[petsc-users] Running an iterative method for a large number of iterations: Possible blow up?

Barry Smith bsmith at mcs.anl.gov
Fri Feb 18 08:00:12 CST 2011

   Are you using petsc-dev? If not you should switch to it, it has an additional convergence test based on the residual of the normal equations.


   One possible explanation for the large values is that they are in the null space of the operator and though they don't increase the residual norm the solution just accumulates them after a large number of iterations.

On Feb 18, 2011, at 1:26 AM, Gaurish Telang wrote:

> Hi,
> I was trying to use LSQR algorithm for solving a least squares problem of size 2683x1274. I notice that if I allow the iterative method to run for a large number of iterations 
> after it has converged (i.e. output of -ksp_monitor KSPresidualnorm seems constant upto the 4th digit) , some numbers in the answer vector seem to get inordinately large. 
> I seem to get my answer comparable to Matlab after 951 iterations, but when I increase the number of iterations to 10000 some numbers seem very large.  
> Is this expected? Also, how do I terminate my iteration when my residual norm  seems constant? 
> Thanks

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