FYI, bench result of USABLE_FRACTION(n) = (n/2):

Report on Linux ip-10-0-1-249 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt25-1 (2016-03-06) x86_64
Total CPU cores: 2
+--------------+---------+------------+--------------+------------------------+
| Benchmark    | default | usable n/2 | Change       | Significance           |
+==============+=========+============+==============+========================+
| 2to3         | 5.18    | 5.17       | 1.00x faster | Not significant        |
+--------------+---------+------------+--------------+------------------------+
| chameleon_v2 | 4.03    | 3.87       | 1.04x faster | Significant (t=61.85)  |
+--------------+---------+------------+--------------+------------------------+
| django_v3    | 0.4     | 0.39       | 1.01x faster | Not significant        |
+--------------+---------+------------+--------------+------------------------+
| fastpickle   | 0.33    | 0.33       | 1.00x slower | Not significant        |
+--------------+---------+------------+--------------+------------------------+
| fastunpickle | 0.41    | 0.42       | 1.04x slower | Significant (t=-56.83) |
+--------------+---------+------------+--------------+------------------------+
| json_dump_v2 | 1.93    | 1.94       | 1.01x slower | Not significant        |
+--------------+---------+------------+--------------+------------------------+
| json_load    | 0.33    | 0.33       | 1.01x faster | Not significant        |
+--------------+---------+------------+--------------+------------------------+
| nbody        | 0.19    | 0.19       | 1.00x slower | Not significant        |
+--------------+---------+------------+--------------+------------------------+
| regex_v8     | 0.04    | 0.04       | 1.00x faster | Not significant        |
+--------------+---------+------------+--------------+------------------------+
| tornado_http | 0.19    | 0.2        | 1.01x slower | Not significant        |
+--------------+---------+------------+--------------+------------------------+


I've changed recommended usable fraction from 5/8~2/3 to 1/2~2/3.