Limit how often GC happens by time.

docs/sample_config.yaml

@@ -155,10 +155,10 @@ presence:
 # 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 `[1, 10, 30]` indicates that a second must pass between consecutive
+# A value of `[1s, 10s, 30s]` indicates that a second must pass between consecutive
 # generation 0 GCs, etc.
 #
-# gc_min_seconds_between: [1, 10, 30]
+#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/config/server.py

@@ -572,7 +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 = read_gc_thresholds(config.get("gc_min_seconds_between", None))
+        self.gc_seconds = self.read_gc_intervals(config.get("gc_min_interval", None))
 
         @attr.s
         class LimitRemoteRoomsConfig:
@@ -921,10 +921,10 @@ def generate_config_section(
         # 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 `[1, 10, 30]` indicates that a second must pass between consecutive
+        # A value of `[1s, 10s, 30s]` indicates that a second must pass between consecutive
         # generation 0 GCs, etc.
         #
-        # gc_min_seconds_between: [1, 10, 30]
+        #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.
@@ -1314,6 +1314,23 @@ def add_arguments(parser):
             help="Turn on the twisted telnet manhole service on the given port.",
         )
 
+    def read_gc_intervals(self, durations):
+        """Reads the three durations for the GC min interval option."""
+        if durations is None:
+            return None
+        try:
+            if len(durations) != 3:
+                raise ValueError()
+            return (
+                self.parse_duration(durations[0]),
+                self.parse_duration(durations[1]),
+                self.parse_duration(durations[2]),
+            )
+        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

@@ -537,9 +537,9 @@ def collect(self):
 
 # The minimum time in seconds between GCs for each generation, regardless of the current GC
 # thresholds and counts.
-MIN_TIME_BETWEEN_GCS = [1, 10, 30]
+MIN_TIME_BETWEEN_GCS = (1, 10, 30)
 
-# The time in seconds of the last time we did a GC for each generation.
+# The time (in seconds since the epoch) of the last time we did a GC for each generation.
 _last_gc = [0, 0, 0]