Sudden random deviation of results

[Barry Smith]

  Ben,

   Try running with a direct solver. -pc_type lu and see how it
goes along. (on one process).

   Barry

On Fri, 8 Dec 2006, Ben Tay wrote:

> Hi,
> 
> I have tested my code again, using the checks suggested by Barry. Using
> KSPGetConvergedReason shows that it has converged. Changing the tol from
> 1.e-5 to -6 or -7 or even -4 eliminated the explosion of value at that
> particular time step.
> 
> 
> However, that is expected since the values are now different. Moreover,  for
> other linear solver package, it does not happen at this particular time step
> as well.
> 
> Another strange thing is that suppose the explosion of value happen at t=50.
> If I store my all my relevent values at t=40 into files and restart my
> simulation from t=40(instead of t=1) by reading the stored values from the
> files, the explosion will now not happen at t=50. Hence it seems very random
> too....
> 
> 
> 
> On 12/7/06, Barry Smith <bsmith at mcs.anl.gov> wrote:
> > 
> > 
> > Ben,
> > 
> >   First make sure that the residual has actually gotten
> > to the tolerance you want. PETSc KSP does NOT stop if the
> > linear system has not converged; you should call KSPGetConvergedReason()
> > after each solve to make sure it has converged (as a quick check
> > you can run with -ksp_converged_reason.)
> > 
> > Next try decreasing the KSP tolerance factor a great deal. Does
> > 1) the solution you get for your physics problem look the same for small
> >   time (after a few time-steps they will start to drift).
> > 2) the residual blow up at the same time, or later?
> > 
> >   Barry
> > 
> > 
> > On Thu, 7 Dec 2006, Ben Tay wrote:
> > 
> > > Hi,
> > >
> > > I have been using a few different Krylov linear solver package such as
> > > nspcg,sparsekit and now petsc to solve the linear eqns for my NS solver
> > > momentum and poisson eqn. For moving bodies simulation, while solving
> > the
> > > poisson eqn, sometimes the result suddenly changes drastically, although
> > the
> > > iterations seems to have converged e.g. at the previous time step,
> > > everything is fine, but at the next time step, the velocity suddenly
> > > explodes from 2-3 to 20-30+.
> > >
> > > The strange thing is that this deviation happens at different time step
> > for
> > > different linear solver package ie for nspcg, it happens at time=20 but
> > for
> > > petsc, it happens at time=40. It seems "random" in this sense. Changing
> > the
> > > type of solver such as gmres or bcgs does not change the time of
> > deviation.
> > >
> > > Does anyone know why this is happening? Is it a characteristic of krylov
> > > solver to happen once in a while? It does not happen in the stationary
> > body
> > > simulation though.
> > >
> > 
> > 
>