[petsc-users] -ksp_rtol problem

[Qiyue Lu]

Dear All:

I am working on a linear A*x=b system.
When I use *-ksp_rtol 1.0e-5 *option in the command line, I am not very
sure how the relative tolerance is defined.

In my understanding:

At the beginning, r= b-A*x_i, so rtol = ||r|| / ||b|| with 2-norm has a
value. with the calculation, rtol gets smaller, when the rtol_new /
rtol_original < 1.0e-5, the calculation will stop.

Is my understanding correct?

If so, how to decide the rtol at the first iteration? If initial solution
values are all zeros, then rtol should be 1.0. In my output file, it's a
value related to the matrix and can be at 1.0e-4  scale. Is there any
initial guess solution from preconditioner used there?

Could you help to clarify this definition? Thanks


Qiyue Lu
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20131111/bb28867f/attachment-0001.html>