Threads performance comparison between JRuby and MRI

example.rb

require 'nokogiri'
require 'benchmark'
require 'thread'
require 'open-uri'

# for JRUBY: export JRUBY_OPTS="--1.9 -J-Xmx2048m"
# JRuby 1.6.7 doesn't support File.write yet in 1.9 mode. Just doing some cache here - not important
File.open('test.html', 'w'){|f| f.write(open('http://www.sec.gov/Archives/edgar/data/1129623/000095012310064472/d73737ddefm14a.htm').read) } unless File.exists?('test.html')
content = File.read('test.html')

class Executor
  def initialize(pool_size)
    @available_threads = Queue.new
    pool_size.times { @available_threads << nil }
    @threads = []
  end

  def execute
    @available_threads.pop
    @threads << Thread.start {
      yield
      @available_threads << nil
    }
  end

  def shutdown
    @threads.each &:join
  end
end

Benchmark.bmbm do |x|
  x.report('Parse HTML') do
    executor = Executor.new(10)
    50.times { executor.execute { Nokogiri.HTML(content) } }
    executor.shutdown
  end  
end

# Results for JRuby 1.6.7:
# Rehearsal ----------------------------------------------
# Parse HTML   4.501000   0.000000   4.501000 (  4.501000)
# ------------------------------------- total: 4.501000sec
# 
#                  user     system      total        real
# Parse HTML   0.927000   0.000000   0.927000 (  0.928000)

# Results for MRI 1.9.3-p
# Rehearsal ----------------------------------------------
# Parse HTML   3.800000   0.050000   3.850000 (  3.853381)
# ------------------------------------- total: 3.850000sec
# 
#                  user     system      total        real
# Parse HTML   3.780000   0.010000   3.790000 (  3.788918)

# They perform about the same if I change to Executor.new(1), ie, using a single thread

# Am I doing any mistake in this code?