make it possible to change event_cache_size with a SIGHUP

[richvdh]

Most cache settings can be reloaded by sending Synapse a SIGHUP. Apparently not so event_cache_size. (Why does that even have a separate setting, rather than just being an entry under per_cache_factors ?)

[DMRobertson]

Apparently not so event_cache_size

There are some clues in this docstring:

synapse/synapse/app/_base.py

Lines 534 to 553 in 898fef2

	def reload_cache_config(config: HomeServerConfig) -> None:
	"""Reload cache config from disk and immediately apply it.resize caches accordingly.
	If the config is invalid, a `ConfigError` is logged and no changes are made.
	Otherwise, this:
	- replaces the `caches` section on the given `config` object,
	- resizes all caches according to the new cache factors, and
	Note that the following cache config keys are read, but not applied:
	- event_cache_size: used to set a max_size and _original_max_size on
	EventsWorkerStore._get_event_cache when it is created. We'd have to update
	the _original_max_size (and maybe
	- sync_response_cache_duration: would have to update the timeout_sec attribute on
	HomeServer -> SyncHandler -> ResponseCache.
	- track_memory_usage. This affects synapse.util.caches.TRACK_MEMORY_USAGE which
	influences Synapse's self-reported metrics.
	Also, the HTTPConnectionPool in SimpleHTTPClient sets its maxPersistentPerHost
	parameter based on the global_factor. This won't be applied on a config reload.

But I think the docstring is wrong about track_memory_usage given the line

synapse/synapse/app/_base.py

Line 567 in 898fef2

synapse.util.caches.TRACK_MEMORY_USAGE = config.caches.track_memory_usage