[petsc-users] Bug in VecNorm, 3.20.3

[Junchao Zhang]

Yes, VecRestoreArray[Read,Write] should be called after you finish using
the array.   This casting to (const double **) is bad. Otherwise, compiler
could catch the error when you use a const pointer in a non-const way.

PetscCall(VecGetArrayRead(ref, (const double **)(&pr))) ;


After I used the right VecGet/RestoreArrayWrite(), the code worked.

--Junchao Zhang


On Fri, Jan 26, 2024 at 8:40 AM Pierre Jolivet <pierre at joliv.et> wrote:

>
> On 26 Jan 2024, at 3:11 PM, Pierre Jolivet <pierre at joliv.et> wrote:
>
>
> On 26 Jan 2024, at 3:03 PM, michael at paraffinalia.co.uk wrote:
>
> On 2024-01-23 18:09, Junchao Zhang wrote:
>
> Do you have an example to reproduce it?
> --Junchao Zhang
>
>
> I have put a minimum example on github:
>
> https://github.com/mjcarley/petsc-test
>
> It does seem that the problem occurs if I do not use the PETSc interface
> to do a matrix multiplication.
>
> In the original code, the PETSc matrix is a wrapper for a Fast Multipole
> Method evaluation; in the minimum example I have simulated this by using an
> array as a matrix. The sample code generates a randomised matrix A and
> reference solution vector ref, and generates a right hand side
>
> b = A*ref
>
> which is then supplied as the right hand side for the GMRES solver. If I
> use the PETSc matrix multiplication, the solver behaves as expected; if I
> generate b directly from the underlying array for the matrix, I get the
> result
>
>
> You should not use VecGetArrayRead() if you change the Vec, but instead,
> VecGetArrayWrite().
> Does that solve the issue?
>
>
> Sorry, I sent the message too fast, you are also missing a couple of calls
> to VecRestoreArray[Read,Write]().
> These are the ones which will let PETSc know that the Vec has had its
> state being increased, and that the cached norm are not valid anymore, see
> https://petsc.org/release/src/vec/vec/interface/rvector.c.html#line2177
>
> Thanks,
> Pierre
>
> Thanks,
> Pierre
>
> 0 KSP Residual norm < 1.e-11
> Linear solve converged due to CONVERGED_ATOL iterations 0
>
>
>
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