PERF: cut down on memory usage allowed to redis

This limits the amount of backlog message bus channels can have.

config/discourse_defaults.conf

@@ -130,3 +130,8 @@ connection_reaper_max_age = 600
 # IMPORTANT: path must not include a trailing /
 # EG: /forum
 relative_url_root =
+
+# increasing this number will increase redis memory use
+# this ensures backlog (ability of channels to catch up are capped)
+# message bus default cap is 1000, we are winding it down to 100
+message_bus_max_backlog_size = 100

config/initializers/004-message_bus.rb

@@ -71,11 +71,10 @@ end
 
 # Point at our redis
 MessageBus.redis_config = GlobalSetting.redis_config
+MessageBus.reliable_pub_sub.max_backlog_size = GlobalSetting.message_bus_max_backlog_size
 
 MessageBus.long_polling_enabled = SiteSetting.enable_long_polling
 MessageBus.long_polling_interval = SiteSetting.long_polling_interval
-
-
 MessageBus.cache_assets = !Rails.env.development?
 MessageBus.enable_diagnostics