[petsc-users] Solutions different after program restart

[Bojan Niceno]

Dear all,


I have a rather strange issue with the incompressible Navier-Stokes 
solver I am developing.  It works fine with PETSc, unless I do a /restart/.

By restart I mean loading the values of dependent variables from 
previously performed simulation.  After such a restart, results obtained 
from PETSc linear solver are slightly different than those obtained from 
the previous simulation.

For example, I make a first run with 100 time steps, save dependent 
values at 50.

I run another simulation, which starts from the results saved at 50, but 
the results in the first time step in the new run will be slightly 
different than those of step 51 in the previous run.  The differences 
are small, but still give an uneasy feeling.

I suspect the difference /must /stem from different values of:
1 -dependent variables (x), or
2 - sources (b), or
3 - system matrices (A).

In order to pinpoint the cause of the differences, I print norms of  A, 
x, b before and after restart.  They are exactly the same which, to my 
understanding, should ensure the results from a call to linear solver 
are the same.  Yet, that doesn't seem to be the case.

Can anybody give me an advice on how to find the cause of these differences?

Is there something inside PETSc objects (such as helping vectors or 
alike) which might need cleaning in order to ensure exactly the same 
results after the program restart?



     Kind regards,


     Bojan
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