diff -r e4ac5a899657 Lib/test/libregrtest/cmdline.py
--- a/Lib/test/libregrtest/cmdline.py	Wed Sep 30 22:06:51 2015 +0200
+++ b/Lib/test/libregrtest/cmdline.py	Wed Sep 30 22:36:23 2015 +0200
@@ -312,8 +312,6 @@ def _parse_args(args, **kwargs):
         if ns.use_mp <= 0:
             # Use all cores + extras for tests that like to sleep
             ns.use_mp = 2 + (os.cpu_count() or 1)
-        if ns.use_mp == 1:
-            ns.use_mp = None
     if ns.use:
         for a in ns.use:
             for r in a:
diff -r e4ac5a899657 Tools/buildbot/test.bat
--- a/Tools/buildbot/test.bat	Wed Sep 30 22:06:51 2015 +0200
+++ b/Tools/buildbot/test.bat	Wed Sep 30 22:36:23 2015 +0200
@@ -4,7 +4,7 @@ setlocal
 
 set here=%~dp0
 set rt_opts=-q -d
-set regrtest_args=
+set regrtest_args=-j1
 
 :CheckOpts
 if "%1"=="-x64" (set rt_opts=%rt_opts% %1) & shift & goto CheckOpts