[petsc-users] On QN + Fieldsplit
Matthew Knepley
knepley at gmail.com
Tue Oct 12 13:24:33 CDT 2021
On Tue, Oct 12, 2021 at 10:27 AM Nicolás Barnafi <nabw91 at gmail.com> wrote:
> Thank you for the support. I rewrote the initialization in a simpler way,
> now it works as expected:
>
> > dofmap_s = V.sub(0).dofmap().dofs(); is_s =
> PETSc.IS().createGeneral(dofmap_s)
> > dofmap_p = V.sub(1).dofmap().dofs(); is_p =
> PETSc.IS().createGeneral(dofmap_p)
> > snes = PETSc.SNES().create(MPI.COMM_WORLD)
> > snes.setFunction(problem.F, b.vec()); snes.setJacobian(problem.J,
> J_mat.mat())
> > pc = snes.ksp.getPC()
> > pc.setType('fieldsplit')
> > pc.setFieldSplitIS((None, is_s), (None, is_p))
> > snes.setFromOptions()
> > snes.solve(None, problem.u.vector().vec())
>
> Apparently trying to setup the solver's internals is not recommended. As a
> side note, I tried also setting up the KSP using 'SNESSetKSP', but this
> solution is not so good as giving the command 'snes_ksp_ew' does nothing,
> even though it gets correctly read as shown by snes.view().
>
I think I can explain this. The Eisenstat-Walker scheme is a way to set
tolerances for the linear solves inside a Newton iteration. The goal is
to avoid over-solving the linear systems, meaning that far away from the
solution accurate linear solves have no advantage over inaccurate ones.
Implementing this involves coordination with the linear solver since we are
setting the convergence tolerance. When you replace the linear solve,
that setup is discarded. Thus, when configuring things, we recommend that
you pull out the existing object
SNESGetKSP()
and customize it, rather than creating a new object and setting it.
Thanks,
Matt
> Thanks for the help!
> Best,
> Nicolas
>
> On Tue, Oct 12, 2021 at 4:23 PM Matthew Knepley <knepley at gmail.com> wrote:
>
>> I looked over every place we use that error code. I do not think it is
>> coming from PETSc, but rather from petsc4py. However, something
>> is eating the error message, and I think Stefano indicated. My first step
>> would be to get the FEniCS folks to display the error message.
>>
>> Another option is to just run it in Firedrake since I think we can see
>> the stack properly there.
>>
>> Thanks,
>>
>> Matt
>>
>> On Tue, Oct 12, 2021 at 8:37 AM Nicolás Barnafi <nabw91 at gmail.com> wrote:
>>
>>> Thank you Stefano for the help. I added the lines you indicated, but the
>>> error remains the same, here goes snes.view() + error
>>>
>>> > SNES Object: 1 MPI processes
>>> > type: qn
>>> > SNES has not been set up so information may be incomplete
>>> > type is BROYDEN, restart type is DEFAULT, scale type is JACOBIAN
>>> > Stored subspace size: 10
>>> > Using the single reduction variant.
>>> > maximum iterations=10000, maximum function evaluations=30000
>>> > tolerances: relative=1e-08, absolute=1e-50, solution=1e-08
>>> > total number of function evaluations=0
>>> > norm schedule ALWAYS
>>> > SNESLineSearch Object: 1 MPI processes
>>> > type: basic
>>> > maxstep=1.000000e+08, minlambda=1.000000e-12
>>> > tolerances: relative=1.000000e-08, absolute=1.000000e-15,
>>> lambda=1.000000e-08
>>> > maximum iterations=1
>>> > Traceback (most recent call last):
>>> > File "Twist.py", line 234, in <module>
>>> > snes.setUp()
>>> > File "PETSc/SNES.pyx", line 530, in petsc4py.PETSc.SNES.setUp
>>> > petsc4py.PETSc.Error: error code 83
>>>
>>> On Tue, Oct 12, 2021 at 2:07 PM Stefano Zampini <
>>> stefano.zampini at gmail.com> wrote:
>>>
>>>>
>>>>
>>>> Il giorno mar 12 ott 2021 alle ore 13:56 Nicolás Barnafi <
>>>> nabw91 at gmail.com> ha scritto:
>>>>
>>>>> Hello PETSc users,
>>>>>
>>>>> first email sent!
>>>>> I am creating a SNES solver using fenics, my example runs smoothly
>>>>> with 'newtonls', but gives a strange missing function error (error 83):
>>>>>
>>>>>
>>>> Dolphin swallows any useful error information returned from PETSc. You
>>>> can try using the below code snippet at the beginning of your script
>>>>
>>>> from petsc4py import PETSc
>>>> from dolfin import *
>>>> # Remove the dolfin error handler
>>>> PETSc.Sys.pushErrorHandler('python')
>>>>
>>>>
>>>>
>>>>>
>>>>> these are the relevant lines of code where I setup the solver:
>>>>>
>>>>> > problem = SNESProblem(Res, sol, bcs)
>>>>> > b = PETScVector() # same as b = PETSc.Vec()
>>>>> > J_mat = PETScMatrix()
>>>>> > snes = PETSc.SNES().create(MPI.COMM_WORLD)
>>>>> > snes.setFunction(problem.F, b.vec())
>>>>> > snes.setJacobian(problem.J, J_mat.mat())
>>>>> > # Set up fieldsplit
>>>>> > ksp = snes.ksp
>>>>> > ksp.setOperators(J_mat.mat())
>>>>> > pc = ksp.pc
>>>>> > pc.setType('fieldsplit')
>>>>> > dofmap_s = V.sub(0).dofmap().dofs()
>>>>> > dofmap_p = V.sub(1).dofmap().dofs()
>>>>> > is_s = PETSc.IS().createGeneral(dofmap_s)
>>>>> > is_p = PETSc.IS().createGeneral(dofmap_p)
>>>>> > pc.setFieldSplitIS((None, is_s), (None, is_p))
>>>>> > pc.setFromOptions()
>>>>> > snes.setFromOptions()
>>>>> > snes.setUp()
>>>>>
>>>>>
>>>> If it can be useful, this are the outputs of snes.view(), ksp.view()
>>>>> and pc.view():
>>>>>
>>>>> > type: qn
>>>>> > SNES has not been set up so information may be incomplete
>>>>> > type is BROYDEN, restart type is DEFAULT, scale type is JACOBIAN
>>>>> > Stored subspace size: 10
>>>>> > Using the single reduction variant.
>>>>> > maximum iterations=10000, maximum function evaluations=30000
>>>>> > tolerances: relative=1e-08, absolute=1e-50, solution=1e-08
>>>>> > total number of function evaluations=0
>>>>> > norm schedule ALWAYS
>>>>> > SNESLineSearch Object: 4 MPI processes
>>>>> > type: basic
>>>>> > maxstep=1.000000e+08, minlambda=1.000000e-12
>>>>> > tolerances: relative=1.000000e-08, absolute=1.000000e-15,
>>>>> lambda=1.000000e-08
>>>>> > maximum iterations=1
>>>>> > KSP Object: 4 MPI processes
>>>>> > type: gmres
>>>>> > restart=1000, using Modified Gram-Schmidt Orthogonalization
>>>>> > happy breakdown tolerance 1e-30
>>>>> > maximum iterations=1000, initial guess is zero
>>>>> > tolerances: relative=1e-05, absolute=1e-50, divergence=10000.
>>>>> > left preconditioning
>>>>> > using UNPRECONDITIONED norm type for convergence test
>>>>> > PC Object: 4 MPI processes
>>>>> > type: fieldsplit
>>>>> > PC has not been set up so information may be incomplete
>>>>> > FieldSplit with Schur preconditioner, factorization FULL
>>>>>
>>>>> I know that PC is not setup, but if I do it before setting up the
>>>>> SNES, the error persists. Thanks in advance for your help.
>>>>>
>>>>> Best,
>>>>> Nicolas
>>>>> --
>>>>> Nicolás Alejandro Barnafi Wittwer
>>>>>
>>>>
>>>>
>>>> --
>>>> Stefano
>>>>
>>>
>>>
>>> --
>>> Nicolás Alejandro Barnafi Wittwer
>>>
>>
>>
>> --
>> 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/>
>>
>
>
> --
> Nicolás Alejandro Barnafi Wittwer
>
--
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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