clarification about convergence
dominik at itis.ethz.ch
Wed Dec 9 11:45:43 CST 2009
I am using
-ksp_rtol 0 -ksp_atol 1e-6 (defaults for all the rest)
and observe for most of the times convergence, as expected.
0 KSP Residual norm 6.315132950112e-04
1 KSP Residual norm 9.139539245601e-05
2 KSP Residual norm 1.869750068013e-05
3 KSP Residual norm 6.788145356909e-06
4 KSP Residual norm 1.729115712274e-06
5 KSP Residual norm 7.214445966641e-07
solved in 2.328146e+00/4.144644e+00 s
solution converged in 5 iterations
||Ax-b|| = 7.21445e-07
with ||...|| being residue computed on my own for a check.
However, SOMETIMES, I do observe:
0 KSP Residual norm 1.228562818225e-02
1 KSP Residual norm 4.045833318123e-04
2 KSP Residual norm 6.589999324657e-05
3 KSP Residual norm 1.494849372638e-05
4 KSP Residual norm 4.311592172896e-06
solved in 1.368086e+00/2.235830e+00 s
solution converged in 4 iterations
||Ax-b|| = 4.31159e-06
which means that the iterations break sooner than they should. Again,
this is not often, but worries me why this should happen... Using or not
-ksp_converged_use_initial_residual_norm does not make any difference to
this behavior: the last output Residual norm is sometimes higher than
the requested atol.
Any clarifications are highly appreciated.
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