[petsc-users] How to do a precondition in SNES flow

[Barry Smith]

  Simply evaluate the Jacobian during your SNESComputeFunction and save it for SNESComputeJacobian.



> On Oct 3, 2023, at 1:51 PM, Gong Ding <gongding at cn.cogenda.com> wrote:
> 
> Hi all,
> 
> I'd like to do a  special jacobian precondition during the snes iteration, for which jacobian matrix and RHS vector must be modified explicitly.
> 
> In the SNESComputeJacobian, the preconditioner P is built after assembly of jacobian matrix.
> 
> I need to multiply P to J and RHS vector  explicitly as left precondition before the solve stage of J*dx = rhs. 
> 
> However, I find that petsc evaluates function before jacobian, so P*RHS vector can not be processed at SNESComputeFunction.
> 
> As a result,  I must find a hook function after SNESComputeJacobian and before the solve stage. 
> 
> Any suggest? 
> 
> Gong Ding

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