test item scheduling does not schedule N items to N workers

I may just be misunderstanding how xdist works, but I can't seem to get the number of workers I requested to all accept tasks in parallel:

$ py.test -v -n 8
================================ test session starts =================================
platform darwin -- Python 3.5.1, pytest-2.8.5, py-1.4.31, pluggy-0.3.1 -- .../pytest-xdist-testcase/venv/bin/python3
cachedir: .cache
rootdir: .../pytest-xdist-testcase, inifile: 
plugins: xdist-1.13.1
[gw0] darwin Python 3.5.1 cwd: .../pytest-xdist-testcase
[gw1] darwin Python 3.5.1 cwd: .../pytest-xdist-testcase
[gw2] darwin Python 3.5.1 cwd: .../pytest-xdist-testcase
[gw3] darwin Python 3.5.1 cwd: .../pytest-xdist-testcase
[gw4] darwin Python 3.5.1 cwd: .../pytest-xdist-testcase
[gw5] darwin Python 3.5.1 cwd: .../pytest-xdist-testcase
[gw6] darwin Python 3.5.1 cwd: .../pytest-xdist-testcase
[gw7] darwin Python 3.5.1 cwd: .../pytest-xdist-testcase
[gw0] Python 3.5.1 (default, Dec  7 2015, 21:59:10)  -- [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)]
[gw1] Python 3.5.1 (default, Dec  7 2015, 21:59:10)  -- [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)]
[gw2] Python 3.5.1 (default, Dec  7 2015, 21:59:10)  -- [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)]
[gw3] Python 3.5.1 (default, Dec  7 2015, 21:59:10)  -- [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)]
[gw4] Python 3.5.1 (default, Dec  7 2015, 21:59:10)  -- [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)]
[gw5] Python 3.5.1 (default, Dec  7 2015, 21:59:10)  -- [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)]
[gw6] Python 3.5.1 (default, Dec  7 2015, 21:59:10)  -- [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)]
[gw7] Python 3.5.1 (default, Dec  7 2015, 21:59:10)  -- [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)]
gw0 [8] / gw1 [8] / gw2 [8] / gw3 [8] / gw4 [8] / gw5 [8] / gw6 [8] / gw7 [8]
scheduling tests via LoadScheduling

test_tests.py::test_1[0] 
test_tests.py::test_1[2] 
test_tests.py::test_2[0] 
test_tests.py::test_2[2] 
[gw0] PASSED test_tests.py::test_1[2] 
[gw7] PASSED test_tests.py::test_1[0] 
[gw2] PASSED test_tests.py::test_2[0] 
[gw4] PASSED test_tests.py::test_2[2] 
test_tests.py::test_2[1] 
test_tests.py::test_1[3] 
test_tests.py::test_1[1] 
test_tests.py::test_2[3] 
[gw7] PASSED test_tests.py::test_1[1] 
[gw0] PASSED test_tests.py::test_1[3] 
[gw4] PASSED test_tests.py::test_2[3] 
[gw2] PASSED test_tests.py::test_2[1] 

============================= 8 passed in 64.12 seconds ==============================

Watching the output as it comes, it looks like the second block of 4 tests doesn't start running until the first block of 4 completes.

Am I misunderstanding something? Why don't all 8 workers start running tests at the same time? Why doesn't gw1, for example, get any tests?

Reproduce the report with py.test -v -n 8 and the eight parametrized tests shown in the issue. Start by tracing the LoadScheduling behavior and worker assignments; done means explaining or correcting why only four tests run initially and why gw1 receives no test.