How can I plot a function only once in parallel

Question

when running

mpirun -n 4 demo_poisson.py

the function u is plotted by each processes. How can I plot the whole function (only once) when running in parrallel?

Answer 1

You cannot.

There is at least one reason: off-process dofs (corresponding to some owned mesh entity) must be updated (passed from owner process to some other processes). In principle responsibility for this update could be delegated to user, hence something like this would be possible:

u.update()
if MPI.process_number() == 0:
    plot(u, update=False)

You can file a proposal to DOLFIN issues but I'm afraid that it will be rejected.

Comments

Really, Naively I tried

if MPI.process_number() == 0:
plot(u)

but nothing plots. Why is that and in general why is it not possible?

---

Why is that

It deadlocks because plot(u) is currently implemented as a collective operation (it must be executed by every process).

and in general why is not possible?

See updated answer.