diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index ef14bcd84..b29cec3de 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -154,6 +154,7 @@ tick_time = reactor_metrics.register_distribution("tick_time")
 pending_calls_metric = reactor_metrics.register_distribution("pending_calls")
 
 gc_time = reactor_metrics.register_distribution("gc_time", labels=["gen"])
+gc_unreachable = reactor_metrics.register_counter("gc_unreachable", labels=["gen"])
 
 
 def runUntilCurrentTimer(func):
@@ -186,15 +187,16 @@ def runUntilCurrentTimer(func):
         # one if necessary.
         threshold = gc.get_threshold()
         counts = gc.get_count()
-        for i in (0, 1, 2):
+        for i in (2, 1, 0):
             if threshold[i] < counts[i]:
                 logger.info("Collecting gc %d", i)
 
                 start = time.time() * 1000
-                gc.collect(i)
+                unreachable = gc.collect(i)
                 end = time.time() * 1000
 
                 gc_time.inc_by(end - start, i)
+                gc_unreachable.inc_by(unreachable, i)
 
         return ret