diff --git a/synapse/config/cache.py b/synapse/config/cache.py
new file mode 100644
index 0000000000..bb68551954
--- /dev/null
+++ b/synapse/config/cache.py
@@ -0,0 +1,112 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+from typing import Dict
+
+from ._base import Config, ConfigError
+
+_CACHES = {}
+_CACHE_PREFIX = "SYNAPSE_CACHE_FACTOR"
+DEFAULT_CACHE_SIZE_FACTOR = float(os.environ.get(_CACHE_PREFIX, 0.5))
+
+_DEFAULT_CONFIG = """\
+# Cache configuration
+#
+# 'global_factor' controls the global cache factor. This overrides the
+# "SYNAPSE_CACHE_FACTOR" environment variable.
+#
+# 'per_cache_factors' is a dictionary of cache name to cache factor for that
+# individual cache.
+#
+#caches:
+# global_factor: 0.5
+# per_cache_factors:
+# get_users_who_share_room_with_user: 2
+#
+"""
+
+# Callback to ensure that all caches are the correct size, registered when the
+# configuration has been loaded.
+_ENSURE_CORRECT_CACHE_SIZING = None
+
+
+def add_resizable_cache(cache_name, cache_resize_callback):
+ _CACHES[cache_name.lower()] = cache_resize_callback
+ if _ENSURE_CORRECT_CACHE_SIZING:
+ _ENSURE_CORRECT_CACHE_SIZING()
+
+
+class CacheConfig(Config):
+ section = "caches"
+ _environ = os.environ
+
+ @staticmethod
+ def _reset():
+ global DEFAULT_CACHE_SIZE_FACTOR
+ global _ENSURE_CORRECT_CACHE_SIZING
+
+ DEFAULT_CACHE_SIZE_FACTOR = float(os.environ.get(_CACHE_PREFIX, 0.5))
+ _ENSURE_CORRECT_CACHE_SIZING = None
+ _CACHES.clear()
+
+ def read_config(self, config, **kwargs):
+ self.event_cache_size = self.parse_size(config.get("event_cache_size", "10K"))
+
+ global DEFAULT_CACHE_SIZE_FACTOR
+ global _ENSURE_CORRECT_CACHE_SIZING
+
+ cache_config = config.get("caches", {})
+
+ self.global_factor = cache_config.get(
+ "global_factor", DEFAULT_CACHE_SIZE_FACTOR
+ )
+ if not isinstance(self.global_factor, (int, float)):
+ raise ConfigError("caches.global_factor must be a number.")
+
+ # Set the global one so that it's reflected in new caches
+ DEFAULT_CACHE_SIZE_FACTOR = self.global_factor
+
+ # Load cache factors from the environment, but override them with the
+ # ones in the config file if they exist
+ individual_factors = {
+ key[len(_CACHE_PREFIX) + 1 :].lower(): float(val)
+ for key, val in self._environ.items()
+ if key.startswith(_CACHE_PREFIX + "_")
+ }
+
+ individual_factors_config = cache_config.get("per_cache_factors", {}) or {}
+ if not isinstance(individual_factors_config, dict):
+ raise ConfigError("caches.per_cache_factors must be a dictionary")
+
+ individual_factors.update(individual_factors_config)
+
+ self.cache_factors = dict() # type: Dict[str, float]
+
+ for cache, factor in individual_factors.items():
+ if not isinstance(factor, (int, float)):
+ raise ConfigError(
+ "caches.per_cache_factors.%s must be a number" % (cache.lower(),)
+ )
+ self.cache_factors[cache.lower()] = factor
+
+ # Register the global callback so that the individual cache sizes get set.
+ def ensure_cache_sizes():
+ for cache_name, callback in _CACHES.items():
+ new_factor = self.cache_factors.get(cache_name, self.global_factor)
+ callback(new_factor)
+
+ _ENSURE_CORRECT_CACHE_SIZING = ensure_cache_sizes
+ _ENSURE_CORRECT_CACHE_SIZING()
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