clarification about convergence

[Dominik Szczerba]

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.

Starting KSPSolve
   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:

Starting KSPSolve
   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.

Dominik