[petsc-users] Recommended examples for using KSP and multigrid with DM

Matthew Knepley knepley at gmail.com
Sun Apr 12 04:38:52 CDT 2015

On Sat, Apr 11, 2015 at 7:03 PM, TAY wee-beng <zonexo at gmail.com> wrote:

> Hi,
> I'm using DMDA. The LHS of the poisson eqn is fixed, only the RHS changes.
> I thought I can't find ex29.c in the current ver of PETSc, so I thought
> it's no longer valid. Do I always have to get the jacobian as in ex29 to
> solve the Poisson eqn?

It is there:


> What about a good ex. using KSPBCGS to solve my momentum eqn with DMDA?

All examples are independent of the precise Krylov method. You can use that

  cd src/snes/examples/tutorials
  make ex5
  ./ex5 -ksp_type bcgs



> Thank you.
> Yours sincerely,
> TAY wee-beng
> On 4/11/2015 6:43 AM, Barry Smith wrote:
>>    Are you using DMDA or DMPLEX or some other DM? Does the Poisson
>> equation change (variable coefficient) or is it a constant coefficient with
>> the same values for all solves? ex29.c could still possibly serve as a
>> model for the Poisson subproblem, why do you say it is no longer valid?
>>    Barry
>>  On Apr 8, 2015, at 10:18 PM, TAY wee-beng <zonexo at gmail.com> wrote:
>>> Hi,
>>> I was following example ex29 in the past to write the code to solve my
>>> Poisson eqn using DM. Now it seems that ex29 is no longer valid. Which
>>> example should I follow now?
>>> Also, I am using KSPBCGS to solve my momentum eqn in the past. Do I need
>>> to change much if I'm now using DM? If so, what is a good example?
>>> --
>>> Thank you
>>> Yours sincerely,
>>> TAY wee-beng

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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