[firedrake] tracking write accesses to dats/functions

[Florian Rathgeber]

On 23/11/14 12:11, Eike Mueller wrote:
> Dear firedrakers,
> 
> can I somehow check whether a dat/function has been written to since I last performed a given operation on it?
> 
> I have a banded matrix A stored in each cell on mesh, and whenever I solve the equation A.x = b (separately in each column, of course) I want to do this in two steps:
> 
> (1) calculate the LU decomposition (lapack dgbtrf)
> (2) solve based on that LU decomposition (lapack dgbtrs)
> 
> I have to do a lot of solves, so for efficiency I want to only do (1) if my matrix A has changed. So when I do a solve, I check if a flag lu_decomp is True (and this flag is set to False upon matrix creation). If it is, then I go straight to (2), otherwise I do (1) first. But I need to make sure that whenever my banded matrix has been written to lu_decomp is set back to False.

I'm not exactly sure I understand what you're trying to do. Are those
matrices represented by firedrake.Matrix/pyop2.Mat? If so, they're only
ever written to when you call assemble on the Matrix object. You can
check the assembled property to see whether the Matrix is assembled and
_needs_reassembly to check whether it needs to be reassembled.

If you're interested whether or not PETSc has written to the matrix
during solve I'm not sure. I don't think an assembled matrix is written
to at all, but maybe Lawrence can clarify.

Hope this helps,
Florian

> Thanks,
> 
> Eike

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