diff -r 3bb2a9f25fe9 -r 3c9691728237 mercurial/localrepo.py
--- a/mercurial/localrepo.py	Thu Sep 21 05:54:34 2017 -0700
+++ b/mercurial/localrepo.py	Wed Sep 13 17:26:26 2017 +0000
@@ -604,6 +604,10 @@
         chainspan = self.ui.configbytes('experimental', 'maxdeltachainspan', -1)
         if 0 <= chainspan:
             self.svfs.options['maxdeltachainspan'] = chainspan
+        mmapindexthreshold = self.ui.configbytes('experimental',
+                                                 'mmapindexthreshold', None)
+        if mmapindexthreshold is not None:
+            self.svfs.options['mmapindexthreshold'] = mmapindexthreshold
 
         for r in self.requirements:
             if r.startswith('exp-compression-'):