diff --git a/mercurial/configitems.toml b/mercurial/configitems.toml
--- a/mercurial/configitems.toml
+++ b/mercurial/configitems.toml
@@ -2163,7 +2163,7 @@ experimental = true  # experimental as l
 [[items]]
 section = "storage"
 name = "revbranchcache.mmap"
-default = false
+default = true
 
 [[items]]
 section = "storage"