Manually run GC on reactor tick.

This also adds a metric for amount of time spent in GC.

synapse/metrics/__init__.py

@@ -22,6 +22,7 @@ import functools
 import os
 import stat
 import time
+import gc
 
 from twisted.internet import reactor
 
@@ -155,6 +156,7 @@ get_metrics_for("process").register_callback("fds", _process_fds, labels=["type"
 reactor_metrics = get_metrics_for("reactor")
 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")
 
 
 def runUntilCurrentTimer(func):
@@ -182,6 +184,18 @@ def runUntilCurrentTimer(func):
         end = time.time() * 1000
         tick_time.inc_by(end - start)
         pending_calls_metric.inc_by(num_pending)
+
+        threshold = gc.get_threshold()
+        counts = gc.get_count()
+
+        start = time.time() * 1000
+        for i in [2, 1, 0]:
+            if threshold[i] < counts[i]:
+                logger.info("Collecting gc %d", i)
+                gc.collect(i)
+        end = time.time() * 1000
+        gc_time.inc_by(end - start)
+
         return ret
 
     return f
@@ -196,5 +210,6 @@ try:
     # runUntilCurrent is called when we have pending calls. It is called once
     # per iteratation after fd polling.
     reactor.runUntilCurrent = runUntilCurrentTimer(reactor.runUntilCurrent)
+    gc.disable()
 except AttributeError:
     pass