Legacy FEniCS Parallel Execution

General
1

I’m using FEniCS 2019.1.0. To test the parallel functionality of legacy FEniCS, I’m running a slightly modified version of the heat equation tutorial program (I know the program is trivial, that’s not the point).

When I run the program serially, XDMF files are indeed written to the directory ./figures as expected. However, If I run the program using mpirun -np N python heat.py, (where N is a positive integer), nothing is written to file and the program runs until I manually halt execution.

What’s going on here? Program is pasted below:

from __future__ import print_function
from fenics import *
import time
import os

comm = MPI.comm_world
rank = MPI.rank(comm)

workdir = os.getcwd()
outDirName = os.path.join(workdir, "figures")
os.makedirs(outDirName, exist_ok=True)

T = 2.0            # final time
num_steps = 50     # number of time steps
dt = T / num_steps # time step size

# Create mesh and define function space
nx = ny = 30
mesh = RectangleMesh(comm, Point(-2, -2), Point(2, 2), nx, ny)
V = FunctionSpace(mesh, 'P', 1)

# Define boundary condition
def boundary(x, on_boundary):
    return on_boundary

bc = DirichletBC(V, Constant(0), boundary)

# Define initial value
u_0 = Expression('exp(-a*pow(x[0], 2) - a*pow(x[1], 2))',
                 degree=2, a=5)
u_n = interpolate(u_0, V)

# Define variational problem
u = TrialFunction(V)
v = TestFunction(V)
f = Constant(0)

F = u*v*dx + dt*dot(grad(u), grad(v))*dx - (u_n + dt*f)*v*dx
a, L = lhs(F), rhs(F)

# Create XDMF file for saving solution
solnfile = XDMFFile(outDirName + "/soln.xdmf")

# Time-stepping
u = Function(V)
t = 0
for n in range(num_steps):

    # Update current time
    t += dt

    # Compute solution
    solve(a == L, u, bc)

    # Write to file
    if rank == 0:
        solnfile.write(u_n, t)

    # Update previous solution
    u_n.assign(u)
2

The file shouldn’t be written on rank 0 only.
You should remove this if-clause and just use

for n in range(num_steps):

    # Update current time
    t += dt

    # Compute solution
    solve(a == L, u, bc)

    # Write to file
    solnfile.write(u_n, t)

    # Update previous solution
    u_n.assign(u)
2 Likes