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-rw-r--r--synapse/app/generic_worker.py3
-rw-r--r--synapse/app/homeserver.py3
-rw-r--r--synapse/config/server.py31
-rw-r--r--synapse/metrics/__init__.py18
4 files changed, 52 insertions, 3 deletions
diff --git a/synapse/app/generic_worker.py b/synapse/app/generic_worker.py
index 1a15ceee81..a3fe9a3f38 100644
--- a/synapse/app/generic_worker.py
+++ b/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,
diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index 8e78134bbe..6a823da10d 100644
--- a/synapse/app/homeserver.py
+++ b/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,
diff --git a/synapse/config/server.py b/synapse/config/server.py
index 21ca7b33e3..c290a35a92 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -19,7 +19,7 @@ import logging
 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 @@ class ServerConfig(Config):
             _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 @@ class ServerConfig(Config):
         #
         #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 @@ class ServerConfig(Config):
             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
diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index 31b7b3c256..e671da26d5 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -535,6 +535,13 @@ class ReactorLastSeenMetric:
 
 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 runUntilCurrentTimer(reactor, func):
             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 runUntilCurrentTimer(reactor, func):
                 unreachable = gc.collect(i)
                 end = time.time()
 
+                _last_gc[i] = end
+
                 gc_time.labels(i).observe(end - start)
                 gc_unreachable.labels(i).set(unreachable)