Add in flight request metrics

This tracks CPU and DB usage while requests are in flight, rather than
when we write the response.

synapse/http/request_metrics.py

@@ -98,14 +98,76 @@ response_size = metrics.register_counter(
     "response_size", labels=["method", "servlet", "tag"]
 )
 
+# In flight metrics are incremented while the requests are in flight, rather
+# than when the response was written.
+
+in_flight_requests_ru_utime = metrics.register_counter(
+    "in_flight_requests_ru_utime_seconds", labels=["method", "servlet"],
+)
+
+in_flight_requests_ru_stime = metrics.register_counter(
+    "in_flight_requests_ru_stime_seconds", labels=["method", "servlet"],
+)
+
+in_flight_requests_db_txn_count = metrics.register_counter(
+    "in_flight_requests_db_txn_count", labels=["method", "servlet"],
+)
+
+# seconds spent waiting for db txns, excluding scheduling time, when processing
+# this request
+in_flight_requests_db_txn_duration = metrics.register_counter(
+    "in_flight_requests_db_txn_duration_seconds", labels=["method", "servlet"],
+)
+
+# seconds spent waiting for a db connection, when processing this request
+in_flight_requests_db_sched_duration = metrics.register_counter(
+    "in_flight_requests_db_sched_duration_seconds", labels=["method", "servlet"]
+)
+
+_in_flight_requests_count = metrics.register_gauge(
+    "in_flight_requests_count", labels=["method", "servlet"]
+)
+
+
+# The set of all in flight requests.
+_in_flight_requests = set()
+
+
+def _collect_in_flight():
+    """Called just before metrics are collected, so we use it to update all
+    the in flight request metrics
+    """
+
+    # Map from (method, name) -> int, the number of in flight requests of that
+    # type
+    counts = {}
+
+    for rm in _in_flight_requests:
+        rm.update_metrics()
+        key = (rm.method, rm.name,)
+        counts[key] = counts.get(key, 0) + 1
+
+    for (method, name), count in counts.iteritems():
+        _in_flight_requests_count.set(count, method, name)
+
+
+metrics.register_collector(_collect_in_flight)
+
 
 class RequestMetrics(object):
-    def start(self, time_msec, name):
+    def start(self, time_msec, name, method):
         self.start = time_msec
         self.start_context = LoggingContext.current_context()
         self.name = name
+        self.method = method
+
+        self._request_stats = _RequestStats.from_context(self.start_context)
+
+        _in_flight_requests.add(self)
 
     def stop(self, time_msec, request):
+        _in_flight_requests.discard(self)
+
         context = LoggingContext.current_context()
 
         tag = ""
@@ -147,3 +209,91 @@ class RequestMetrics(object):
         )
 
         response_size.inc_by(request.sentLength, request.method, self.name, tag)
+
+        # We always call this at the end to ensure that we update the metrics
+        # regardless of whether a call to /metrics while the request was in
+        # flight.
+        self.update_metrics()
+
+    def update_metrics(self):
+        """Updates the in flight metrics with values from this request.
+        """
+
+        diff = self._request_stats.update()
+
+        in_flight_requests_ru_utime.inc_by(
+            diff.ru_utime, self.method, self.name,
+        )
+
+        in_flight_requests_ru_stime.inc_by(
+            diff.ru_stime, self.method, self.name,
+        )
+
+        in_flight_requests_db_txn_count.inc_by(
+            diff.db_txn_count, self.method, self.name,
+        )
+
+        in_flight_requests_db_txn_duration.inc_by(
+            diff.db_txn_duration_ms / 1000., self.method, self.name,
+        )
+
+        in_flight_requests_db_sched_duration.inc_by(
+            diff.db_sched_duration_ms / 1000., self.method, self.name,
+        )
+
+
+class _RequestStats(object):
+    """Keeps tracks of various metrics for an in flight request.
+    """
+
+    __slots__ = [
+        "context", "ru_utime", "ru_stime",
+        "db_txn_count", "db_txn_duration_ms", "db_sched_duration_ms",
+    ]
+
+    def __init__(self, context, ru_utime, ru_stime, db_txn_count,
+                 db_txn_duration_ms, db_sched_duration_ms):
+        self.context = context
+        self.ru_utime = ru_utime
+        self.ru_stime = ru_stime
+        self.db_txn_count = db_txn_count
+        self.db_txn_duration_ms = db_txn_duration_ms
+        self.db_sched_duration_ms = db_sched_duration_ms
+
+    @staticmethod
+    def from_context(context):
+        ru_utime, ru_stime = context.get_resource_usage()
+
+        return _RequestStats(
+            context,
+            ru_utime, ru_stime,
+            context.db_txn_count,
+            context.db_txn_duration_ms,
+            context.db_sched_duration_ms,
+        )
+
+    def update(self):
+        """Updates the current values and returns the difference between the
+        old and new values.
+
+        Returns:
+            _RequestStats: The difference between the old and new values
+        """
+        new = _RequestStats.from_context(self.context)
+
+        diff = _RequestStats(
+            self.context,
+            new.ru_utime - self.ru_utime,
+            new.ru_stime - self.ru_stime,
+            new.db_txn_count - self.db_txn_count,
+            new.db_txn_duration_ms - self.db_txn_duration_ms,
+            new.db_sched_duration_ms - self.db_sched_duration_ms,
+        )
+
+        self.ru_utime = new.ru_utime
+        self.ru_stime = new.ru_stime
+        self.db_txn_count = new.db_txn_count
+        self.db_txn_duration_ms = new.db_txn_duration_ms
+        self.db_sched_duration_ms = new.db_sched_duration_ms
+
+        return diff

synapse/http/site.py

@@ -85,7 +85,9 @@ class SynapseRequest(Request):
     def _started_processing(self, servlet_name):
         self.start_time = int(time.time() * 1000)
         self.request_metrics = RequestMetrics()
-        self.request_metrics.start(self.start_time, name=servlet_name)
+        self.request_metrics.start(
+            self.start_time, name=servlet_name, method=self.method,
+        )
 
         self.site.access_logger.info(
             "%s - %s - Received request: %s %s",