[petsc-users] difference between solns for np=1 and np>1.

[Jed Brown]

On Thu, 18 Mar 2010 11:53:27 -0400, "(Rebecca) Xuefei YUAN" <xy2102 at columbia.edu> wrote:
> Dear all,
> 
> I have a piece of code working right for np=1, but when I use np>1(for  
> example, np=2), the solution is different from the one with np=1.
> 
> I checked the residual function(f_np1_itsi vs f_np2_itsi) and Jacobian  
> matrix(J_np1_itsi,J_np2_itsi), it turns out that for different initial  
> guesses,
> 
> f_np1_its0 = f_np2_its0;
> J_np1_its0 = J_np2_its0;
> 
> although two Jacobians are the same, they have different nonzero structures.

The format is different.

> After 1 nonlinear iterations, the solution is not the same.
> 
> I do not understand that if I have F(u^{m}) and J(u^{m}) the same, how  
> could I get different \delta u^{m} in np=1 and np=2 situations?

How much different?  Is it much different with -pc_type jacobi or
-pc_type none?

http://www.mcs.anl.gov/petsc/petsc-as/documentation/faq.html#differentiterations

Jed