petsc-users < petsc-users@mcs.anl.gov> ha scritto: > Ok I get it, Vec.assemble() is mandatory whenever MPI communications are > required to get the values to appropriate processes. > > > > To provide more information, I am in a situation where I perform a > A.multTranspose(x, y), where y is a vector that has been filled in with > values earlier in the process. At this stage, I no longer care of these > values and I expect the multTranspose() to override the values. But what I > get is: > > · if I do y.assemble() before the transpose multiplication, then > y is filled-in with the correct result. > > · if I do not perform y.assemble() before the transpose > multiplication, I actually get y = y + A^T(x), i.e. a result rather of the > form multTransposeAdd(). > > · If I do A.multTranspose(x, y) *twice*, then I get the correct > result y = A^T(x). > > > > This makes me think that I am misusing something at some points, but it is > not clear what. If someone has a hint to explain this behaviour that would > help me better understand how to properly use PETSc! > > > > Regards, > Alexandre. > > > > *De :* Matthew Knepley <knep...@gmail.com> > *Envoyé :* mardi 15 juillet 2025 14:18 > *À :* SCOTTO Alexandre <alexandre.sco...@irt-saintexupery.com> > *Cc :* Jose E. Roman <jro...@dsic.upv.es>; petsc-users@mcs.anl.gov > *Objet :* Re: [petsc-users] When to perform PETSc.Vec assembly with > petsc4py > > > > On Tue, Jul 15, 2025 at 8:13 AM SCOTTO Alexandre via petsc-users < > petsc-users@mcs.anl.gov> wrote: > > Hello Jose, > > Thanks for your answer. Then it seems that I have under the hood usages of > setValues() in my code since I do not explicitly make use of it but still > has problems when not assembling my vector. Do I need to assemble vectors > after Mat.mult or Scatter.scatter? > > > > No. The purpose of VecAsseblyBegin/End() is to move values from processes > that do not own them to the processes that do. PETSc does this > automatically for MatMult() and VecScatter routines because we know exactly > where values are headed. However, when users call VecSetValues(), they may > set locations that are owned by other processes. We could communicate these > immediately, but that might be expensive for a series of VecSetValues() > calls, so we wait until you call VecAssembly(). > > > > Note that direct assignment to the array can only set local values. This > is equivalent to VecGetArray(). > > > > Thanks, > > > > Matt > > > > By the way, I did not know that PETSc.Vec objects supported direct > assignments like x[i] = ..., so I rather use assignments of the form > x.array = ... > > From what I understand from the GitHub page ( > https://urldefense.us/v3/__https://gitlab.com/petsc/petsc/-/blob/main/src/binding/petsc4py/src/petsc4py/PETSc/Vec.pyx__;!!G_uCfscf7eWS!f3M5SmfhSqew4tgfvFDjdLnd3q3kT_KzcCitJTJC0HRl1YvbSNBr5Uuzv6OWP1pVQBgbi_WJlekerylpGcBHQ9vsbzL1suFBQHkoPIwhdw$ > <https://urldefense.us/v3/__https:/gitlab.com/petsc/petsc/-/blob/main/src/binding/petsc4py/src/petsc4py/PETSc/Vec.pyx__;!!G_uCfscf7eWS!f3M5SmfhSqew4tgfvFDjdLnd3q3kT_KzcCitJTJC0HRl1YvbSNBr5Uuzv6OWP1pVQBgbi_WJlekerylpGcBHQ9vsbzL1suFBQHkoPIwhdw$> > ) it seems, that doing so, we only access the local portion of the array > which should not require any assemble() am I right? > > Best regards, > Alexandre. > > > -----Message d'origine----- > De : Jose E. Roman <jro...@dsic.upv.es> > Envoyé : mardi 15 juillet 2025 12:21 > À : SCOTTO Alexandre <alexandre.sco...@irt-saintexupery.com> > Cc : petsc-users@mcs.anl.gov > Objet : Re: [petsc-users] When to perform PETSc.Vec assembly with petsc4py > > Assembly is needed after a call to x.setValues() or any of its variants. > > https://urldefense.us/v3/__https://petsc.org/main/petsc4py/reference/petsc4py.PETSc.Vec.html*petsc4py.PETSc.Vec.setValue__;Iw!!G_uCfscf7eWS!f3M5SmfhSqew4tgfvFDjdLnd3q3kT_KzcCitJTJC0HRl1YvbSNBr5Uuzv6OWP1pVQBgbi_WJlekerylpGcBHQ9vsbzL1suFBQHlKEPOyQg$ > <https://urldefense.us/v3/__https:/petsc.org/main/petsc4py/reference/petsc4py.PETSc.Vec.html*petsc4py.PETSc.Vec.setValue__;Iw!!G_uCfscf7eWS!f3M5SmfhSqew4tgfvFDjdLnd3q3kT_KzcCitJTJC0HRl1YvbSNBr5Uuzv6OWP1pVQBgbi_WJlekerylpGcBHQ9vsbzL1suFBQHlKEPOyQg$> > Take into account that in python the notation x[i] = ... with call > x.setValues() under the hood. > > Jose > > > > El 15 jul 2025, a las 12:02, SCOTTO Alexandre via petsc-users < > petsc-users@mcs.anl.gov> escribió: > > > > Dear PETSc community, > > As a beginner in the MPI world and with the PETSc library, I come with > a possibly very naive question. > > I know from the documentation that assembling vectors must be done, but > it is not clear to me when to perform this operation. > > Is there a simple way to know when a vector need be assembled and when > it is not? > > Thanks in advance. > > Regards, > > Alexandre. > > > > > -- > > What most experimenters take for granted before they begin their > experiments is infinitely more interesting than any results to which their > experiments lead. > -- Norbert Wiener > > > > https://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!dhEH-iztM8EuysrQ-SA3_P74GVXmb3CXNESdTF-hHBHpAa-fQbeM0Lf27pHtwK5mW_VFNYIFPHdQbhUpPxZdxitcsLpPCdI$ > > <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!cspsHbAhId4QoslCKt3i_holgL7kvzmVSbBmROUEJ9HuA2YEjyfDurQ0XiBEpAaqohjglNK31xfzm4zAiM3wDxpywkRKnJnXtifrP1eVXw$> > -- Stefano
