[petsc-users] different convergence behaviour

[Barry Smith]

  Use -ksp_type gmres to get it to print the residuals. With preonly it doesn't compute or print them.


> On Jul 15, 2016, at 11:28 AM, Hoang Giang Bui <hgbk2008 at gmail.com> wrote:
> 
> I used
> 
> -ksp_monitor_true_residual
> -ksp_monitor_true_solution
> -ksp_converged_reason
> 
> with MUMPS but it does not compute the true residual. Should I compute that myself?
> 
> Below is a sample for a full log of MUMPS
> https://www.dropbox.com/s/fy5uknooxw77r19/log13Jun16_mumps?dl=0
> 
> 
> Giang
> 
> On Fri, Jul 15, 2016 at 2:52 AM, Mark Adams <mfadams at lbl.gov> wrote:
> 
> 
> On Thu, Jul 14, 2016 at 7:29 PM, Matthew Knepley <knepley at gmail.com> wrote:
> On Thu, Jul 14, 2016 at 6:27 PM, Mark Adams <mfadams at lbl.gov> wrote:
> 
> 
> Notice that there are 7 orders of magnitude between the apparent residual (using the preconditioner), and the actual residual, Ax - b.
> You are using Hypre, and this generally means the Hypre coarse grid operator is crap. Please
> 
> 
> Huh?, this data looks fine, both the true and preconditioned residual stay separated by about 9 orders of magnitude. This just tells you that the norm of A (or is it A^-1) is 10^9.  Am I misunderstanding this?
> 
> This is why Barry and I asked for a comparsion with MUMPS. If you are right, and its just the condition number,
> 
> I said norm not condition number. I trust I'm missing something in this thread.
>  
> the LU
> will not be any more accurate.
> 
>    Matt
> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener
> 
>