Multiple Program Multiple Data (MPMD)

The AMReX-MPMD interface is used to send data to another program or application. In order to enable this feature, the executable has to be built with -DMFIX_MPMD = yes.

Input parameters controlling the AMReX-MPMD interface are defined in the User Guide run-time inputs section.

Sample Python Program

A sample python script that gathers and plots velocity statistics for the case of fluid flow through a pipe can be found in tutorials/mpmd/main.py. The script can be divided into the following sections:

Initialize

Initialize AMReX::MPMD and leverage MPI from mpi4py to perform communication split.

amr.MPMD_Initialize_without_split([])
app_comm = MPI.COMM_WORLD.Split(amr.MPMD_AppNum(), amr.MPMD_MyProc())
app_world_size = app_comm.Get_size()
app_rank = app_comm.Get_rank()
amr.initialize_when_MPMD([], app_comm)

Determine the C++ app’s root process

if app_rank == 0:
    if amr.MPMD_MyProc() == app_rank: # first program
        other_root = app_comm.Get_size()
    print(f'other_root = {other_root}')

Create an MPMD::Copier object that gets the BoxArray information from the C++ app.
copr = amr.MPMD_Copier(True)

Receive Once

Receive the Header information as a json string on the python root from the C++ root and broadcast to all python ranks.

header_json = ""

if app_rank == 0:
    buf = bytearray(10000)  # Create a buffer to receive the message
    MPI.COMM_WORLD.Recv([buf, MPI.CHAR], source=other_root)
    header_json = buf.decode().strip('\x00')  # Decode and strip null characters

header_json = app_comm.bcast(header_json, root=0)

Receive all the static Multifab data.

my_static_data = MyData()
for mf in header["data"]["static_mfs"]:
    my_static_data.define_mf(copr, mf["n"], mf["c"])
    my_static_data.copy_mf(copr, mf["n"], mf["c"])

Receive Until End

Receive End Flag on the python root from the C++ root and broadcast to all python ranks. If the flag is 1, break out of the loop.

if app_rank == 0:
    int_flags = np.empty(len(header["data"]["int_flags_root"]), dtype='i')
    MPI.COMM_WORLD.Recv(int_flags, source=other_root)
    print(f"app_rank = {app_rank}, int_flags = {int_flags})")
    end = int_flags[0]

end = app_comm.bcast(end, root=0)

if end == 1:
    break

Receive Reals on the python root from the C++ root and broadcast to all python ranks. Save time to an array on the python root for plotting.

if app_rank == 0:
    reals = np.empty(len(header["data"]["reals_root"]), dtype=np.double)
    MPI.COMM_WORLD.Recv(reals, source=other_root)
    print(f"app_rank = {app_rank}, reals = {reals})")
    time = reals[0]
    time_arr.append(time)

time = app_comm.bcast(time, root=0)

Receive MultiFab data and store to arrays on the python root as necessary. In this example, the centerline u-velocity and the data needed to compute the mean and variance of u-velocity on the central y-plane are stored as an array in time.

for mf in header["data"]["mfs"]:
    my_data.copy_mf(copr, mf["n"], mf["c"])

for mfi in my_data.mfs["vel_g"]:
    bx = mfi.validbox()
    y_intrst_exists = True
    z_intrst_exists = True

    if (j_intrst < bx.small_end[1] or j_intrst > bx.big_end[1]):
        y_intrst_exists = False

    if (k_intrst < bx.small_end[2] or k_intrst > bx.big_end[2]):
        z_intrst_exists = False

    if (not((y_intrst_exists or z_intrst_exists))):
        continue

    ###..............

    if ( y_intrst_exists and z_intrst_exists ):
        np_array = np.array(vel_g_array[0,k_intrst,j_intrst,:])
        u_centerline[bx.small_end[0]:bx.small_end[0] + np_array.size] = np_array

    if (y_intrst_exists):

    ###..............

        y_pl_npts += np.sum(y_volfrac_array)
        y_pl_u_mn += np.sum(y_vel_g_array[0,:,:])
        y_pl_u2_mn += np.sum(y_vel_g_array[0,:,:]*y_vel_g_array[0,:,:])

# Reduce from all python ranks
y_pl_npts = app_comm.reduce(y_pl_npts,op=MPI.SUM,root=0)
y_pl_u_mn = app_comm.reduce(y_pl_u_mn,op=MPI.SUM,root=0)
y_pl_u2_mn = app_comm.reduce(y_pl_u2_mn,op=MPI.SUM,root=0)
u_centerline = app_comm.reduce(u_centerline,op=MPI.SUM,root=0)

#...................

if app_rank == 0:
    y_pl_u_mn /= y_pl_npts
    y_pl_u2_mn /= y_pl_npts

    y_pl_u_mn_arr.append(y_pl_u_mn)
    y_pl_u_var_arr.append(y_pl_u2_mn-y_pl_u_mn*y_pl_u_mn)
    u_centerline_arr.append(u_centerline)

app_comm.barrier()

Plot

Plot figures on the python root using the collected arrays.

if app_rank == 0:
    fig, (ax1, ax2, ax3) = plt.subplots(1,3)

    ax1.plot(time_arr, y_pl_u_mn_arr)
    ax1.set_title('mean')
    ax1.set_xlabel('Time (s)')

    ax2.plot(time_arr, y_pl_u_var_arr)
    ax2.set_title('var')
    ax2.set_xlabel('Time (s)')

    ax3.plot(range(xlen), np.array(u_centerline_arr).mean(axis=0))
    ax3.set_title('centerline U (m/s)')
    ax3.set_xlabel('i')

    plt.savefig('my_plot.png')

Finalize

Finalize AMReX and AMReX::MPMD.
amr.finalize() amr.MPMD_Finalize()