def kill_all(signal)
      idle, running = partition_workers
      log "kill -#{signal} #{running.map { |r| r.pid }.inspect}"
      running.each { |worker| worker.kill(signal) }
    end
def kill_all(signal)
      idle, running = partition_workers
      log "kill -#{signal} #{running.map { |r| r.pid }.inspect}"
      running.each { |worker| worker.kill(signal) }
    end
Private: send a signal to all the workers.