Add method for deciding how many concurrent make jobs to run.

2 files changed, 30 insertions, 0 deletions

diff --git a/morphlib/util.py b/morphlib/util.py
index 430fdc20..4558d8b8 100644
--- a/morphlib/util.py
+++ b/morphlib/util.py
@@ -44,3 +44,18 @@ def indent(string, spaces=4):
     lines = ['%*s%s' % (spaces, '', line) for line in lines]
     return '\n'.join(lines)
 
+
+
+def make_concurrency(cores=None):
+    '''Return the number of concurrent jobs for make.
+    
+    This will be given to make as the -j argument.
+
+    '''
+
+    n = multiprocessing.cpu_count() if cores is None else cores
+    # Experimental results (ref. Kinnison) says a factor of 1.5
+    # gives about the optimal result for build times, since much of
+    # builds are I/O bound, not CPU bound.
+    return max(int(n * 1.5 + 0.5), 1)
+
diff --git a/morphlib/util_tests.py b/morphlib/util_tests.py
index 6637f571..e65d4507 100644
--- a/morphlib/util_tests.py
+++ b/morphlib/util_tests.py
@@ -42,3 +42,18 @@ class IndentTests(unittest.TestCase):
         self.assertEqual(morphlib.util.indent('foo\nbar\n'), 
                          '    foo\n    bar')
 
+
+class MakeConcurrencyTests(unittest.TestCase):
+
+    def test_returns_2_for_1_core(self):
+        self.assertEqual(morphlib.util.make_concurrency(cores=1), 2)
+
+    def test_returns_3_for_2_cores(self):
+        self.assertEqual(morphlib.util.make_concurrency(cores=2), 3)
+
+    def test_returns_5_for_3_cores(self):
+        self.assertEqual(morphlib.util.make_concurrency(cores=3), 5)
+
+    def test_returns_6_for_4_cores(self):
+        self.assertEqual(morphlib.util.make_concurrency(cores=4), 6)
+