diff --git a/gclient_scm.py b/gclient_scm.py
index 26845b828..9ccc76f21 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -116,6 +116,9 @@ class SCMWrapper(object):
 
   This is the abstraction layer to bind to different SCM.
   """
+  nag_timer = 30
+  nag_max = 3
+
   def __init__(self, url=None, root_dir=None, relpath=None):
     self.url = url
     self._root_dir = root_dir
@@ -195,6 +198,8 @@ class GitWrapper(SCMWrapper):
     gclient_utils.CheckCallAndFilter(
         ['git', 'diff', merge_base],
         cwd=self.checkout_path,
+        nag_timer=self.nag_timer,
+        nag_max=self.nag_max,
         filter_fn=GitDiffFilterer(self.relpath).Filter)
 
   def UpdateSubmoduleConfig(self):
@@ -208,6 +213,8 @@ class GitWrapper(SCMWrapper):
     cmd4 = ['git', 'config', 'fetch.recurseSubmodules', 'false']
     kwargs = {'cwd': self.checkout_path,
               'print_stdout': False,
+              'nag_timer': self.nag_timer,
+              'nag_max': self.nag_max,
               'filter_fn': lambda x: None}
     try:
       gclient_utils.CheckCallAndFilter(cmd, **kwargs)
@@ -852,6 +859,8 @@ class GitWrapper(SCMWrapper):
     return subprocess2.check_output(
         ['git'] + args,
         stderr=subprocess2.PIPE,
+        nag_timer=self.nag_timer,
+        nag_max=self.nag_max,
         cwd=self.checkout_path).strip()
 
   def _UpdateBranchHeads(self, options, fetch=False):
@@ -879,6 +888,8 @@ class GitWrapper(SCMWrapper):
   def _Run(self, args, options, **kwargs):
     kwargs.setdefault('cwd', self.checkout_path)
     kwargs.setdefault('print_stdout', True)
+    kwargs.setdefault('nag_timer', self.nag_timer)
+    kwargs.setdefault('nag_max', self.nag_max)
     stdout = kwargs.get('stdout', sys.stdout)
     stdout.write('\n________ running \'git %s\' in \'%s\'\n' % (
                  ' '.join(args), kwargs['cwd']))
@@ -928,6 +939,8 @@ class SVNWrapper(SCMWrapper):
         ['svn', 'diff', '-x', '--ignore-eol-style'] + args,
         cwd=self.checkout_path,
         print_stdout=False,
+        nag_timer=self.nag_timer,
+        nag_max=self.nag_max,
         filter_fn=SvnDiffFilterer(self.relpath).Filter)
 
   def update(self, options, args, file_list):
@@ -1225,6 +1238,8 @@ class SVNWrapper(SCMWrapper):
   def _Run(self, args, options, **kwargs):
     """Runs a commands that goes to stdout."""
     kwargs.setdefault('cwd', self.checkout_path)
+    kwargs.setdefault('nag_timer', self.nag_timer)
+    kwargs.setdefault('nag_max', self.nag_max)
     gclient_utils.CheckCallAndFilterAndHeader(['svn'] + args,
         always=options.verbose, **kwargs)
 
diff --git a/gclient_utils.py b/gclient_utils.py
index bf9803e86..d0fa201b2 100644
--- a/gclient_utils.py
+++ b/gclient_utils.py
@@ -375,7 +375,7 @@ class GClientChildren(object):
 
 def CheckCallAndFilter(args, stdout=None, filter_fn=None,
                        print_stdout=None, call_filter_on_first_line=False,
-                       **kwargs):
+                       nag_timer=None, nag_max=None, **kwargs):
   """Runs a command and calls back a filter function if needed.
 
   Accepts all subprocess2.Popen() parameters plus:
@@ -399,6 +399,21 @@ def CheckCallAndFilter(args, stdout=None, filter_fn=None,
   # Do a flush of stdout before we begin reading from the subprocess2's stdout
   stdout.flush()
 
+  nag = None
+  if nag_timer:
+    # Hack thread.index to force correct annotation.
+    index = getattr(threading.currentThread(), 'index', 0)
+    def _nag_cb(elapsed):
+      setattr(threading.currentThread(), 'index', index)
+      stdout.write('  No output for %.0f seconds from command:\n' % elapsed)
+      stdout.write('    %s\n' % kid.cmd_str)
+      if (nag_max and
+          int('%.0f' % (elapsed / nag_timer)) >= nag_max):
+        stdout.write('  ... killing it!\n')
+        kid.kill()
+    nag = subprocess2.NagTimer(nag_timer, _nag_cb)
+    nag.start()
+
   # Also, we need to forward stdout to prevent weird re-ordering of output.
   # This has to be done on a per byte basis to make sure it is not buffered:
   # normally buffering is done for each line, but if svn requests input, no
@@ -406,6 +421,8 @@ def CheckCallAndFilter(args, stdout=None, filter_fn=None,
   try:
     in_byte = kid.stdout.read(1)
     if in_byte:
+      if nag:
+        nag.event()
       if call_filter_on_first_line:
         filter_fn(None)
       in_line = ''
@@ -422,6 +439,8 @@ def CheckCallAndFilter(args, stdout=None, filter_fn=None,
           filter_fn(in_line)
           in_line = ''
         in_byte = kid.stdout.read(1)
+        if in_byte and nag:
+          nag.event()
       # Flush the rest of buffered output. This is only an issue with
       # stdout/stderr not ending with a \n.
       if len(in_line):
@@ -435,6 +454,9 @@ def CheckCallAndFilter(args, stdout=None, filter_fn=None,
   except KeyboardInterrupt:
     print >> sys.stderr, 'Failed while running "%s"' % ' '.join(args)
     raise
+  finally:
+    if nag:
+      nag.cancel()
 
   if rv:
     raise subprocess2.CalledProcessError(
diff --git a/subprocess2.py b/subprocess2.py
index e81798613..ac44555dc 100644
--- a/subprocess2.py
+++ b/subprocess2.py
@@ -132,6 +132,42 @@ def get_english_env(env):
   return env
 
 
+class NagTimer(object):
+  """
+  Triggers a callback when a time interval passes without an event being fired.
+
+  For example, the event could be receiving terminal output from a subprocess;
+  and the callback could print a warning to stderr that the subprocess appeared
+  to be hung.
+  """
+  def __init__(self, interval, cb):
+    self.interval = interval
+    self.cb = cb
+    self.timer = threading.Timer(self.interval, self.fn)
+    self.last_output = self.previous_last_output = 0
+
+  def start(self):
+    self.last_output = self.previous_last_output = time.time()
+    self.timer.start()
+
+  def event(self):
+    self.last_output = time.time()
+
+  def fn(self):
+    now = time.time()
+    if self.last_output == self.previous_last_output:
+      self.cb(now - self.previous_last_output)
+    # Use 0.1 fudge factor, just in case
+    #   (self.last_output - now) is very close to zero.
+    sleep_time = (self.last_output - now - 0.1) % self.interval
+    self.previous_last_output = self.last_output
+    self.timer = threading.Timer(sleep_time + 0.1, self.fn)
+    self.timer.start()
+
+  def cancel(self):
+    self.timer.cancel()
+
+
 class Popen(subprocess.Popen):
   """Wraps subprocess.Popen() with various workarounds.
 
@@ -192,6 +228,7 @@ class Popen(subprocess.Popen):
     self.start = time.time()
     self.timeout = None
     self.nag_timer = None
+    self.nag_max = None
     self.shell = kwargs.get('shell', None)
     # Silence pylint on MacOSX
     self.returncode = None
@@ -230,8 +267,7 @@ class Popen(subprocess.Popen):
     # because of memory exhaustion.
     queue = Queue.Queue()
     done = threading.Event()
-    timer = []
-    last_output = [time.time()] * 2
+    nag = None
 
     def write_stdin():
       try:
@@ -253,28 +289,12 @@ class Popen(subprocess.Popen):
           data = pipe.read(1)
           if not data:
             break
-          last_output[0] = time.time()
+          if nag:
+            nag.event()
           queue.put((name, data))
       finally:
         queue.put(name)
 
-    def nag_fn():
-      now = time.time()
-      if done.is_set():
-        return
-      if last_output[0] == last_output[1]:
-        logging.warn('  No output for %.0f seconds from command:' % (
-            now - last_output[1]))
-        logging.warn('    %s' % self.cmd_str)
-      # Use 0.1 fudge factor in case:
-      #   now ~= last_output[0] + self.nag_timer
-      sleep_time = self.nag_timer + last_output[0] - now - 0.1
-      while sleep_time < 0:
-        sleep_time += self.nag_timer
-      last_output[1] = last_output[0]
-      timer[0] = threading.Timer(sleep_time, nag_fn)
-      timer[0].start()
-
     def timeout_fn():
       try:
         done.wait(self.timeout)
@@ -313,8 +333,15 @@ class Popen(subprocess.Popen):
       t.start()
 
     if self.nag_timer:
-      timer.append(threading.Timer(self.nag_timer, nag_fn))
-      timer[0].start()
+      def _nag_cb(elapsed):
+        logging.warn('  No output for %.0f seconds from command:' % elapsed)
+        logging.warn('    %s' % self.cmd_str)
+        if (self.nag_max and
+            int('%.0f' % (elapsed / self.nag_timer)) >= self.nag_max):
+          queue.put('timeout')
+          done.set()  # Must do this so that timeout thread stops waiting.
+      nag = NagTimer(self.nag_timer, _nag_cb)
+      nag.start()
 
     timed_out = False
     try:
@@ -327,20 +354,22 @@ class Popen(subprocess.Popen):
           self.stderr_cb(item[1])
         else:
           # A thread terminated.
-          threads[item].join()
-          del threads[item]
+          if item in threads:
+            threads[item].join()
+            del threads[item]
           if item == 'wait':
             # Terminate the timeout thread if necessary.
             done.set()
           elif item == 'timeout' and not timed_out and self.poll() is None:
-            logging.debug('Timed out after %fs: killing' % self.timeout)
+            logging.debug('Timed out after %.0fs: killing' % (
+                time.time() - self.start))
             self.kill()
             timed_out = True
     finally:
       # Stop the threads.
       done.set()
-      if timer:
-        timer[0].cancel()
+      if nag:
+        nag.cancel()
       if 'wait' in threads:
         # Accelerate things, otherwise it would hang until the child process is
         # done.
@@ -353,7 +382,8 @@ class Popen(subprocess.Popen):
         self.returncode = TIMED_OUT
 
   # pylint: disable=W0221,W0622
-  def communicate(self, input=None, timeout=None, nag_timer=None):
+  def communicate(self, input=None, timeout=None, nag_timer=None,
+                  nag_max=None):
     """Adds timeout and callbacks support.
 
     Returns (stdout, stderr) like subprocess.Popen().communicate().
@@ -365,6 +395,7 @@ class Popen(subprocess.Popen):
     """
     self.timeout = timeout
     self.nag_timer = nag_timer
+    self.nag_max = nag_max
     if (not self.timeout and not self.nag_timer and
         not self.stdout_cb and not self.stderr_cb):
       return super(Popen, self).communicate(input)
@@ -393,7 +424,7 @@ class Popen(subprocess.Popen):
     return (stdout, stderr)
 
 
-def communicate(args, timeout=None, nag_timer=None, **kwargs):
+def communicate(args, timeout=None, nag_timer=None, nag_max=None, **kwargs):
   """Wraps subprocess.Popen().communicate() and add timeout support.
 
   Returns ((stdout, stderr), returncode).
diff --git a/tests/gclient_scm_test.py b/tests/gclient_scm_test.py
index f9a29bbfa..3640a3e25 100755
--- a/tests/gclient_scm_test.py
+++ b/tests/gclient_scm_test.py
@@ -107,6 +107,8 @@ class SVNWrapperTestCase(BaseTestCase):
         'RunCommand',
         'cleanup',
         'diff',
+        'nag_max',
+        'nag_timer',
         'pack',
         'relpath',
         'revert',
@@ -496,6 +498,8 @@ class SVNWrapperTestCase(BaseTestCase):
     gclient_scm.gclient_utils.CheckCallAndFilterAndHeader(
         ['svn', 'checkout', '--depth', 'empty', self.url, self.base_path],
         always=True,
+        nag_max=3,
+        nag_timer=30,
         cwd=self.root_dir)
     gclient_scm.scm.SVN.RunAndGetFileList(
         options.verbose,
@@ -530,7 +534,7 @@ class SVNWrapperTestCase(BaseTestCase):
     files_list = self.mox.CreateMockAnything()
     gclient_scm.gclient_utils.CheckCallAndFilterAndHeader(
         ['svn', 'export', join(self.url, 'DEPS'), join(self.base_path, 'DEPS')],
-        always=True, cwd=self.root_dir)
+        nag_timer=30, nag_max=3, always=True, cwd=self.root_dir)
 
     self.mox.ReplayAll()
     scm = self._scm_wrapper(url=self.url, root_dir=self.root_dir,
@@ -563,6 +567,8 @@ class SVNWrapperTestCase(BaseTestCase):
     gclient_scm.gclient_utils.CheckCallAndFilterAndHeader(
         ['svn', 'checkout', '--depth', 'empty', self.url, self.base_path],
         always=True,
+        nag_max=3,
+        nag_timer=30,
         cwd=self.root_dir)
     gclient_scm.scm.SVN.RunAndGetFileList(
         options.verbose,
@@ -787,6 +793,8 @@ class ManagedGitWrapperTestCase(BaseGitWrapperTestCase):
         'RunCommand',
         'cleanup',
         'diff',
+        'nag_max',
+        'nag_timer',
         'pack',
         'UpdateSubmoduleConfig',
         'relpath',
diff --git a/tests/subprocess2_test.py b/tests/subprocess2_test.py
index 62034adf5..dc020f2b8 100755
--- a/tests/subprocess2_test.py
+++ b/tests/subprocess2_test.py
@@ -78,7 +78,7 @@ class DefaultsTest(auto_stub.TestCase):
         results['args'] = args
       @staticmethod
       # pylint: disable=W0622
-      def communicate(input=None, timeout=None, nag_timer=None):
+      def communicate(input=None, timeout=None, nag_max=None, nag_timer=None):
         return None, None
     self.mock(subprocess2, 'Popen', fake_Popen)
     return results