Track number of hosts affected by the rate limiter

changelog.d/13534.misc

@@ -0,0 +1 @@
+Add metrics to track how the rate limiter is affecting requests (sleep/reject).

synapse/metrics/__init__.py

@@ -78,6 +78,17 @@ def collect() -> Iterable[Metric]:
 # TODO Do something nicer about this.
 RegistryProxy = cast(CollectorRegistry, _RegistryProxy)
 
+T = TypeVar("T")
+
+
+def count(func: Callable[[T], bool], it: Iterable[T]) -> int:
+    """Return the number of items in it for which func returns true."""
+    n = 0
+    for x in it:
+        if func(x):
+            n += 1
+    return n
+
 
 @attr.s(slots=True, hash=True, auto_attribs=True)
 class LaterGauge(Collector):
@@ -475,6 +486,7 @@ def register_threadpool(name: str, threadpool: ThreadPool) -> None:
     "MetricsResource",
     "generate_latest",
     "start_http_server",
+    count,
     "LaterGauge",
     "InFlightGauge",
     "GaugeBucketCollector",

synapse/notifier.py

@@ -40,7 +40,7 @@
 from synapse.logging import issue9533_logger
 from synapse.logging.context import PreserveLoggingContext
 from synapse.logging.opentracing import log_kv, start_active_span
-from synapse.metrics import LaterGauge
+from synapse.metrics import count, LaterGauge
 from synapse.streams.config import PaginationConfig
 from synapse.types import (
     JsonDict,
@@ -68,16 +68,6 @@
 T = TypeVar("T")
 
 
-# TODO(paul): Should be shared somewhere
-def count(func: Callable[[T], bool], it: Iterable[T]) -> int:
-    """Return the number of items in it for which func returns true."""
-    n = 0
-    for x in it:
-        if func(x):
-            n += 1
-    return n
-
-
 class _NotificationListener:
     """This represents a single client connection to the events stream.
     The events stream handler will have yielded to the deferred, so to

synapse/util/ratelimitutils.py

@@ -18,6 +18,8 @@
 import typing
 from typing import Any, DefaultDict, Iterator, List, Set
 
+from prometheus_client.core import Counter, Gauge
+
 from twisted.internet import defer
 
 from synapse.api.errors import LimitExceededError
@@ -27,6 +29,7 @@
     make_deferred_yieldable,
     run_in_background,
 )
+from synapse.metrics import count, LaterGauge
 from synapse.util import Clock
 
 if typing.TYPE_CHECKING:
@@ -35,6 +38,11 @@
 logger = logging.getLogger(__name__)
 
 
+# Track how much the ratelimiter is affecting requests
+rate_limit_sleep_counter = Counter("synapse_rate_limit_sleep", "")
+rate_limit_reject_counter = Counter("synapse_rate_limit_reject", "")
+
+
 class FederationRateLimiter:
     def __init__(self, clock: Clock, config: FederationRatelimitSettings):
         def new_limiter() -> "_PerHostRatelimiter":
@@ -44,6 +52,34 @@ def new_limiter() -> "_PerHostRatelimiter":
             str, "_PerHostRatelimiter"
         ] = collections.defaultdict(new_limiter)
 
+        # We track the number of affected hosts per time-period so we can
+        # differentiate one really noisy homeserver from a general
+        # ratelimit tuning problem across the federation.
+        LaterGauge(
+            "synapse_rate_limit_sleep_affected_hosts",
+            "Number of hosts that had requests put to sleep",
+            [],
+            lambda: count(
+                bool,
+                [
+                    ratelimiter.should_sleep()
+                    for ratelimiter in self.ratelimiters.values()
+                ],
+            ),
+        )
+        LaterGauge(
+            "synapse_rate_limit_reject_affected_hosts",
+            "Number of hosts that had requests rejected",
+            [],
+            lambda: count(
+                bool,
+                [
+                    ratelimiter.should_reject()
+                    for ratelimiter in self.ratelimiters.values()
+                ],
+            ),
+        )
+
     def ratelimit(self, host: str) -> "_GeneratorContextManager[defer.Deferred[None]]":
         """Used to ratelimit an incoming request from a given host
 
@@ -59,7 +95,7 @@ def ratelimit(self, host: str) -> "_GeneratorContextManager[defer.Deferred[None]
         Returns:
             context manager which returns a deferred.
         """
-        return self.ratelimiters[host].ratelimit()
+        return self.ratelimiters[host].ratelimit(host)
 
 
 class _PerHostRatelimiter:
@@ -94,19 +130,32 @@ def __init__(self, clock: Clock, config: FederationRatelimitSettings):
         self.request_times: List[int] = []
 
     @contextlib.contextmanager
-    def ratelimit(self) -> "Iterator[defer.Deferred[None]]":
+    def ratelimit(self, host: str) -> "Iterator[defer.Deferred[None]]":
         # `contextlib.contextmanager` takes a generator and turns it into a
         # context manager. The generator should only yield once with a value
         # to be returned by manager.
         # Exceptions will be reraised at the yield.
 
+        self.host = host
+
         request_id = object()
         ret = self._on_enter(request_id)
         try:
             yield ret
         finally:
             self._on_exit(request_id)
 
+    def should_reject(self):
+        """
+        Reject the request if we already have too many queued up (either
+        sleeping or in the ready queue).
+        """
+        queue_size = len(self.ready_request_queue) + len(self.sleeping_requests)
+        return queue_size > self.reject_limit
+
+    def should_sleep(self):
+        return len(self.request_times) > self.sleep_limit
+
     def _on_enter(self, request_id: object) -> "defer.Deferred[None]":
         time_now = self.clock.time_msec()
 
@@ -117,8 +166,9 @@ def _on_enter(self, request_id: object) -> "defer.Deferred[None]":
 
         # reject the request if we already have too many queued up (either
         # sleeping or in the ready queue).
-        queue_size = len(self.ready_request_queue) + len(self.sleeping_requests)
-        if queue_size > self.reject_limit:
+        if self.should_reject():
+            logger.debug("Ratelimiter(%s): rejecting request", self.host)
+            rate_limit_reject_counter.inc()
             raise LimitExceededError(
                 retry_after_ms=int(self.window_size / self.sleep_limit)
             )
@@ -130,7 +180,8 @@ def queue_request() -> "defer.Deferred[None]":
                 queue_defer: defer.Deferred[None] = defer.Deferred()
                 self.ready_request_queue[request_id] = queue_defer
                 logger.info(
-                    "Ratelimiter: queueing request (queue now %i items)",
+                    "Ratelimiter(%s): queueing request (queue now %i items)",
+                    self.host,
                     len(self.ready_request_queue),
                 )
 
@@ -139,19 +190,28 @@ def queue_request() -> "defer.Deferred[None]":
                 return defer.succeed(None)
 
         logger.debug(
-            "Ratelimit [%s]: len(self.request_times)=%d",
+            "Ratelimit(%s) [%s]: len(self.request_times)=%d",
+            self.host,
             id(request_id),
             len(self.request_times),
         )
 
-        if len(self.request_times) > self.sleep_limit:
-            logger.debug("Ratelimiter: sleeping request for %f sec", self.sleep_sec)
+        if self.should_sleep():
+            logger.debug(
+                "Ratelimiter(%s) [%s]: sleeping request for %f sec",
+                self.host,
+                id(request_id),
+                self.sleep_sec,
+            )
+            rate_limit_sleep_counter.inc()
             ret_defer = run_in_background(self.clock.sleep, self.sleep_sec)
 
             self.sleeping_requests.add(request_id)
 
             def on_wait_finished(_: Any) -> "defer.Deferred[None]":
-                logger.debug("Ratelimit [%s]: Finished sleeping", id(request_id))
+                logger.debug(
+                    "Ratelimit(%s) [%s]: Finished sleeping", self.host, id(request_id)
+                )
                 self.sleeping_requests.discard(request_id)
                 queue_defer = queue_request()
                 return queue_defer
@@ -161,7 +221,9 @@ def on_wait_finished(_: Any) -> "defer.Deferred[None]":
             ret_defer = queue_request()
 
         def on_start(r: object) -> object:
-            logger.debug("Ratelimit [%s]: Processing req", id(request_id))
+            logger.debug(
+                "Ratelimit(%s) [%s]: Processing req", self.host, id(request_id)
+            )
             self.current_processing.add(request_id)
             return r
 
@@ -183,7 +245,7 @@ def on_both(r: object) -> object:
         return make_deferred_yieldable(ret_defer)
 
     def _on_exit(self, request_id: object) -> None:
-        logger.debug("Ratelimit [%s]: Processed req", id(request_id))
+        logger.debug("Ratelimit(%s) [%s]: Processed req", self.host, id(request_id))
         self.current_processing.discard(request_id)
         try:
             # start processing the next item on the queue.