_INTRO.sh

$ ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]
$ rails -v
Rails 3.2.6
$ mkdir memory_leak
$ cd memory_leak
$ rails new .
$ rails g model Stub
$ bundle exec rake db:create db:migrate
$ rails runner ml.rb
In loop: 0
Not in loop: 10

case-1.rb

require "./ml.rb"
1.times {stub_all}
GC.start
puts base_count #=> 0

case-2.rb

require "./ml.rb"
stub_all
GC.start
puts base_count #=> 10

case-3.rb

require "./ml.rb"
begin stub_all end
GC.start
puts base_count #=> 10

ml.rb

def stub_all
  Stub.all.each {}
  nil
end

def base_count
  ObjectSpace.each_object(ActiveRecord::Base).count
end