[Fixes #11512] improves cache size calculation in ActiveSupport::Cache::MemoryStore

Previously, the cache size of `ActiveSupport::Cache::MemoryStore` was calculated
as the sum of the size of its entries, ignoring the size of keys and any data
structure overhead. This could lead to the calculated cache size sometimes being
10-100x smaller than the memory used, e.g., in the case of small values.

The size of a key/entry pair is now calculated via `#cached_size`:

    def cached_size(key, entry)
      key.to_s.bytesize + entry.size + PER_ENTRY_OVERHEAD
    end

The value of `PER_ENTRY_OVERHEAD` is 240 bytes based on an [empirical
estimation](https://gist.github.com/ssimeonov/6047200) for 64-bit MRI on
1.9.3 and 2.0.

Fixes GH#11512 https://github.com/rails/rails/issues/11512

1 files changed, 14 insertions, 3 deletions

diff --git a/activesupport/lib/active_support/cache/memory_store.rb b/activesupport/lib/active_support/cache/memory_store.rb
index 4d26fb7e42..b0aae54aa4 100644
--- a/activesupport/lib/active_support/cache/memory_store.rb
+++ b/activesupport/lib/active_support/cache/memory_store.rb
@@ -122,6 +122,14 @@ module ActiveSupport
       end
 
       protected
+
+        # See https://gist.github.com/ssimeonov/6047200
+        PER_ENTRY_OVERHEAD = 240
+
+        def cached_size(key, entry)
+          key.to_s.bytesize + entry.size + PER_ENTRY_OVERHEAD
+        end
+
         def read_entry(key, options) # :nodoc:
           entry = @data[key]
           synchronize do
@@ -139,8 +147,11 @@ module ActiveSupport
           synchronize do
             old_entry = @data[key]
             return false if @data.key?(key) && options[:unless_exist]
-            @cache_size -= old_entry.size if old_entry
-            @cache_size += entry.size
+            if old_entry
+              @cache_size -= (old_entry.size - entry.size)
+            else
+              @cache_size += cached_size(key, entry)
+            end
             @key_access[key] = Time.now.to_f
             @data[key] = entry
             prune(@max_size * 0.75, @max_prune_time) if @cache_size > @max_size
@@ -152,7 +163,7 @@ module ActiveSupport
           synchronize do
             @key_access.delete(key)
             entry = @data.delete(key)
-            @cache_size -= entry.size if entry
+            @cache_size -= cached_size(key, entry) if entry
             !!entry
           end
         end