a question about tolerance

Jed Brown jed at 59A2.org
Tue Sep 15 17:27:00 CDT 2009

Ryan Yan wrote:
> How can I explain the relative tolerance is function correctly? as you can
> see from the term ||Ae||/||Ax||, neither of my testruns seems to stop at the
> right place(should be 10^(-2), 10^(-3) respectively).

The ||Ae||/||Ax|| column is not the preconditioned norm.  You are using
left-preconditioned GMRES (the default) which uses the preconditioned
norm.  Use

  -ksp_right_pc -ksp_norm_type unpreconditioned

for the behavior you seem to expect.


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