[petsc-users] Field dependent stencil in DAs

[Kevin Green]

Greetings,

I have been playing around with PetSc for the past couple of weeks, and have a question to which I have not found an answer in either the examples or the mailing list archives.

Essentially, I'm (currently) looking to numerically solve a system of 5 time dependent equations, but only 1 of the fields is involved with spatial derivatives.  Now, I've found a couple of ways to do this, either by introducing a Field struct cf. ${PETSC_DIR}/src/snes/examples/tutorials/ex19.c, or by changing one of the various ts examples to use DAVecGetArrayDOF(...).  Would there be any performance difference between introducing a Field struct compared to DAVecGetArrayDOF(...)?  Intuitively, I would think not...but I suppose I should ask since I'm here anyway.

Now the real reason I came:  Is there some sort of mask that can be applied to the DAs that would turn off the communication of ghost points for some degrees of freedom?  I assume that the communication structures are set up so that only a single message needs to be passed between communicating processes on update, but this still results in passing 5X the data that I have to.  Any comment on this would be much appreciated.

Thank you,
Kevin