A wannabe GenServer.

counter.rb

class Counter
  def initialize(initial_count = 0)
    @mailbox = Queue.new
    @replies = Queue.new
    @state = initial_count
    @server = process_messages
  end

  def increment
    mailbox.push([:handle_increment, 1])
  end

  def read
    mailbox.push([:handle_read])
    replies.pop
  end

  private

  attr_reader :mailbox, :replies

  def process_messages
    Thread.new do
      loop do
        message = mailbox.deq
        @state = send(*message)
      end
    end
  end

  def handle_increment(amount)
    @state + amount
  end

  def handle_read
    replies.push(@state)
    @state
  end
end

def time_intensive_operation(counter)
  sleep(0.005)
  counter.increment
end

counter = Counter.new

threads = Array.new(1000) do
  Thread.new do
    time_intensive_operation(counter)
  end
end
threads.each(&:join)

puts counter.read

# ~/Desktop [ruby 2.3.1p112]$ time ruby counter.rb
# 1000
#
# real    0m0.214s
# user    0m0.108s
# sys     0m0.144s