[petsc-users] Réf. : Re: Unreproducible number of iterations for consecutives solves (GCR + ML)

Thu Jun 27 11:30:20 CDT 2013

Dear PETSc developers,

As usual your answers are concise and precise. Thanks a lot for the 
explanation !
Browsing the mailing list archive a bit has made me aware of the native 
PCGAMG, so we'll update PETSc and give it a try.

Keep up the fantastic work,
Thomas





jedbrown at mcs.anl.gov
Envoyé par : five9a2 at gmail.com
27/06/2013 18:13
 
        Pour :  rupp at mcs.anl.gov, thomas.de-soza at edf.fr
        cc :    petsc-users at mcs.anl.gov
        Objet : Re: [petsc-users] Unreproducible number of iterations for 
consecutives solves (GCR + ML)




NOTE : La(les) pièce(s) jointe(s) a(ont) été supprimée(s) du message 
initial. Le nom est conservé pour information.
Le : 27/06/2013, à 18:18
Par : Thomas DE-SOZA
Nom(s) du(des) fichiers(s) : 
C.DTF


Karl Rupp <rupp at mcs.anl.gov> writes:

> Thus, it seems that there is a little share of randomness involved,
> which may only be significant when the solver needs to run a large
> number of iterations. So, the fluctuation of iterations when using ML
> does not seem to be something you need to be worried about.

Yes, ML has some randomness.  It sets a deterministic seed so the first
solve is reproducible, but multiple solves will vary some:

diff --git i/src/ksp/ksp/examples/tutorials/ex56.c 
w/src/ksp/ksp/examples/tutorials/ex56.c
index 91fad9d..fa8860c 100644
--- i/src/ksp/ksp/examples/tutorials/ex56.c
+++ w/src/ksp/ksp/examples/tutorials/ex56.c
@@ -284,7 +284,7 @@ int main(int argc,char **args)
PetscReal emax, emin;
ierr = MaybeLogStagePush(stage[2]);CHKERRQ(ierr);
/* PC setup basically */
-    ierr = MatScale(Amat, 100000.0);CHKERRQ(ierr);
+    //ierr = MatScale(Amat, 100000.0);CHKERRQ(ierr);
ierr = KSPSetOperators(ksp, Amat, Amat, 
SAME_NONZERO_PATTERN);CHKERRQ(ierr);
ierr = KSPSetUp(ksp);CHKERRQ(ierr);

@@ -297,7 +297,7 @@ int main(int argc,char **args)
ierr = MaybeLogStagePush(stage[4]);CHKERRQ(ierr);

/* 3rd solve */
-    ierr = MatScale(Amat, 100000.0);CHKERRQ(ierr);
+    //ierr = MatScale(Amat, 100000.0);CHKERRQ(ierr);
ierr = KSPSetOperators(ksp, Amat, Amat, 
SAME_NONZERO_PATTERN);CHKERRQ(ierr);
ierr = KSPSetUp(ksp);CHKERRQ(ierr);

$ mpiexec -n 8 ./ex56 -ne 19 -alpha 1.e-3 -ksp_monitor -ksp_type cg 
-ksp_max_it 50 -pc_type ml -ksp_converged_reason -mg_levels_ksp_type 
chebyshev -mg_levels_ksp_chebyshev_estimate_eigenvalues 0,0.05,0,1.05 
-mg_levels_pc_type jacobi -two_solves -ksp_norm_type unpreconditioned
0 KSP Residual norm 3.916254888964e+00
1 KSP Residual norm 2.881812838115e+01
2 KSP Residual norm 1.275966743250e+01
3 KSP Residual norm 4.773599170045e+00
4 KSP Residual norm 1.935268484585e+00
5 KSP Residual norm 4.209580628708e-01
6 KSP Residual norm 8.134351555855e-02
7 KSP Residual norm 2.105221695641e-02
8 KSP Residual norm 6.251570402725e-03
9 KSP Residual norm 1.693062086081e-03
10 KSP Residual norm 4.088111376292e-04
11 KSP Residual norm 8.282470497154e-05
12 KSP Residual norm 2.910468516666e-05
Linear solve converged due to CONVERGED_RTOL iterations 12
0 KSP Residual norm 3.916254888964e+00
1 KSP Residual norm 2.884447434423e+01
2 KSP Residual norm 1.279637972963e+01
3 KSP Residual norm 4.778606784425e+00
4 KSP Residual norm 1.945306501447e+00
5 KSP Residual norm 4.252458287438e-01
6 KSP Residual norm 8.200469618341e-02
7 KSP Residual norm 2.128564125382e-02
8 KSP Residual norm 6.325422255484e-03
9 KSP Residual norm 1.722774707230e-03
10 KSP Residual norm 4.155306725796e-04
11 KSP Residual norm 8.432468795266e-05
12 KSP Residual norm 2.965043662043e-05
Linear solve converged due to CONVERGED_RTOL iterations 12
0 KSP Residual norm 3.916254888964e+00
1 KSP Residual norm 2.884581541125e+01
2 KSP Residual norm 1.281469034767e+01
3 KSP Residual norm 4.787719947229e+00
4 KSP Residual norm 1.954021039200e+00
5 KSP Residual norm 4.286214813381e-01
6 KSP Residual norm 8.267288000027e-02
7 KSP Residual norm 2.150072898753e-02
8 KSP Residual norm 6.392547836067e-03
9 KSP Residual norm 1.749098960784e-03
10 KSP Residual norm 4.214004987050e-04
11 KSP Residual norm 8.561869518870e-05
12 KSP Residual norm 3.013123608130e-05
Linear solve converged due to CONVERGED_RTOL iterations 12
[0]main |b-Ax|/|b|=7.693890e-06, |b|=3.916255e+00, emax=9.962246e-01




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