> On Apr 1, 2015, at 5:09 PM, Juris Vencels <[email protected]> wrote:
>
> On 03/31/2015 08:26 PM, Barry Smith wrote:
>> No, the model is that each time the matrix changes the PCSetUp is called
>> automatically and so is a shell set up if you have provided it, so you need
>> to change your PCShellSetUp() function so that it can be called repeatedly
>> for each new matrix and call
>>
>>
>>> >
>>> call PCShellSetApply(pc,PCShellSetUp,ierr)
>>>
>> when you create the Shell PC.
>>
>> Then make a test run in the debugger to make sure that your setup is being
>> called each time the matrix changes.
>>
>>
>
> It works, thank you!
>
>> Note: also if you are using ILU preconditioning, you do not need to use
>> PCSHELL, you can just use PCSetType(pc,PCILU) or -pc_type ilu.
>
> I am confused about this, I am using matrix-free SNES and Manual Section 5.5.
> says:
>
>> The matrix-free variant is allowed only when the linear systems are solved
>> by an it-
>> erative method in combination with no preconditioning (PCNONE or -pc_type
>> none), a user-provided
>> preconditioner matrix, or a user-provided preconditioner shell (PCSHELL,
>> discussed in Section 4.4); ...
>
> PETSc has function "PCShellSetApply " which sets routine to use as
> preconditioner.
> How can I set a routine if I do not use PCSHELL? There is no such function as
> " PCSetApply" anymore.
One can use -snes_mf_operator this causes the operator that DEFINES the
linear system to be matrix free. Meanwhile one still builds an explicit
approximation to the Jacobian as the SECOND matrix argument to
SNESSetJacobian(). The preconditioner is then buildt from this second matrix.
So if one does -pc_type ilu the preconditioner is built by doing ILU on that
second matrix.
If you use -snes_mf you need to use a PCSHELL but if you have an explicit
matrix from which you are going to build the preconditioner then there is no
reason to use PCSHELL (which is more complicated) because you can use
-snes_mf_operator instead and have PETSc manage the preconditioner in the usual
manner with, for example, -pc_type ilu etc
Barry
