pytest-mpi

Pytest plugin that lets you run tests in parallel with MPI.

Installation

To install pytest-mpi simply run:

$ python -m pip install /path/to/pytest-mpi-repo

Usage

Writing a parallel test simply requires marking the test with the parallel marker:

@pytest.mark.parallel(nprocs=5)  # run in parallel with 5 processes
def test_my_code_on_5_procs():
    ...

@pytest.mark.parallel(5)  # the "nprocs" kwarg is optional
def test_my_code_on_5_procs_again():
    ...

@pytest.mark.parallel  # run in parallel with the default number of processes (3)
def test_my_code_on_3_procs():
    ...

@pytest.mark.parallel()  # the brackets are optional
def test_my_code_on_3_procs_again():
    ...

One can also mark a test with a sequence of values for nprocs:

@pytest.mark.parallel(nprocs=[1, 2, 3])  # run in parallel on 1, 2 and 3 processes
def test_my_code_on_variable_nprocs():
    ...

@pytest.mark.parallel([1, 2, 3])  # again the "nprocs" kwarg is optional
def test_my_code_on_variable_nprocs_again():
    ...

If multiple numbers of processes are requested then the tests are parametrised and renamed to, in this case, test_my_code_on_variable_nprocs[nprocs=1], test_my_code_on_variable_nprocs[nprocs=2] and test_my_code_on_variable_nprocs[nprocs=3].

Extra markers

When running the code with these parallel markers, pytest-mpi adds extra markers to each test to allow one to select all tests with a particular number of processors. For example, to select all parallel tests on 3 processors, one should run

$ pytest -m "parallel[3]"

For serial tests - those either unmarked or marked @pytest.mark.parallel(1) - one can select these by running

$ pytest -m "not parallel or parallel[1]"

Forking mode

pytest-mpi can be run in one of two modes: forking or non-forking. The former works as follows:

1. The user calls pytest (not mpiexec -n <# proc> pytest!). This launches the "parent" pytest process.
2. This parent pytest process collects all the tests and begins to run them.
3. When a test is found with the parallel marker, rather than executing the function as before, a subprocess is forked calling mpiexec -np <# proc> pytest this_specific_test_file.py::this_specific_test. This produces <# proc> "child" pytest processes that execute the test together.
4. If this terminates successfully then the test is considered to have passed.

This is convenient for development for a number of reasons:

• The plugin composes better with other pytest plugins like pytest-xdist.
• It is not necessary to wrap pytest invocations with mpiexec calls, and all parallel and serial tests can be run at once.

However, the forking mode of pytest-mpi is restricted in that only one mainstream MPI distribution (MPICH) supports nested calls to MPI_Init. If your "parent" pytest process initialises MPI (for instance by executing from petsc4py import PETSc) then this will cause non-MPICH MPI distributions to crash. Further, forking a subprocess can be expensive since a completely fresh Python interpreter is launched each time.

Non-forking mode

With these significant limitations in mind, pytest-mpi therefore also supports a non-forking mode. To use it, one simply needs to wrap the pytest invocation with mpiexec, no additional configuration is necessary. For example, to run all of the parallel tests on 2 ranks one needs to execute:

$ mpiexec -n 2 pytest -m "parallel[2]"

This approach is agnostic to MPI distribution, and free from the forking startup overhead, but has a number of disadvantages:

• pytest-xdist is strictly disallowed as threading would lead to deadlocks. It is therefore impossible to take full advantage of machines with many cores.
• A different mpiexec instruction is required for each level of parallelism. Attempting to run with mpiexec with a mismatching number of processes to the parallel marker will result in an error.

Configuration

pytest-mpi respects the environment variable PYTEST_MPI_MAX_NPROCS, which defines the maximum number of processes that can be requested by a parallel marker. If this value is exceeded an error will be raised.