Thanks, Barry,
when you said,
> x is computed in the code as the difference between the "exact" solution and
> the "computed" solution.
Does it mean x is not the solution?
In the webpage document for KSPSolve, the x is defined as :
Parameter
ksp
- iterative context obtained from KSPCreate()
b
- the right hand side vector
x
- the solution
Why is x the difference in this example code? what is the solution,then?
> From: bsmith at mcs.anl.gov
> Date: Sun, 2 Jan 2011 22:55:00 -0600
> To: petsc-users at mcs.anl.gov
> Subject: Re: [petsc-users] result of ex2f.F in
> petsc-3.1-p5\src\ksp\ksp\examples\tutorials
>
>
> On Jan 2, 2011, at 9:32 PM, Peter Wang wrote:
>
> > I added VecView() in the ex2f.F to check the three vectors (u,b, and x) in
> > the code.
> >
> > -----The vectors of u and b are same. BUt, the value of vector x is
> > different with different processes. For example,
> >
> > with only 1 process, the vector x is:
> >
> > Process [0]
> > 2.72322e-07
> > 3.81437e-07
> > 1.58922e-07
> > 3.81437e-07
> > 2.38878e-07
> > -6.65645e-07
> > 1.58922e-07
> > -6.65645e-07
> > -2.51219e-07
> >
> > with 2 processes, the vector is:
> >
> > Process [0]
> > -1.11022e-16
> > 0
> > 2.22045e-16
> > 2.22045e-16
> > 0
> >
> > Process [1]
> > 2.22045e-16
> > 2.22045e-16
> > 0
> > 2.22045e-16
> void PETSC_STDCALL kspbuildsolution_(KSP *ksp,Vec *v,Vec *V, int *ierr )
> {
> Vec vp = 0;
> CHKFORTRANNULLOBJECT(v);
> CHKFORTRANNULLOBJECT(V);
> if (v) vp = *v;
> *ierr = KSPBuildSolution(*ksp,vp,V);
> }
>
> void PETSC_STDCALL kspbuildresidual_(KSP *ksp,Vec *t,Vec *v,Vec *V, int *ierr
> )
> {
> Vec tp = 0,vp = 0;
> CHKFORTRANNULLOBJECT(t);
> CHKFORTRANNULLOBJECT(v);
> CHKFORTRANNULLOBJECT(V);
> if (t) tp = *t;
> if (v) vp = *v;
> *ierr = KSPBuildResidual(*ksp,tp,vp,V);
> }
>
> Barry
>
> x is computed in the code as the difference between the "exact" solution and
> the "computed" solution. Since this example uses iterative solvers, which by
> default do not compute the solution to full accuracy, the "error" will be
> different for different number of processes. It is just a fluke that the
> error is smaller with two processes instead of one.
>
> >
> > The example is supposed to get a vector x similar to u. Why the result is
> > different with differnt number of processes used?
> >
> > ----Also, if the runtime option -my_ksp_monitor is used, there is a error
> > showing:
> >
> > [0]PETSC ERROR:
> > ------------------------------------------------------------------------
> > [0]PETSC ERROR: Caught signal number 11 SEGV: Segmentation Violation,
> > probably memory access out of range
> >
> > It seems there is something wrong with calling 'call
> > KSPBuildSolution(ksp,PETSC_NULL_OBJECT,x,ierr)' in user defined function
> > MyKSPMonitor().
> >
> > Any hints for this error? Thanks a lots.
>
> Bug in our Fortran Interface for KSPBuildSolution() fortran interface in that
> case. If you replace the two functions in
> src/ksp/ksp/interface/ftn-custom/zitclf.c with the ones below and run make in
> that directory this monitor routine will work.
>
>
> >
> >
> >
> >
> >
> >
> >
>
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