[petsc-users] Bug in VecNorm, 3.20.3

[Pierre Jolivet]

> 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?

Thanks,
Pierre

>  0 KSP Residual norm < 1.e-11
> Linear solve converged due to CONVERGED_ATOL iterations 0