--- a/hgext/fsmonitor/__init__.py	Fri Oct 06 06:48:43 2017 -0700
+++ b/hgext/fsmonitor/__init__.py	Wed Oct 18 22:57:15 2017 +0200
@@ -18,6 +18,9 @@
 repository as necessary. You'll need to install Watchman from
 https://facebook.github.io/watchman/ and make sure it is in your PATH.
 
+fsmonitor is incompatible with the largefiles and eol extensions, and
+will disable itself if any of those are active.
+
 The following configuration options exist:
 
 ::
@@ -58,9 +61,22 @@
 that can outpace the IPC overhead of getting the result data for the full repo
 from Watchman. Defaults to false.
 
-fsmonitor is incompatible with the largefiles and eol extensions, and
-will disable itself if any of those are active.
+::
+
+    [fsmonitor]
+    warn_when_unused = (boolean)
+
+Whether to print a warning during certain operations when fsmonitor would be
+beneficial to performance but isn't enabled.
 
+::
+
+    [fsmonitor]
+    warn_update_file_count = (integer)
+
+If ``warn_when_unused`` is set and fsmonitor isn't enabled, a warning will
+be printed during working directory updates if this many files will be
+created.
 '''
 
 # Platforms Supported

--- a/mercurial/configitems.py	Fri Oct 06 06:48:43 2017 -0700
+++ b/mercurial/configitems.py	Wed Oct 18 22:57:15 2017 +0200
@@ -488,6 +488,12 @@
 coreconfigitem('format', 'usestore',
     default=True,
 )
+coreconfigitem('fsmonitor', 'warn_when_unused',
+    default=True,
+)
+coreconfigitem('fsmonitor', 'warn_update_file_count',
+    default=50000,
+)
 coreconfigitem('hooks', '.*',
     default=dynamicdefault,
     generic=True,

--- a/mercurial/merge.py	Fri Oct 06 06:48:43 2017 -0700
+++ b/mercurial/merge.py	Wed Oct 18 22:57:15 2017 +0200
@@ -25,6 +25,7 @@
 from . import (
     copies,
     error,
+    extensions,
     filemerge,
     match as matchmod,
     obsutil,
@@ -1943,6 +1944,38 @@
             # note that we're in the middle of an update
             repo.vfs.write('updatestate', p2.hex())
 
+        # Advertise fsmonitor when its presence could be useful.
+        #
+        # We only advertise when performing an update from an empty working
+        # directory. This typically only occurs during initial clone.
+        #
+        # We give users a mechanism to disable the warning in case it is
+        # annoying.
+        #
+        # We only allow on Linux and MacOS because that's where fsmonitor is
+        # considered stable.
+        fsmonitorwarning = repo.ui.configbool('fsmonitor', 'warn_when_unused')
+        fsmonitorthreshold = repo.ui.configint('fsmonitor',
+                                               'warn_update_file_count')
+        try:
+            extensions.find('fsmonitor')
+            fsmonitorenabled = repo.ui.config('fsmonitor', 'mode') != 'off'
+            # We intentionally don't look at whether fsmonitor has disabled
+            # itself because a) fsmonitor may have already printed a warning
+            # b) we only care about the config state here.
+        except KeyError:
+            fsmonitorenabled = False
+
+        if (fsmonitorwarning
+                and not fsmonitorenabled
+                and p1.node() == nullid
+                and len(actions['g']) >= fsmonitorthreshold
+                and pycompat.sysplatform.startswith(('linux', 'darwin'))):
+            repo.ui.warn(
+                _('(warning: large working directory being used without '
+                  'fsmonitor enabled; enable fsmonitor to improve performance; '
+                  'see "hg help -e fsmonitor")\n'))
+
         stats = applyupdates(repo, actions, wc, p2, overwrite, labels=labels)
         wc.flushall()
 

--- a/tests/hghave.py	Fri Oct 06 06:48:43 2017 -0700
+++ b/tests/hghave.py	Wed Oct 18 22:57:15 2017 +0200
@@ -559,6 +559,11 @@
     except ImportError:
         return False
 
+@check('linuxormacos', 'Linux or MacOS')
+def has_linuxormacos():
+    # This isn't a perfect test for MacOS. But it is sufficient for our needs.
+    return sys.platform.startswith(('linux', 'darwin'))
+
 @check("docker", "docker support")
 def has_docker():
     pat = br'A self-sufficient runtime for'

--- a/tests/test-clone.t	Fri Oct 06 06:48:43 2017 -0700
+++ b/tests/test-clone.t	Wed Oct 18 22:57:15 2017 +0200
@@ -1177,3 +1177,80 @@
 We should not have created a file named owned - if it exists, the
 attack succeeded.
   $ if test -f owned; then echo 'you got owned'; fi
+
+Cloning without fsmonitor enabled does not print a warning for small repos
+
+  $ hg clone a fsmonitor-default
+  updating to bookmark @ on branch stable
+  3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+Lower the warning threshold to simulate a large repo
+
+  $ cat >> $HGRCPATH << EOF
+  > [fsmonitor]
+  > warn_update_file_count = 2
+  > EOF
+
+We should see a warning about no fsmonitor on supported platforms
+
+#if linuxormacos no-fsmonitor
+  $ hg clone a nofsmonitor
+  updating to bookmark @ on branch stable
+  (warning: large working directory being used without fsmonitor enabled; enable fsmonitor to improve performance; see "hg help -e fsmonitor")
+  3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+#else
+  $ hg clone a nofsmonitor
+  updating to bookmark @ on branch stable
+  3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+#endif
+
+We should not see warning about fsmonitor when it is enabled
+
+#if fsmonitor
+  $ hg clone a fsmonitor-enabled
+  updating to bookmark @ on branch stable
+  3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+#endif
+
+We can disable the fsmonitor warning
+
+  $ hg --config fsmonitor.warn_when_unused=false clone a fsmonitor-disable-warning
+  updating to bookmark @ on branch stable
+  3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+Loaded fsmonitor but disabled in config should still print warning
+
+#if linuxormacos fsmonitor
+  $ hg --config fsmonitor.mode=off clone a fsmonitor-mode-off
+  updating to bookmark @ on branch stable
+  (warning: large working directory being used without fsmonitor enabled; enable fsmonitor to improve performance; see "hg help -e fsmonitor") (fsmonitor !)
+  3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+#endif
+
+Warning not printed if working directory isn't empty
+
+  $ hg -q clone a fsmonitor-update
+  (warning: large working directory being used without fsmonitor enabled; enable fsmonitor to improve performance; see "hg help -e fsmonitor") (?)
+  $ cd fsmonitor-update
+  $ hg up acb14030fe0a
+  1 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  (leaving bookmark @)
+  $ hg up cf0fe1914066
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+`hg update` from null revision also prints
+
+  $ hg up null
+  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+
+#if linuxormacos no-fsmonitor
+  $ hg up cf0fe1914066
+  (warning: large working directory being used without fsmonitor enabled; enable fsmonitor to improve performance; see "hg help -e fsmonitor")
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+#else
+  $ hg up cf0fe1914066
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+#endif
+
+  $ cd ..
+