diff --git a/eventlet/debug.py b/eventlet/debug.py
index 50586b3..76528ed 100644
--- a/eventlet/debug.py
+++ b/eventlet/debug.py
@@ -134,10 +134,23 @@ def tpool_exceptions(state = False):
 
 def hub_blocking_detection(state = False, resolution = 1):
     """Toggles whether Eventlet makes an effort to detect blocking
-    behavior in other code.  It does this by setting a SIGALARM with a short 
-    timeout.
+    behavior in an application.
+
+    It does this by telling the kernel to raise a SIGALARM after a
+    short timeout, and clearing the timeout every time the hub
+    greenlet is resumed.  Therefore, any code that runs for a long
+    time without yielding to the hub will get interrupted by the
+    blocking detector (don't use it in production!).
+
+    The *resolution* argument governs how long the SIGALARM timeout
+    waits in seconds.  If on Python 2.6 or later, the implementation
+    uses :func:`signal.setitimer` and can be specified as a
+    floating-point value.  On 2.5 or earlier, 1 second is the minimum.
+    The shorter the resolution, the greater the chance of false
+    positives.
     """
     from eventlet import hubs
+    assert resolution > 0
     hubs.get_hub().debug_blocking = state
     hubs.get_hub().debug_blocking_resolution = resolution
     if(not state):
diff --git a/eventlet/hubs/hub.py b/eventlet/hubs/hub.py
index 333b3d3..c759d81 100644
--- a/eventlet/hubs/hub.py
+++ b/eventlet/hubs/hub.py
@@ -1,20 +1,23 @@
 import heapq
-import sys
+import math
 import traceback
-import warnings
 import signal
+import sys
+import warnings
 
-HAS_ITIMER = False
-alarm_func = signal.alarm
+arm_alarm = None
 if hasattr(signal, 'setitimer'):
-    HAS_ITIMER = True
+    def alarm_itimer(seconds):
+        signal.setitimer(signal.ITIMER_REAL, seconds)
+    arm_alarm = alarm_itimer
 else:
     try:
         import itimer
-        HAS_ITIMER = True
-        alarm_func = itimer.alarm
+        arm_alarm = itimer.alarm
     except ImportError:
-        pass
+        def alarm_signal(seconds):
+            signal.alarm(math.ceil(seconds))
+        arm_alarm = alarm_signal
 
 from eventlet.support import greenlets as greenlet, clear_sys_exc_info
 from eventlet.hubs import timer
@@ -57,7 +60,7 @@ class DebugListener(FdListener):
 
 def alarm_handler(signum, frame):
     import inspect
-    raise RuntimeError("ALARMED at" + str(inspect.getframeinfo(frame)))
+    raise RuntimeError("Blocking detector ALARMED at" + str(inspect.getframeinfo(frame)))
 
 
 class BaseHub(object):
@@ -91,8 +94,7 @@ class BaseHub(object):
         if tmp != alarm_handler:
             self._old_signal_handler = tmp
 
-        if HAS_ITIMER:
-            alarm_func(self.debug_blocking_resolution)
+        arm_alarm(self.debug_blocking_resolution)
 
     def block_detect_post(self):
         if (hasattr(self, "_old_signal_handler") and