bench_rails4.0.txt

$ bundle exec ruby per_thread_registry_benchmark.rb
Calculating -------------------------------------
         thread-safe    25.176k i/100ms
       thread-unsafe    30.069k i/100ms
-------------------------------------------------
         thread-safe      1.480M (± 6.5%) i/s -      7.377M
       thread-unsafe      2.480M (± 7.2%) i/s -     12.328M

bench_rails4.2.txt

ruby per_thread_registry_benchmark.rb
Calculating -------------------------------------
         thread-safe    25.525k i/100ms
       thread-unsafe    29.837k i/100ms
-------------------------------------------------
         thread-safe      1.691M (± 7.5%) i/s -      8.398M
       thread-unsafe      2.440M (± 7.8%) i/s -     12.114M

monkey_patch.rb

require 'active_support/all'
require 'benchmark/ips'

class ThreadSafeRegistry
  extend ActiveSupport::PerThreadRegistry
  attr_accessor :foo
end

module GlobalRegistry
  include ActiveSupport::PerThreadRegistry

  private
  def instance
    @instance ||= new
  end

  def per_thread_registry_instance
    @instance ||= new
  end
end

class ThreadUnsafeRegistry
  extend GlobalRegistry
  attr_accessor :foo
end

ActiveSupport::PerThreadRegistry.send(:define_method, :instance, GlobalRegistry.instance_method(:instance))
ActiveSupport::PerThreadRegistry.module_eval { private :instance }

Benchmark.ips do |x|
  x.report('thread-safe') { ThreadSafeRegistry.foo }
  x.report('thread-unsafe') { ThreadUnsafeRegistry.foo }
end

monkey_patch_rails_4.2.txt

$ ruby monkey_patch.rb 
Calculating -------------------------------------
         thread-safe    30.865k i/100ms
       thread-unsafe    30.602k i/100ms
-------------------------------------------------
         thread-safe      2.725M (± 7.8%) i/s -     13.519M
       thread-unsafe      2.765M (± 7.3%) i/s -     13.740M

per_thread_registry_benchmark.rb

require 'active_support/all'
require 'benchmark/ips'

class ThreadSafeRegistry
  extend ActiveSupport::PerThreadRegistry
  attr_accessor :foo
end

module GlobalRegistry
  include ActiveSupport::PerThreadRegistry

  private
  def instance
    @instance ||= new
  end

  def per_thread_registry_instance
    @instance ||= new
  end
end

class ThreadUnsafeRegistry
  extend GlobalRegistry
  attr_accessor :foo
end

Benchmark.ips do |x|
  x.report('thread-safe') { ThreadSafeRegistry.foo }
  x.report('thread-unsafe') { ThreadUnsafeRegistry.foo }
end