Configuration
Free-space threshold
One ini option sets how much free space /dev/shm needs before the plugin uses it:
The default is 1.
Set it above your suite's peak usage, which you can watch with df -h /dev/shm during a run.
Override it for one run with -o shm_min_free_gib=8.
pytest 9's native [tool.pytest] table accepts the number as well.
Turning it off
Export TMPDIR to the directory you want, or pass -o shm_min_free_gib=inf.
-p no:shm works too, but pytest then warns about the unknown shm_min_free_gib option if you configured it, and --strict-config makes that an error.
Measuring the difference
Compare a plain pytest run with pytest -o shm_min_free_gib=inf, which keeps the plugin off.
--durations 20 shows which tests gained the most.
Plugin autoloading
pytest loads the plugin through its pytest11 entry point.
With PYTEST_DISABLE_PLUGIN_AUTOLOAD set, pass -p shm to load it.