Avoided running more test processes than necessary.

This reduces the time spent cloning databases.

Thanks Tim for the suggestion.

django/test/runner.py

@@ -453,7 +453,17 @@ class DiscoverRunner(object):
         suite = reorder_suite(suite, self.reorder_by, self.reverse)
 
         if self.parallel > 1:
-            suite = self.parallel_test_suite(suite, self.parallel, self.failfast)
+            parallel_suite = self.parallel_test_suite(suite, self.parallel, self.failfast)
+
+            # Since tests are distributed across processes on a per-TestCase
+            # basis, there's no need for more processes than TestCases.
+            parallel_units = len(parallel_suite.subsuites)
+            if self.parallel > parallel_units:
+                self.parallel = parallel_units
+
+            # If there's only one TestCase, parallelization isn't needed.
+            if self.parallel > 1:
+                suite = parallel_suite
 
         return suite
 

docs/ref/django-admin.txt

@@ -1274,6 +1274,10 @@ By default ``--parallel`` runs one process per core according to
 either by providing it as the option's value, e.g. ``--parallel=4``, or by
 setting the ``DJANGO_TEST_PROCESSES`` environment variable.
 
+Django distributes test cases — :class:`unittest.TestCase` subclasses — to
+subprocesses. If there are fewer test cases than configured processes, Django
+will reduce the number of processes accordingly.
+
 Each process gets its own database. You must ensure that different test cases
 don't access the same resources. For instance, test cases that touch the
 filesystem should create a temporary directory for their own use.