[petsc-users] Solving with SNES

[Jed Brown]

On Sat, Nov 12, 2011 at 11:50, behzad baghapour
<behzad.baghapour at gmail.com>wrote:

> The nonlinear residual and Jacobian matrix are evaluated with a face-based
> method. Two integrals are involved (the effect of element internal
> connections and face flux connections). All of my calculations of flow
> states are solved and stored in arrays of objects called elements.


Are you talking about intermediate quantities like Roe averages on face
quadrature points? The state X at which the residual is evaluated is
different each time, so you would be using old values if you stashed them.
The residual might be evaluated multiple times before a Jacobian is
requested. Normally the residual is much less expensive than the Jacobian,
so it's not worth stashing the intermediate quantities. See also this FAQ:

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