--- a/mercurial/changelog.py	Thu Sep 21 05:54:34 2017 -0700
+++ b/mercurial/changelog.py	Wed Sep 13 17:26:26 2017 +0000
@@ -277,7 +277,7 @@
 
         datafile = '00changelog.d'
         revlog.revlog.__init__(self, opener, indexfile, datafile=datafile,
-                               checkambig=True)
+                               checkambig=True, mmaplargeindex=True)
 
         if self._initempty:
             # changelogs don't benefit from generaldelta