Thanks Barry, sorry for careless reading the code.
> From: bsmith at mcs.anl.gov
> Date: Mon, 3 Jan 2011 10:32:17 -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
>
>
> call KSPSolve(ksp,b,x,ierr)
>
> ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> ! Check solution and clean up
> ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>
> ! Check the error
> call VecAXPY(x,neg_one,u,ierr)
> ^^^^^^^^^^^^^^^^^^^
> x = x - 1.0* u; and hence changes x
>
>
>
> On Jan 2, 2011, at 11:35 PM, Peter Wang wrote:
>
> > 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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