a question about tolerance

Matthew Knepley knepley at gmail.com
Tue Sep 15 19:29:04 CDT 2009

On Tue, Sep 15, 2009 at 6:04 PM, Ryan Yan <vyan2000 at gmail.com> wrote:

> Thanks Jed.
> And in the term ||Ae||/||Ax||,  PETSc impilictly assume that we use zero as
> initial guess and here the x really means exact solution, right?

This has nothing to do with the initial guess. Ae is the residual, and x is
the current guess. We do not have the exact solution,
but I guess A x^* = b, so that could have been used.


> Yan
> On Tue, Sep 15, 2009 at 6:41 PM, Jed Brown <jed at 59a2.org> wrote:
>> Ryan Yan wrote:
>> > But can you say a little more about what happened in the first scenario,
>> I
>> > did not quite fellow you. Let's say, If I set -ksp_rtol 1e-2, what does
>> this
>> > mean in the left PC case. More specifically, which term will be
>> considered
>> > as a stop creteria for the KSP solve.
>> Look at the first column (preconditioned residual norm).  Notice that
>> this is decreasing by 1e-2 and 1e-3 respectively in your examples.
>> Jed

What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
-- Norbert Wiener
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