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<div class="moz-cite-prefix">Hi Matthew,<br>
<br>
Thanks so much. It works out now. <br>
<br>
Danyang<br>
<br>
On 14/08/2013 11:14 AM, Matthew Knepley wrote:<br>
</div>
<blockquote
cite="mid:CAMYG4G=OkWBa6u8hR=wor6wz2tO7wgWu1weqtRh=2gTr1dU=aQ@mail.gmail.com"
type="cite">
<div dir="ltr">On Wed, Aug 14, 2013 at 12:28 PM, Danyang Su <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:danyang.su@gmail.com" target="_blank">danyang.su@gmail.com</a>></span>
wrote:<br>
<div class="gmail_extra">
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF"> 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">
<div text="#000000" bgcolor="#FFFFFF"> 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">
<div text="#000000" bgcolor="#FFFFFF"> 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>
</blockquote>
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