module_inclusion_memsize.rb

# Method tables and constant tables are lazily allocated.  When a module is
# mixed in to a class, it creates an ICLASS which is the object used to
# represent the module in the inheritance hierarchy.  The ICLASS object shares
# some tables with the source module, so when it gets created Ruby will
# allocate these tables to share even if they didn't exist previously.
#
# You can see the tables get allocated in this function:
#   https://github.com/ruby/ruby/blob/962c302a1ae8e50738c36adb61c8ec9c9fa5a49b/class.c#L832-L862
#
# Output on my machine:
#
# [aaron@tc-lan-adapter ~/g/ruby (auto-compact)]$ ./ruby -v x.rb
# ruby 2.8.0dev (2020-05-22T14:54:34Z master 8d798e7c53) [x86_64-darwin19]
# Memsize before inclusion
# {:Maximal=>168}
# {:None=>560}
# {:Partial=>472}
# #########
# Memsize after inclusion
# {:Maximal=>496}
# {:None=>560}
# {:Partial=>496}
# [aaron@tc-lan-adapter ~/g/ruby (master)]$ 

require "objspace"

class Bar
  # Maximal laziness
  maximal = Module.new { }

  # No laziness
  module None
    BAR = 10
  end

  # Partial laziness
  module Partial
  end

  puts "Memsize before inclusion"
  p Maximal: ObjectSpace.memsize_of(maximal)
  p None: ObjectSpace.memsize_of(None)
  p Partial: ObjectSpace.memsize_of(Partial)

  puts "#########"
  include maximal
  include None
  include Partial

  puts "Memsize after inclusion"
  p Maximal: ObjectSpace.memsize_of(maximal)
  p None: ObjectSpace.memsize_of(None)
  p Partial: ObjectSpace.memsize_of(Partial)
end