Limit default MAX_CONCURRENCY to 4.

.travis.yml

@@ -3,8 +3,6 @@ language: python
 notifications:
   irc: "chat.freenode.net#pil"
 
-env: MAX_CONCURRENCY=4
-
 # Run slow PyPy* first, to give them a headstart and reduce waiting time.
 # Run latest 3.x and 2.x next, to get quick compatibility results.
 # Then run the remainder.

docs/installation.rst

@@ -97,11 +97,9 @@ Build Options
 -------------
 
 * Environment Variable: ``MAX_CONCURRENCY=n``. By default, Pillow will
-  use multiprocessing to build the extension in parallel. This may not
+  use multiprocessing to build the extension on all available CPU,
-  be ideal for machines that report a large number of cores compared
+  but not more than 4. Setting ``MAX_CONCURRENCY`` to 1 will disable
-  to the actual processor power. Set ``MAX_CONCURRENCY`` to 1 to disable
+  parallel building.
-  parallel building, or to a larger number to limit to that number of
-  parallel tasks.
 
 * Build flags: ``--disable-zlib``, ``--disable-jpeg``,
   ``--disable-tiff``, ``--disable-freetype``, ``--disable-tcl``,

mp_compile.py

@@ -6,8 +6,8 @@ from distutils.ccompiler import CCompiler
 import os, sys
 
 try:
-    MAX_PROCS = int(os.environ.get('MAX_CONCURRENCY', cpu_count()))
+    MAX_PROCS = int(os.environ.get('MAX_CONCURRENCY', min(4, cpu_count())))
-except:
+except NotImplementedError:
     MAX_PROCS = None