[Capistrano]単一サーバに対しシェルコマンドを並行して実行したい

def run_parallel(parallel_size)
  threads = []
  parallel_size.times do |i|
    threads << Thread.new do
      logger.info "starting concurrent task in #{parallel_size} threads"
      logger.info "started - thread #{i}"
      yield i
      logger.info "finished - thread #{i}"
    end
  end
  threads.each {|t| t.join }
end

run_parallel 4 do |i|
  run 'sleep 120'
end