Track DB scheduling delay per-request

For each request, track the amount of time spent waiting for a db
connection. This entails adding it to the LoggingContext and we may as well add
metrics for it while we are passing.

synapse/http/server.py

@@ -102,6 +102,10 @@ response_db_txn_duration = metrics.register_counter(
     ),
 )
 
+# seconds spent waiting for a db connection, when processing this request
+response_db_sched_duration = metrics.register_counter(
+    "response_db_sched_duration_seconds", labels=["method", "servlet", "tag"]
+)
 
 _next_request_id = 0
 
@@ -381,6 +385,9 @@ class RequestMetrics(object):
         response_db_txn_duration.inc_by(
             context.db_txn_duration_ms / 1000., request.method, self.name, tag
         )
+        response_db_sched_duration.inc_by(
+            context.db_sched_duration_ms / 1000., request.method, self.name, tag
+        )
 
 
 class RootRedirect(resource.Resource):

synapse/http/site.py

@@ -67,13 +67,14 @@ class SynapseRequest(Request):
             ru_utime, ru_stime = context.get_resource_usage()
             db_txn_count = context.db_txn_count
             db_txn_duration_ms = context.db_txn_duration_ms
+            db_sched_duration_ms = context.db_sched_duration_ms
         except Exception:
             ru_utime, ru_stime = (0, 0)
             db_txn_count, db_txn_duration_ms = (0, 0)
 
         self.site.access_logger.info(
             "%s - %s - {%s}"
-            " Processed request: %dms (%dms, %dms) (%dms/%d)"
+            " Processed request: %dms (%dms, %dms) (%dms/%dms/%d)"
             " %sB %s \"%s %s %s\" \"%s\"",
             self.getClientIP(),
             self.site.site_tag,
@@ -81,6 +82,7 @@ class SynapseRequest(Request):
             int(time.time() * 1000) - self.start_time,
             int(ru_utime * 1000),
             int(ru_stime * 1000),
+            db_sched_duration_ms,
             db_txn_duration_ms,
             int(db_txn_count),
             self.sentLength,

synapse/storage/_base.py

@@ -347,7 +347,9 @@ class SQLBaseStore(object):
 
         def inner_func(conn, *args, **kwargs):
             with LoggingContext("runWithConnection") as context:
-                sql_scheduling_timer.inc_by(time.time() * 1000 - start_time)
+                sched_duration_ms = time.time() * 1000 - start_time
+                sql_scheduling_timer.inc_by(sched_duration_ms)
+                current_context.add_database_scheduled(sched_duration_ms)
 
                 if self.database_engine.is_connection_closed(conn):
                     logger.debug("Reconnecting closed database connection")

synapse/util/logcontext.py

@@ -59,7 +59,8 @@ class LoggingContext(object):
 
     __slots__ = [
         "previous_context", "name", "ru_stime", "ru_utime",
-        "db_txn_count", "db_txn_duration_ms", "usage_start", "usage_end",
+        "db_txn_count", "db_txn_duration_ms", "db_sched_duration_ms",
+        "usage_start", "usage_end",
         "main_thread", "alive",
         "request", "tag",
     ]
@@ -86,6 +87,9 @@ class LoggingContext(object):
         def add_database_transaction(self, duration_ms):
             pass
 
+        def add_database_scheduled(self, sched_ms):
+            pass
+
         def __nonzero__(self):
             return False
 
@@ -101,6 +105,9 @@ class LoggingContext(object):
         # ms spent waiting for db txns, excluding scheduling time
         self.db_txn_duration_ms = 0
 
+        # ms spent waiting for db txns to be scheduled
+        self.db_sched_duration_ms = 0
+
         self.usage_start = None
         self.usage_end = None
         self.main_thread = threading.current_thread()
@@ -210,6 +217,15 @@ class LoggingContext(object):
         self.db_txn_count += 1
         self.db_txn_duration_ms += duration_ms
 
+    def add_database_scheduled(self, sched_ms):
+        """Record a use of the database pool
+
+        Args:
+            sched_ms (int): number of milliseconds it took us to get a
+                connection
+        """
+        self.db_sched_duration_ms += sched_ms
+
 
 class LoggingContextFilter(logging.Filter):
     """Logging filter that adds values from the current logging context to each

synapse/util/metrics.py

@@ -80,6 +80,11 @@ block_db_txn_duration = metrics.register_counter(
     ),
 )
 
+# seconds spent waiting for a db connection, in this block
+block_db_sched_duration = metrics.register_counter(
+    "block_db_sched_duration_seconds", labels=["block_name"],
+)
+
 
 def measure_func(name):
     def wrapper(func):
@@ -96,7 +101,9 @@ def measure_func(name):
 class Measure(object):
     __slots__ = [
         "clock", "name", "start_context", "start", "new_context", "ru_utime",
-        "ru_stime", "db_txn_count", "db_txn_duration_ms", "created_context"
+        "ru_stime",
+        "db_txn_count", "db_txn_duration_ms", "db_sched_duration_ms",
+        "created_context",
     ]
 
     def __init__(self, clock, name):
@@ -117,6 +124,7 @@ class Measure(object):
         self.ru_utime, self.ru_stime = self.start_context.get_resource_usage()
         self.db_txn_count = self.start_context.db_txn_count
         self.db_txn_duration_ms = self.start_context.db_txn_duration_ms
+        self.db_sched_duration_ms = self.start_context.db_sched_duration_ms
 
     def __exit__(self, exc_type, exc_val, exc_tb):
         if isinstance(exc_type, Exception) or not self.start_context:
@@ -149,6 +157,10 @@ class Measure(object):
             (context.db_txn_duration_ms - self.db_txn_duration_ms) / 1000.,
             self.name
         )
+        block_db_sched_duration.inc_by(
+            (context.db_sched_duration_ms - self.db_sched_duration_ms) / 1000.,
+            self.name
+        )
 
         if self.created_context:
             self.start_context.__exit__(exc_type, exc_val, exc_tb)