问题 单项选择题

若操作系统中有n个作业Ji(i=1,2,…,n),分别需要Ti(i=1,2,…,n)的运行时间,采用()的作业调度算法可以使平均周转时间最短。

A.先来先服务

B.最短作业优先

C.响应比高者优先

D.优先级

答案

参考答案:B

解析:

[要点解析] 这是一道考查作业管理中作业调度算法性能衡量的试题。 先来先服务(FCFS)调度算法是指按照用户作业到达的先后顺序进行调度处理。它优先考虑在系统中等待时间最长的作业,而不管要求运行时间的长短。 最短作业优先(SJF)调度算法是指对短作业优先调度的算法。作业调度程序每次是从后备作业队列中选择一个作业投入运行。该算法对于长作业可能会有一个较长的延迟时间。 响应比高者优先(HRN)调度算法是指调度时既考虑作业估计运行时间,又考虑作业等待时间,响应比是HRN=(估计运行时间+等待时间)/估计运行时间。 优先级调度是指根据作业的优先级别,优先级高者首先调度。 对于最短作业优先(SJF)调度算法可使系统在同一时间内处理的作业个数最多,即可以使平均周转时间最短。

阅读理解

阅读下面短文,并根据短文后的要求答题。

    Walking is a popular form of exercise. It is an easy activity and offers a good way

to improve physical fitness. Walking also gives many of the same benefits as other kinds

of exercise.

    Regular, brisk walks help a person's body work better. Walking builds a stronger heart

and healthier lungs. The heart pumps blood through the body. It gets more rest between

beats. Walking also seems to help protect the heart from heart diseases. The lungs work

better because they take in and use oxygen more effectively. Walking can help , too. A

quick fifteen-minute walk buns as many calories as jogging the same distance in half the

time. Walking causes very few injuries. So there is also a lower "dropout" rate among

walkers than among runners. People are more likely to continue a walking program. This

gives a better chance for success.

    Walking offers some mental benefits, too. It seems to make people feel better. Many

walkers say they sleep better at night when they take regular walks. Others say they have

a better attitude about life.

    Walking offers many of the same physical and mental benefits as other forms of

exercise, but walking offers some special advantages, too.

    ★Almost everyone can walk. There are no special lessons of coaching. To become a

serious walker, a person only needs to walk faster, farther and more often.

    ★People can walk almost anywhere. There are no special playing fields or courts for

walking. Sidewalks, streets, parks, fields and malls are excellent places for walking.

    ★People can walk almost anytime. A person doesn't need a team or a partner for

walking. There is no "season" for walking. Most walkers walk in all kinds of weather.

    ★Walking doesn't cost anything. There are no special fees for walking. Good walking

shoes and comfortable clothes are only equipment that a walker needs.

    Walking offers a form of exercise within the reach of nearly everyone. With a little

time and effort, people can rediscover a valuable form of exercise and improve their

fitness.

1. What's the best title of the passage? (within 10 words)

   _____________________________________________________________________________________

2. Which sentence in the passage can be replaced by the following one?  

   Comparing with running, more people can continue walking as a form of exercise.

   _____________________________________________________________________________________

3. Please fill in the blank in the 2nd paragraph with proper words or phrases. (within 10

   words)

   _____________________________________________________________________________________

4. Among the special advantages of walking, which one do you think is the most important?

   Why (within 30 words)

   _____________________________________________________________________________________

5. Translate the underlined sentence into Chinese.

   _____________________________________________________________________________________

单项选择题