--- a/mercurial/manifest.py	Thu Sep 21 05:54:34 2017 -0700
+++ b/mercurial/manifest.py	Wed Sep 13 17:26:26 2017 +0000
@@ -1231,7 +1231,8 @@
 
         super(manifestrevlog, self).__init__(opener, indexfile,
                                              # only root indexfile is cached
-                                             checkambig=not bool(dir))
+                                             checkambig=not bool(dir),
+                                             mmaplargeindex=True)
 
     @property
     def fulltextcache(self):