diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -599,6 +599,8 @@
         chainspan = self.ui.configbytes('experimental', 'maxdeltachainspan', -1)
         if 0 <= chainspan:
             self.svfs.options['maxdeltachainspan'] = chainspan
+        self.svfs.options['mmapindex'] = self.ui.configbool('experimental',
+                                                            'mmapindex')
 
         for r in self.requirements:
             if r.startswith('exp-compression-'):
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -19,6 +19,7 @@
 import hashlib
 import os
 import struct
+import sys
 import zlib
 
 # import stuff from node for others to import from revlog
@@ -300,6 +301,7 @@
         self._nodepos = None
         self._compengine = 'zlib'
         self._maxdeltachainspan = -1
+        self._mmapindex = False
 
         v = REVLOG_DEFAULT_VERSION
         opts = getattr(opener, 'options', None)
@@ -323,6 +325,8 @@
                 self._compengine = opts['compengine']
             if 'maxdeltachainspan' in opts:
                 self._maxdeltachainspan = opts['maxdeltachainspan']
+            if sys.version_info >= (2, 7) and 'mmapindex' in opts:
+                self._mmapindex = opts['mmapindex']
 
         if self._chunkcachesize <= 0:
             raise RevlogError(_('revlog chunk cache size %r is not greater '
@@ -335,7 +339,10 @@
         self._initempty = True
         try:
             f = self.opener(self.indexfile)
-            indexdata = f.read()
+            if self._mmapindex:
+                indexdata = util.buffer(util.mmapread(f))
+            else:
+                indexdata = f.read()
             f.close()
             if len(indexdata) > 0:
                 v = versionformat_unpack(indexdata[:4])[0]