numba-mpi

Overview

numba-mpi provides Python wrappers to the C MPI API callable from within Numba JIT-compiled code (@jit mode). For an outline of the project, rationale, architecture, and features, refer to: numba-mpi paper in SoftwareX (open access) (please cite if numba-mpi is used in your research).

Support is provided for a subset of MPI routines covering: size/rank, send/recv, allreduce, reduce, bcast, scatter/gather & allgather, barrier, wtime and basic asynchronous communication with isend/irecv (only for contiguous arrays); for request handling including wait/waitall/waitany and test/testall/testany.

The API uses NumPy and supports both numeric and character datatypes (e.g., broadcast). Auto-generated docstring-based API docs are published on the web: https://numba-mpi.github.io/numba-mpi

Packages can be obtained from PyPI, Conda Forge, Arch Linux or by invoking pip install git+https://github.com/numba-mpi/numba-mpi.git.

numba-mpi is a pure-Python package. The codebase includes a test suite used through the GitHub Actions workflows (thanks to mpi4py's setup-mpi!) for automated testing on: Intel Linux (MPICH, OpenMPI & Intel MPI), ARM Linux (MPICH & OpenMPI), Intel and ARM macOS (MPICH) and Windows (MS MPI). Note, that some of those combinations may not be fully supported yet - see Known Issues for more information.

Features that are not implemented yet include (help welcome!):

• support for non-default communicators
• support for MPI_IN_PLACE in [all]gather/scatter and allreduce
• support for MPI_Type_create_struct (Numpy structured arrays)
• ...

Hello world send/recv example:

import numba, numba_mpi, numpy

@numba.jit()
def hello():
    src = numpy.array([1., 2., 3., 4., 5.])
    dst_tst = numpy.empty_like(src)

    if numba_mpi.rank() == 0:
        numba_mpi.send(src, dest=1, tag=11)
    elif numba_mpi.rank() == 1:
        numba_mpi.recv(dst_tst, source=0, tag=11)

hello()

Example comparing numba-mpi vs. mpi4py performance:

The example below compares Numba+mpi4py vs. Numba+numba-mpi performance. The sample code estimates $\pi$ by numerical integration of $\int_0^1 (4/(1+x^2))dx=\pi$ dividing the workload into n_intervals handled by separate MPI processes and then obtaining a sum using allreduce (see, e.g., analogous Matlab docs example). The computation is carried out in a JIT-compiled function get_pi_part() and is repeated N_TIMES. The repetitions and the MPI-handled reduction are done outside or inside of the JIT-compiled block for mpi4py and numba-mpi, respectively. Timing is repeated N_REPEAT times and the minimum time is reported. The generated plot shown below depicts the speedup obtained by replacing mpi4py with numba_mpi, plotted as a function of N_TIMES / n_intervals - the number of MPI calls per interval. The speedup, which stems from avoiding roundtrips between JIT-compiled and Python code is significant (150%-300%) in all cases. The more often communication is needed (smaller n_intervals), the larger the measured speedup. Note that nothing in the actual number crunching (within the get_pi_part() function) or in the employed communication logic (handled by the same MPI library) differs between the mpi4py or numba-mpi solutions. These are the overhead of mpi4py higher-level abstractions and the overhead of repeatedly entering and leaving the JIT-compiled block if using mpi4py, which can be eliminated by using numba-mpi, and which the measured differences in execution time stem from.

import timeit, mpi4py, numba, numpy as np, numba_mpi

N_TIMES = 10000
RTOL = 1e-3

@numba.jit
def get_pi_part(n_intervals=1000000, rank=0, size=1):
    h = 1 / n_intervals
    partial_sum = 0.0
    for i in range(rank + 1, n_intervals, size):
        x = h * (i - 0.5)
        partial_sum += 4 / (1 + x**2)
    return h * partial_sum

@numba.jit
def pi_numba_mpi(n_intervals):
    pi = np.array([0.])
    part = np.empty_like(pi)
    for _ in range(N_TIMES):
        part[0] = get_pi_part(n_intervals, numba_mpi.rank(), numba_mpi.size())
        numba_mpi.allreduce(part, pi, numba_mpi.Operator.SUM)
        assert abs(pi[0] - np.pi) / np.pi < RTOL

def pi_mpi4py(n_intervals):
    pi = np.array([0.])
    part = np.empty_like(pi)
    for _ in range(N_TIMES):
        part[0] = get_pi_part(n_intervals, mpi4py.MPI.COMM_WORLD.rank, mpi4py.MPI.COMM_WORLD.size)
        mpi4py.MPI.COMM_WORLD.Allreduce(part, (pi, mpi4py.MPI.DOUBLE), op=mpi4py.MPI.SUM)
        assert abs(pi[0] - np.pi) / np.pi < RTOL

plot_x = [x for x in range(1, 11)]
plot_y = {'numba_mpi': [], 'mpi4py': []}
for x in plot_x:
    for impl in plot_y:
        plot_y[impl].append(min(timeit.repeat(
            f"pi_{impl}(n_intervals={N_TIMES // x})",
            globals=locals(),
            number=1,
            repeat=10
        )))

if numba_mpi.rank() == 0:
    from matplotlib import pyplot
    pyplot.figure(figsize=(8.3, 3.5), tight_layout=True)
    pyplot.plot(plot_x, np.array(plot_y['mpi4py'])/np.array(plot_y['numba_mpi']), marker='o')
    pyplot.xlabel('number of MPI calls per interval')
    pyplot.ylabel('mpi4py/numba-mpi wall-time ratio')
    pyplot.title(f'mpiexec -np {numba_mpi.size()}')
    pyplot.grid()
    pyplot.savefig('readme_plot.svg')

Known Issues

NOTE: Issues listed below only relate to combinations of platforms and MPI distributions that we target to support, but due to various reason are currently not working and are temporarily excluded from automated testing:

• tests on MacOS (both Intel- and ARM-based) that use OpenMPI are currently not run due to failures being under investigation (see related issue - TODO #163).

MPI resources on the web:

• MPI standard and general information:
  • https://www.mpi-forum.org/docs
  • https://en.wikipedia.org/wiki/Message_Passing_Interface
• MPI implementations:
  • OpenMPI: https://www.open-mpi.org
  • MPICH: https://www.mpich.org
  • MS MPI: https://learn.microsoft.com/en-us/message-passing-interface
  • Intel MPI: https://intel.com/content/www/us/en/developer/tools/oneapi/mpi-library-documentation.html
• MPI bindings:
  • Python: https://mpi4py.readthedocs.io
  • Python/JAX: https://mpi4jax.readthedocs.io
  • Julia: https://juliaparallel.org/MPI.jl
  • Rust: https://docs.rs/mpi
  • C++: https://www.boost.org/doc/libs/latest/doc/html/mpi.html
  • R: https://cran.r-project.org/web/packages/Rmpi

Acknowledgements:

We thank all contributors and users who reported feedback to the project through GitHub issues.

Development of numba-mpi has been supported by the Polish National Science Centre (grant no. 2020/39/D/ST10/01220), the Max Planck Society and the European Union (ERC, EmulSim, 101044662). We further acknowledge Poland’s high-performance computing infrastructure PLGrid (HPC Centers: ACK Cyfronet AGH) for providing computer facilities and support within computational grant no. PLG/2023/016369.