[CIG-SHORT] Change in dt means new Jacobian

[Charles Williams]

If I'm remembering correctly, this is true for Maxwell and generalized Maxwell viscoelastic models.  It would not be true for elastic models, and for power-law or Drucker Prager models you'll have to reform the Jacobian anyway, no matter if the time step size changes or not.

Cheers,
Charles


On 3/09/2012, at 5:24 AM, Birendra jha wrote:

> Dear developers
> 
> I am trying to run my Pylith simulation faster. Right now it takes about 3 min for Linear solve (using default KSP solver, about 960 iterations) and 3 min for integrating residual. I have about 190000 hex cells (mesh is very refined in the center and becomes coarse outward). No faults. Elastic materials. I used non-uniform user specified timesteps.
> 
> I thought of following options:
> 
> (1) Try different solver. I am thinking of using field split preconditioner with multigrid, as in tet4/step02.cfg. 
> (2) Switch from vtk to hdf5 output
> (3) Run in parallel. I am looking into running it using sge. Or just running it on one multicore machine.
> (4) Increase linear solve tolerance from 1E-12 to 1E-11.
> (5) Fix timestep.
> 
> Are these good options to consider?
> 
> I have a question on (5). Manual says
> Warning: Changing the time step requires recomputing the Jacobian of the system, which can greatly increase the runtime if the time-step size changes frequently.
> 
> Where does this happen in the program, i.e a change in dt setting needNewJacobian=true? I ask this because I could not find message on the stdout about reforming Jacobian when I am using non-uniform dt (it is possible that I missed it, the timesteps remains constant for a while then changes then becomes constant again).
> 
> Thanks and regards
> Birendra
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Charles A. Williams
Scientist
GNS Science
1 Fairway Drive, Avalon
PO Box 30368
Lower Hutt  5040
New Zealand
ph (office): 0064-4570-4566
fax (office): 0064-4570-4600
C.Williams at gns.cri.nz

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