SNES Problem

Fri Mar 17 17:27:39 CST 2006

On 3/17/06, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>    Based on my understanding. This is correct.

I was almost hoping I had missed something fundamental.

>
>    Suggest you run a TINY problem with your "old" code
> and the TS (and or SNES one). Print out everything. The
> current solution the result from calling Get_DRO() and
> compare the runs, when and why do they change? This will
> help understand what is going on.

Well, Get_DRO() is exactly the same in the old and new code, and they
start with the same initial values. Get_DRO() is not producing strange
values, it is crashing because it get strange values fed into it.

A normal run for the formfunction f(in,out) in TS would look something
like this if printed:
in=25560         <- Start value
out=0.0001     <-  du/dt
in=25560         <- Value fed into formfunction after some TS magic
out=0.0001      <- du/dt
...                      <- (a couple iterations)
in=25451          <- Input value after a couple of iterations
out=0.0001      <- du/dt
in=0.23151       <- Value fed into formfunction decided by TS
Petscerror divide by zero...

The problem seems to be that TS decides that the input values to
formfunction should be (in this case) 0.23151 which makes Get_DRO()
crash because of  for example sqrt(in-1).

I have no idea how to fix this. The values from Get_DRO() in TS match
the values when running the old rk code.

/nisse

>
>     Barry
>
>
> On Fri, 17 Mar 2006, Nils Erik Svangård wrote:
>
> > Ok, I managed to compile and link rk.c to my fortran program, I forgot
> > that in C you need a ;  in the end of every statement. (stupid mistake
> > ;) )
> >
> > I have just added one line to see if it works.
> >
> >  /* computing new dt */
> >     dt = dt * dt_fac;
> >
> >     /* Start Nisse stuff */
> >        ierr = PetscPrintf(PETSC_COMM_WORLD,"Nisse prints dt: %f\n",dt);
> >     /* End nisse stuff */
> >
> >
> >     if(ts->ptime+dt > ts->max_time){
> >        dt = ts->max_time - ts->ptime;
> >     }
> >
> > I just try to print the current timestep, however this is never
> > printed. And I'm not really sure that it is the timestep that is
> > causing the problems.
> >
> > I have used call TSGetTimeStep(ts,timestep,ierr) to monitor what
> > timestep TS uses and it seem ok. However after the first iteration of
> > FormFunction everything seems ok, but in start of the second iteration
> > all values are really strange.
> >
> > I see the same thing when using SNES and my back euler implementation,
> > it iterate many more times however, but all of a sudden the all "in"
> > values are in the range 0.2-0.7 (for all 7 equations) and my code
> > bombs because of the strange values.
> >
> > When using TS and  running with  -snes_mf -ts_type beuler -ksp_rtol
> > 1.e-10 this is what printed just before producing strange values:
> > KSP Object:
> >  type: gmres
> >    GMRES: restart=30, using Classical (unmodified) Gram-Schmidt
> > Orthogonalization with no iterative refinement
> >    GMRES: happy breakdown tolerance 1e-30
> >  maximum iterations=10000, initial guess is zero
> >  tolerances:  relative=1e-10, absolute=1e-50, divergence=10000
> >  left preconditioning
> > PC Object:
> >  type: none
> >  linear system matrix = precond matrix:
> >  Matrix Object:
> >    type=mffd, rows=70000, cols=70000
> >        SNES matrix-free approximation:
> >          err=1e-07 (relative error in function evaluation)
> >          Using wp compute h routine
> >          Computes normA
> >
> >
> > And just to make sure that I havent misunderstood how SNES and TS work:
> > If the original 3-stage RK uses (my numbering):
> > 1.  RO0(L)=RO(L)
> >     Get_DRO(RO(L))
> >     RO(L)=RO0(L)+CFL*DRO(L)
> > 2.  RO0(L)=.5*(RO0(L)+RO(L))
> >     Get_DRO(RO(L))
> >     RO(L)=RO0(L)+.5*CFL*DRO(L)
> > 3.  Get_DRO(RO(L))
> >     RO(L)=RO0(L)+.5*CFL*DRO(L)
> >
> > Then this should be in TS which should return du/dt which is DRO:
> >      RO(L)=xx(1,L)
> >      Get_DRO(RO(L))
> >      ff(1,L) = DRO(L)
> >
> > And in SNES with back euler:
> >      (Old RO from previous iteration ORO(L)
> >      RO(L)=xx(1,L)
> >      Get_DRO(RO(L))
> >      ff(1,L)= RO(L)-OLD(1,L)-TSF(L)*DRO(L)
> >
> >
> > This became a long mail, I hope this shows if I missed something vital.
> > /nisse
> > On 3/16/06, Nils Erik Svangård <nilserik at gmail.com> wrote:
> >> Barry,
> >> the problem is making the objectfile, but I'll try again when I have
> >> the code. I will check the makefile for the c-examples.
> >> /nisse
> >>
> >> On 3/16/06, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >>>
> >>>    Nisse,
> >>>
> >>>      Just list it in your makefile with all your other object
> >>> files (that come from Fortran). Send the output if this fails.
> >>>
> >>>     Barry
> >>>
> >>>
> >>> On Thu, 16 Mar 2006, Nils Erik Svangård wrote:
> >>>
> >>>> I havent managed to get rk.c compiled with changes. how do I compile
> >>>> it in my working directory to get a object file. I just realised that
> >>>> I probably forgot to link it against $TSLIB but should I need to that
> >>>> when I dont do any linking, the linking is done when linking the
> >>>> fortran and the c code?
> >>>> Or what am I doing wrong (I not that good with C++ and linking).
> >>>> /nisse
> >>>>
> >>>> On 3/15/06, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >>>>>
> >>>>>    Both
> >>>>>
> >>>>>
> >>>>> On Wed, 15 Mar 2006, Nils Erik Svangård wrote:
> >>>>>
> >>>>>>>
> >>>>>>>    Sorry, I forgot. Is the linear solver converging? If not, then that
> >>>>>>> is the problem? Use a tolerance like -ksp_rtol 1.e-10 and see if the
> >>>>>>> nonlinear solver converges.
> >>>>>>>
> >>>>>>
> >>>>>> I'll try that when I have access to the code. I havent checked if the
> >>>>>> linear solver converges is thera a -kspmonitor or -kspconvergedreason
> >>>>>> I should use?
> >>>>>>
> >>>>>> /nisse
> >>>>>>
> >>>>>>
> >>>>>
> >>>>
> >>>>
> >>>> --
> >>>> Nils-Erik Svangård
> >>>> E-Mail: nilserik at gmail.com
> >>>> MSN: schweingaard at hotmail.com
> >>>> Skype: schweingaard
> >>>> Mobil: +46-(0)70-3612178
> >>>>
> >>>>
> >>>
> >>
> >>
> >> --
> >> Nils-Erik Svangård
> >> E-Mail: nilserik at gmail.com
> >> MSN: schweingaard at hotmail.com
> >> Skype: schweingaard
> >> Mobil: +46-(0)70-3612178
> >>
> >
> >
> > --
> > Nils-Erik Svangård
> > E-Mail: nilserik at gmail.com
> > MSN: schweingaard at hotmail.com
> > Skype: schweingaard
> > Mobil: +46-(0)70-3612178
> >
> >
>

--
Nils-Erik Svangård
E-Mail: nilserik at gmail.com
MSN: schweingaard at hotmail.com
Skype: schweingaard
Mobil: +46-(0)70-3612178