[petsc-users] Simultaneously compute Residual+Jacobian in SNES

[Derek Gaston]

Is there a way to tell SNES to simultaneously compute both the residual and
the Jacobian in one callback?

My code can compute both simultaneously and it will be more efficient
(think FE where you can reuse the shape-functions, variables, material
properties, etc. for both residual and Jacobian computation).  In addition,
I also have automatic differentiation as an option which _definitely_
computes both efficiently (and actually computes residuals, by themselves,
much slower).

I was thinking that I may just save off the Jacobian whenever the initial
residual computation is asked for by SNES... and then just return that
Jacobian when SNES asks for it.  This may be a bit dicey though as SNES can
ask for residual computations at many different points during the solve.

Thanks for any help!

Derek
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