Don't marshal ActiveSupport::Cache::Entry objects twice

When upgrading to Rails 5.2 we're seeing
`ActiveSupport::Cache::Entry#compress` and
`ActiveSupport::Cache::Entry#should_compress?` as the highest usage of
our CPU. At least some part of this is coming from the fact that objects
are being marshaled multiple times. This memoizes the marshaled value to
eliminate half the problem.

1 files changed, 14 insertions, 13 deletions

diff --git a/activesupport/lib/active_support/cache.rb b/activesupport/lib/active_support/cache.rb
index 1ea2d0bbf2..6967c164ab 100644
--- a/activesupport/lib/active_support/cache.rb
+++ b/activesupport/lib/active_support/cache.rb
@@ -714,11 +714,9 @@ module ActiveSupport
       # Creates a new cache entry for the specified value. Options supported are
       # +:compress+, +:compress_threshold+, and +:expires_in+.
       def initialize(value, options = {})
-        if should_compress?(value, options)
-          @value = compress(value)
-          @compressed = true
-        else
-          @value = value
+        @value = value
+        if should_compress?(options)
+          compress!
         end
 
         @version    = options[:version]
@@ -783,28 +781,31 @@ module ActiveSupport
       end
 
       private
-        def should_compress?(value, options)
-          if value && options.fetch(:compress, true)
+        def should_compress?(options)
+          if @value && options.fetch(:compress, true)
             compress_threshold = options.fetch(:compress_threshold, DEFAULT_COMPRESS_LIMIT)
-            serialized_value_size = (value.is_a?(String) ? value : Marshal.dump(value)).bytesize
+            serialized_value_size = (@value.is_a?(String) ? @value : marshaled_value).bytesize
 
-            return true if serialized_value_size >= compress_threshold
+            serialized_value_size >= compress_threshold
           end
-
-          false
         end
 
         def compressed?
           defined?(@compressed) ? @compressed : false
         end
 
-        def compress(value)
-          Zlib::Deflate.deflate(Marshal.dump(value))
+        def compress!
+          @value = Zlib::Deflate.deflate(marshaled_value)
+          @compressed = true
         end
 
         def uncompress(value)
           Marshal.load(Zlib::Inflate.inflate(value))
         end
+
+        def marshaled_value
+          @marshaled_value ||= Marshal.dump(@value)
+        end
     end
   end
 end