[petsc-users] Bug in VecNorm, 3.20.3

[michael at paraffinalia.co.uk]

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

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