<div dir="ltr">On Wed, Aug 14, 2013 at 12:28 PM, Danyang Su <span dir="ltr"><<a href="mailto:danyang.su@gmail.com" target="_blank">danyang.su@gmail.com</a>></span> wrote:<br><div class="gmail_extra"><div class="gmail_quote">
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Hi All,<br>
<br>
I have many linear equations with the same matrix structure (same
non-zero entries) that are derived from a flow problem at different
time steps. I feel puzzled that the results are a little different
when the solver run repeatedly and one by one. Say, I have three
equations, I can get the following results if running three
equations together<br>
<br>
Equation 1: Iterations 1 norm 0.9457E-02 Result error PETSc
vs Solver2, max 0.4362E-02 min -0.2277E-04 norm 0.9458E-02<br>
<font color="#ff0000">Equation 2: Iterations 2 norm
0.2994E-05 Result error PETSc vs Solver2, max 0.1381E-05 min
-0.7209E-08 norm 0.2994E-05<br>
Equation 3: Iterations 2 norm 0.3919E-04 Result error
PETSc vs Solver2, max 0.9435E-07 min -0.1808E-04 norm 0.3919E-04</font><br>
<br>
But if I solve only one equation every time, then restart the
program to run another one, the results are like this:<br>
<br>
Equation 1: Iterations 1 norm 0.9457E-02 Result error PETSc
vs Solver2, max 0.4362E-02 min -0.2277E-04 norm 0.9458E-02<br>
<font color="#ff0000">Equation 2: Iterations 1 norm
0.7949E-05 Result error PETSc vs Solver2, max 0.3501E-05 min
-0.8377E-06 norm 0.7949E-05<br>
Equation 3: Iterations 1 norm 0.1980E-04 Result error
PETSc vs Solver2, max 0.4168E-08 min -0.9085E-05 norm 0.1980E-04</font><br>
<br>
<font color="#000099">Note: Solver2 is the original sequential
solver used in this flow model.</font><br>
<br>
Though there are no big difference in the solution for the above
equations, I want to know why?<br>
<br>
For another large linear equations with more than 400,000 unknowns
and 10,000,000 non-zero entries, if the equations are solved
repeatedly, they need a lot of iterations or fail, but if the
equations are solved one by one, it only needs 1 to 2 iterations.<br>
<br>
How does this difference come from?<br>
<br>
The sample codes are attached bellow.<br>
<br>
Thanks and regards,<br>
<br>
Danyang<br>
<br>
!***************************************************************************!<br>
!Create matrix, rhs and solver<br>
call MatCreateAIJ(Petsc_Comm_World, Petsc_Decide, Petsc_Decide, nb,
nb, nd_nzrow, &<br>
Petsc_Null_Integer, nd_nzrow, Petsc_Null_Integer,
a, ierr) <br>
call MatSetOption(a,Mat_New_Nonzero_Allocation_Err,Petsc_False,ierr)<br>
call VecCreateMPI(Petsc_Comm_World, Petsc_Decide, nb, b, ierr) <br>
call VecDuplicate(b, x, ierr) <br>
call VecDuplicate(x, u, ierr)<br>
call KSPCreate(Petsc_Comm_World,ksp,ierr)<br>
call KSPSetTolerances(ksp,tol, &<br>
PETSC_DEFAULT_DOUBLE_PRECISION, &<br>
PETSC_DEFAULT_DOUBLE_PRECISION, &<br>
100,ierr)<br>
call KSPSetFromOptions(ksp,ierr)<br>
<br>
!Do time loop<br>
do i = 1, nTimeStep<br>
call MatGetOwnershipRange(a,istart,iend,ierr)<br>
do i = istart, iend - 1<br>
ii = ia_in(i+1)<br>
jj = ia_in(i+2)<br>
call MatSetValues(a, ione, i, jj-ii, ja_in(ii:jj-1)-1,
a_in(ii:jj-1), Insert_Values, ierr)<br>
end do <br>
call MatAssemblyBegin(a, Mat_Final_Assembly, ierr) <br>
call MatAssemblyEnd(a, Mat_Final_Assembly, ierr)<br>
<br>
call VecGetOwnershipRange(b,istart,iend,ierr)<br>
call VecSetValues(b, iend-istart, ix(istart+1:iend),
b_in(istart+1:iend), Insert_Values, ierr)<br>
call VecAssemblyBegin(b,ierr)<br>
call VecAssemblyEnd(b,ierr)<br>
<br>
if(i == 1) then<br>
call MatConvert(a,MATSAME,MAT_INITIAL_MATRIX,a2,ierr)<br></div></blockquote><div><br></div><div> Why are you doing this?</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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end if<br>
!call KSPSetOperators(ksp,a,a2,SAME_PRECONDITIONER,ierr) <br></div></blockquote><div><br></div><div>Just use a, a for the matrices</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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call KSPSetOperators(ksp,a,a2,SAME_NONZERO_PATTERN,ierr)
!These three patterns make no difference in current codes<br></div></blockquote><div><br></div><div>This DOES matter here if you are using the default PC which is ILU.</div><div><br></div><div> Matt</div><div> </div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div text="#000000" bgcolor="#FFFFFF">
!call KSPSetOperators(ksp,a,a2,DIFFERENT_NONZERO_PATTERN,ierr)<br>
<br>
call KSPSolve(ksp,b,x,ierr)<br>
<br>
call KSPGetResidualNorm(ksp,norm,ierr)<br>
call KSPGetIterationNumber(ksp,its,ierr) <br>
end do<br>
<br>
!Destroy objects<br>
!...<br>
!***************************************************************************!<br>
</div>
</blockquote></div><br><br clear="all"><div><br></div>-- <br>What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br>
-- Norbert Wiener
</div></div>