2 files changed, 14 insertions, 2 deletions
diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index 2265e6e8d6..e0cfb7d08f 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -146,10 +146,15 @@ def runUntilCurrentTimer(func):
             num_pending += 1
 
         num_pending += len(reactor.threadCallQueue)
-
         start = time.time() * 1000
         ret = func(*args, **kwargs)
         end = time.time() * 1000
+
+        # record the amount of wallclock time spent running pending calls.
+        # This is a proxy for the actual amount of time between reactor polls,
+        # since about 25% of time is actually spent running things triggered by
+        # I/O events, but that is harder to capture without rewriting half the
+        # reactor.
         tick_time.inc_by(end - start)
         pending_calls_metric.inc_by(num_pending)
 
diff --git a/synapse/metrics/metric.py b/synapse/metrics/metric.py
index f480aae614..1e783e5ff4 100644
--- a/synapse/metrics/metric.py
+++ b/synapse/metrics/metric.py
@@ -15,6 +15,9 @@
 
 
 from itertools import chain
+import logging
+
+logger = logging.getLogger(__name__)
 
 
 def flatten(items):
@@ -153,7 +156,11 @@ class CallbackMetric(BaseMetric):
         self.callback = callback
 
     def render(self):
-        value = self.callback()
+        try:
+            value = self.callback()
+        except Exception:
+            logger.exception("Failed to render %s", self.name)
+            return ["# FAILED to render " + self.name]
 
         if self.is_scalar():
             return list(self._render_for_labels([], value))