Add a memory cache local to the thread to reduce Redis load

Loading `ApplicationSetting` from Redis was responsible for at least 50%
of the CPU load of the Redis cluster on GitLab.com. Since these values
generally don't change very much, we can load this from the database and
cache it in memory, skipping Redis altogther. We use
`ActiveSupport::Cache::MemoryStore` as a drop-in replacement for
`RedisCacheStore` even though we probably don't need synchronized access
within `Thread.current`.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63977

1 files changed, 7 insertions, 3 deletions

diff --git a/app/models/concerns/cacheable_attributes.rb b/app/models/concerns/cacheable_attributes.rb
index 3d60f6924c1..8cbf4bcfaf7 100644
--- a/app/models/concerns/cacheable_attributes.rb
+++ b/app/models/concerns/cacheable_attributes.rb
@@ -36,7 +36,7 @@ module CacheableAttributes
     end
 
     def retrieve_from_cache
-      record = Rails.cache.read(cache_key)
+      record = cache_backend.read(cache_key)
       ensure_cache_setup if record.present?
 
       record
@@ -58,7 +58,7 @@ module CacheableAttributes
     end
 
     def expire
-      Rails.cache.delete(cache_key)
+      cache_backend.delete(cache_key)
     rescue
       # Gracefully handle when Redis is not available. For example,
       # omnibus may fail here during gitlab:assets:compile.
@@ -69,9 +69,13 @@ module CacheableAttributes
       # to be loaded when read from cache: https://github.com/rails/rails/issues/27348
       define_attribute_methods
     end
+
+    def cache_backend
+      Rails.cache
+    end
   end
 
   def cache!
-    Rails.cache.write(self.class.cache_key, self, expires_in: 1.minute)
+    self.class.cache_backend.write(self.class.cache_key, self, expires_in: 1.minute)
   end
 end