[petsc-users] Wise usage of user contexts

[Dave May]

On 18 August 2015 at 10:42, Timothée Nicolas <timothee.nicolas at gmail.com>
wrote:

> Hi all,
>
> I am in the process of writing an implicit solver for a set of PDEs
> (namely MHD equations), in FORTRAN. When setting the non-linear function to
> solve via Newton-Krylov, I use a "user defined context", namely the thing
> denoted by "ctx" on the doc page about SNESFunction :
>
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESFunction.html#SNESFunction
>
> In practice ctx is a user defined type which contains everything I need in
> the local routine which sets the function on the local part of the grid,
> FormFunctionLocal. That is, some local/global geometrical information on
> the grid, the physical parameter, and possibly any other thing.
>
> In my case it so happens that due to the scheme I have chosen, when I
> compute my function, I need the full solution of the problem at the last
> two time steps (which are in Vec format). So my ctx contains two Vec
> elements. Since I will work in 3D and intend to use a lot of points in the
> future, I am concerned about memory problems which could occur.
>

In the grand scheme of things, the two vectors in your context aren't
likely to significantly add to the total memory footprint of your code. A
couple of things to note:
* If you run in parallel, only the local part of the vector will be stored
on each MPI process.
* All the KSP methods will allocate auxiliary vectors. Most methods require
more than 2 auxiliary vectors.
* SNES also requires auxiliary vectors. If you use JFNK, that method will
also need some additional temporary vectors.
* If you assemble a Jacobian, this matrix will likely require much more
memory per MPI process than two vectors



> Is there a limit to the size occupied by ctx ?
>

The only limit is defined by the available memory per MPI process you have
on your target machine.


> Would this be better if instead I was declaring global variables in a
> module and using this module inside FormFunctionLocal ? Is this allowed ?
>

What would be the difference in doing that - the memory usage will be
identical.

Cheers
  Dave


>
> Best regards
>
> Timothee NICOLAS
>
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