Trying to understand pools

pool.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'bundler/setup'
require 'celluloid'
require 'chromatic'

$stdout.sync = true
Celluloid.logger = nil

WITH_POOL = true

class Executor
  include Celluloid

  def initialize
    puts self.inspect.magenta
  end

  def run(i)
    print " +#{i}+ ".light_green
    sleep 1
    print " -#{i}- ".light_red
  end
end

# in this case only two executors run in parallel
if WITH_POOL
  supervision_group = Celluloid::SupervisionGroup.run!
  supervision_group.pool(Executor, {
    as: :executor,
    size: 2
  })
else
  # in this case all ten executors run in parallel
  Celluloid::Actor[:executor] = Executor.new
end

start = Time.now
(1..10).map { |i|
  Celluloid::Actor[:executor].future.run(i)
}.map(&:value)
finish = Time.now

puts "\ndone (%.2fs)".green % (finish - start)

• Output with pool:

  $ ./pool.rb 
  #
  #
  +1+  +2+  -2-  -1-  +3+  +4+  -4-  -3-  +5+  +6+  -5-  -6-  +7+  +8+  -8-  -7-  +9+  +10+  -10-  -9- 
  done (5.02s)
  

• Output without pool:

  $ ./pool.rb 
  #
  +1+  +2+  +3+  +4+  +5+  +6+  +7+  +8+  +9+  +10+  -1-  -2-  -3-  -4-  -5-  -6-  -7-  -8-  -9-  -10- 
  done (1.00s)