Limit how often GC happens by time. (#9902)

Synapse can be quite memory intensive, and unless care is taken to tune
the GC thresholds it can end up thrashing, causing noticable performance
problems for large servers. We fix this by limiting how often we GC a
given generation, regardless of current counts/thresholds.

This does not help with the reverse problem where the thresholds are set
too high, but that should only happen in situations where they've been
manually configured.

Adds a `gc_min_seconds_between` config option to override the defaults.

Fixes #9890.

changelog.d/9902.feature

@@ -0,0 +1 @@
+Add limits to how often Synapse will GC, ensuring that large servers do not end up GC thrashing if `gc_thresholds` has not been correctly set.

docs/sample_config.yaml

@@ -152,6 +152,16 @@ presence:
 #
 #gc_thresholds: [700, 10, 10]
 
+# The minimum time in seconds between each GC for a generation, regardless of
+# the GC thresholds. This ensures that we don't do GC too frequently.
+#
+# A value of `[1s, 10s, 30s]` indicates that a second must pass between consecutive
+# generation 0 GCs, etc.
+#
+# Defaults to `[1s, 10s, 30s]`.
+#
+#gc_min_interval: [0.5s, 30s, 1m]
+
 # Set the limit on the returned events in the timeline in the get
 # and sync operations. The default value is 100. -1 means no upper limit.
 #

synapse/app/generic_worker.py

@@ -455,6 +455,9 @@ def start(config_options):
 
     synapse.events.USE_FROZEN_DICTS = config.use_frozen_dicts
 
+    if config.server.gc_seconds:
+        synapse.metrics.MIN_TIME_BETWEEN_GCS = config.server.gc_seconds
+
     hs = GenericWorkerServer(
         config.server_name,
         config=config,

synapse/app/homeserver.py

@@ -342,6 +342,9 @@ def setup(config_options):
 
     events.USE_FROZEN_DICTS = config.use_frozen_dicts
 
+    if config.server.gc_seconds:
+        synapse.metrics.MIN_TIME_BETWEEN_GCS = config.server.gc_seconds
+
     hs = SynapseHomeServer(
         config.server_name,
         config=config,

synapse/config/server.py

@@ -19,7 +19,7 @@
 import os.path
 import re
 from textwrap import indent
-from typing import Any, Dict, Iterable, List, Optional, Set
+from typing import Any, Dict, Iterable, List, Optional, Set, Tuple
 
 import attr
 import yaml
@@ -572,6 +572,7 @@ def read_config(self, config, **kwargs):
             _warn_if_webclient_configured(self.listeners)
 
         self.gc_thresholds = read_gc_thresholds(config.get("gc_thresholds", None))
+        self.gc_seconds = self.read_gc_intervals(config.get("gc_min_interval", None))
 
         @attr.s
         class LimitRemoteRoomsConfig:
@@ -917,6 +918,16 @@ def generate_config_section(
         #
         #gc_thresholds: [700, 10, 10]
 
+        # The minimum time in seconds between each GC for a generation, regardless of
+        # the GC thresholds. This ensures that we don't do GC too frequently.
+        #
+        # A value of `[1s, 10s, 30s]` indicates that a second must pass between consecutive
+        # generation 0 GCs, etc.
+        #
+        # Defaults to `[1s, 10s, 30s]`.
+        #
+        #gc_min_interval: [0.5s, 30s, 1m]
+
         # Set the limit on the returned events in the timeline in the get
         # and sync operations. The default value is 100. -1 means no upper limit.
         #
@@ -1305,6 +1316,24 @@ def add_arguments(parser):
             help="Turn on the twisted telnet manhole service on the given port.",
         )
 
+    def read_gc_intervals(self, durations) -> Optional[Tuple[float, float, float]]:
+        """Reads the three durations for the GC min interval option, returning seconds."""
+        if durations is None:
+            return None
+
+        try:
+            if len(durations) != 3:
+                raise ValueError()
+            return (
+                self.parse_duration(durations[0]) / 1000,
+                self.parse_duration(durations[1]) / 1000,
+                self.parse_duration(durations[2]) / 1000,
+            )
+        except Exception:
+            raise ConfigError(
+                "Value of `gc_min_interval` must be a list of three durations if set"
+            )
+
 
 def is_threepid_reserved(reserved_threepids, threepid):
     """Check the threepid against the reserved threepid config

synapse/metrics/__init__.py

@@ -535,6 +535,13 @@ def collect(self):
 
 REGISTRY.register(ReactorLastSeenMetric())
 
+# The minimum time in seconds between GCs for each generation, regardless of the current GC
+# thresholds and counts.
+MIN_TIME_BETWEEN_GCS = (1.0, 10.0, 30.0)
+
+# The time (in seconds since the epoch) of the last time we did a GC for each generation.
+_last_gc = [0.0, 0.0, 0.0]
+
 
 def runUntilCurrentTimer(reactor, func):
     @functools.wraps(func)
@@ -575,11 +582,16 @@ def f(*args, **kwargs):
             return ret
 
         # Check if we need to do a manual GC (since its been disabled), and do
-        # one if necessary.
+        # one if necessary. Note we go in reverse order as e.g. a gen 1 GC may
+        # promote an object into gen 2, and we don't want to handle the same
+        # object multiple times.
         threshold = gc.get_threshold()
         counts = gc.get_count()
         for i in (2, 1, 0):
-            if threshold[i] < counts[i]:
+            # We check if we need to do one based on a straightforward
+            # comparison between the threshold and count. We also do an extra
+            # check to make sure that we don't a GC too often.
+            if threshold[i] < counts[i] and MIN_TIME_BETWEEN_GCS[i] < end - _last_gc[i]:
                 if i == 0:
                     logger.debug("Collecting gc %d", i)
                 else:
@@ -589,6 +601,8 @@ def f(*args, **kwargs):
                 unreachable = gc.collect(i)
                 end = time.time()
 
+                _last_gc[i] = end
+
                 gc_time.labels(i).observe(end - start)
                 gc_unreachable.labels(i).set(unreachable)