Get global mesh size

[PG20R]

Hi.
Sorry, for this very easy question, but I am a complete beginner and just could not find an answer working for me.
I need the global mesh size for a convergence plot. (In FEniCS one could just type mesh.hmax() for example, but this does not seem to work here). How can I do this for a mesh in Firedrake?

[jwallwork23]

It depends what you mean by "global mesh size". The following will compute the size of each cell and then give you the maximum. Is that what you are looking for?

P0 = FunctionSpace(mesh, "DG", 0)
hmax = project(CellSize(mesh), P0)
print(hmax.vector().gather().max())

[PG20R]

Thanks, that's what I was looking for.

[wence-]

Note this is a non-scalable way of computing the max, you get the entire vector on rank 0. A nicer way:

P0 = FunctionSpace(mesh, "DG", 0)
hmax = project(CellSize(mesh), P0)
with hmax.dat.vec_ro as v:
   global_hmax = v.max()