Imagine you have a Makefile target that runs pytest in addition to other tests:

test: test-python test-mypy

test-python:
    pytest --numprocesses=8 my_app

test-mypy:
    mypy my_app

Conveniently, you just have to run make test to run both test targets. To save time, you can even run make -j8 test (8 being the number of cpu cores that you have at your disposal in this example) to run everything in parallel. But what really happens now is that

• make knows it has 8 slots available, and starts pytest and mypy in parallel

• mypy runs on one cpu core

• pytest-xdist starts its own job server with 8 parallel workers

In effect you now are running 9 parallel processes, one more than anticipated. This does not matter too much in this simplified example, but if you have big testsuites then you would like to avoid that.

The solution to this issue is this pytest-xdist plugin, which makes the pytest-xdist workers communicate with the GNU make jobserver to avoid exceeding the total limit.

Simply pip install pytest-xdist-gnumake, and the plugin should be picked up automatically by pytest-xdist.

The GNU jobserver uses the local file system (a fifo in /tmp), so this will not work with remote xdist workers.

Older make versions relied on passing open file descriptors to child process, but pytest-xdist will always close those. You need GNU make >= 4.4 in order for the communication with the jobserver to work, that is, a version of make that uses the fifo jobserver. See https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html for details.

If your make version is too old then the plugin will print:

$ pytest --numprocesses=8 my_app
pytest-xdist-gnumake using dummy job server: no job server found

and tests will run as if the plugin was not installed. Otherwise, it will print:

$ pytest --numprocesses=8 my_app
pytest-xdist-gnumake using gnu make jobserver client

• GNU Make https://www.gnu.org/software/make/
• https://pytest.org and https://github.com/pytest-dev/pytest-xdist to parallelize Python testsuites
• https://github.com/milahu/gnumake-tokenpool to communicate with GNU Make