fib.rb

def fib( n )
  return  n  if ( 0..1 ).include? n
  ( fib( n - 1 ) + fib( n - 2 ) )
end

threads = []

4.times do
  threads << Thread.new do
    fib(50) while true
  end
end

threads.each do |t|
  t.join
end