Overhead of pytest-xdist is often greater than time saved for parallelization

(I'm pretty sure this is a known issue, but I didn't see an open issue for it, and wanted there to be a canonical place to track it)

Example:

(cryptography) ~/p/cryptography (master) $ py.test tests/test_x509*.py
====================================================== test session starts =======================================================
platform darwin -- Python 2.7.10[pypy-4.0.1-final], pytest-2.8.7, py-1.4.31, pluggy-0.3.1
rootdir: /Users/alex_gaynor/projects/cryptography, inifile: tox.ini
plugins: hypothesis-2.0.0, xdist-1.14
collected 492 items

tests/test_x509.py .........................................................................................................................................................................................
tests/test_x509_crlbuilder.py ............................
tests/test_x509_ext.py .......................................................................................................................................................................................................................................................................
tests/test_x509_revokedcertbuilder.py ................

=================================================== 492 passed in 3.60 seconds ===================================================
(cryptography) ~/p/cryptography (master) $ py.test -n 2 tests/test_x509*.py
====================================================== test session starts =======================================================
platform darwin -- Python 2.7.10[pypy-4.0.1-final], pytest-2.8.7, py-1.4.31, pluggy-0.3.1
rootdir: /Users/alex_gaynor/projects/cryptography, inifile: tox.ini
plugins: hypothesis-2.0.0, xdist-1.14
gw0 [492] / gw1 [492]
scheduling tests via LoadScheduling
............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
=================================================== 492 passed in 5.00 seconds ===================================================
(cryptography) ~/p/cryptography (master) $ py.test -n 2 tests/test_x509*.py
====================================================== test session starts =======================================================
platform darwin -- Python 2.7.10[pypy-4.0.1-final], pytest-2.8.7, py-1.4.31, pluggy-0.3.1
rootdir: /Users/alex_gaynor/projects/cryptography, inifile: tox.ini
plugins: hypothesis-2.0.0, xdist-1.14
gw0 [492] / gw1 [492]
scheduling tests via LoadScheduling
............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
=================================================== 492 passed in 4.62 seconds ===================================================
(cryptography) ~/p/cryptography (master) $ py.test -n 4 tests/test_x509*.py
====================================================== test session starts =======================================================
platform darwin -- Python 2.7.10[pypy-4.0.1-final], pytest-2.8.7, py-1.4.31, pluggy-0.3.1
rootdir: /Users/alex_gaynor/projects/cryptography, inifile: tox.ini
plugins: hypothesis-2.0.0, xdist-1.14
gw0 [492] / gw1 [492] / gw2 [492] / gw3 [492]
scheduling tests via LoadScheduling
............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
=================================================== 492 passed in 6.23 seconds ===================================================
(cryptography) ~/p/cryptography (master) $ py.test -n 4 tests/test_x509*.py
====================================================== test session starts =======================================================
platform darwin -- Python 2.7.10[pypy-4.0.1-final], pytest-2.8.7, py-1.4.31, pluggy-0.3.1
rootdir: /Users/alex_gaynor/projects/cryptography, inifile: tox.ini
plugins: hypothesis-2.0.0, xdist-1.14
gw0 [492] / gw1 [492] / gw2 [492] / gw3 [492]
scheduling tests via LoadScheduling
............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
=================================================== 492 passed in 6.73 seconds ===================================================
(cryptography) ~/p/cryptography (master) $ py.test -n 4 tests/test_x509*.py
====================================================== test session starts =======================================================
platform darwin -- Python 2.7.10[pypy-4.0.1-final], pytest-2.8.7, py-1.4.31, pluggy-0.3.1
rootdir: /Users/alex_gaynor/projects/cryptography, inifile: tox.ini
plugins: hypothesis-2.0.0, xdist-1.14
gw0 [492] / gw1 [492] / gw2 [492] / gw3 [492]
scheduling tests via LoadScheduling
............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
=================================================== 492 passed in 6.53 seconds ===================================================

The issue provides benchmark commands for pytest and pytest-xdist using tests/test_x509*.py, but names no source file or failing test. Start by reproducing the serial and -n 2/-n 4 runs, then inspect pytest-xdist's parallel-test startup and scheduling behavior; completion criteria are not defined beyond addressing the observed overhead.