问题 问答题

在使用磁盘高速缓存的系统中,平均的访问时间是80.6ms,高速缓存的平均访问时间是1ms,磁盘平均访问时间是200ms,系统有8MB的高速缓存。高速缓存增加一倍时,非命中率将降低40%。问需要增加多少高速缓存才能将平均访问时间减少到20ms(假定高速缓存的数据按2的倍数增加)。

答案

参考答案:假定现在访问高速缓存的比率为n,显然
80.6=1×n+(1-n)×200
由此可得n=(200-80.6)/199=0.6。
这说明配置8MB高速缓存时,高速缓存的访问比率为0.6,磁盘的访问比率为0.4。
要使平均访问时间减少到20ms,则有
20=1×n+(1-n)×200
由此可得n=0.9,磁盘的访问比率为0.1。即:
0.1=0.4×(1-0.4)x
x=2.71≈3
解得
8×23=64MB,故应该配置64MB的高速缓存。

阅读理解

阅读理解。

       The English language we know today is about six hundred years old. For half that time, English was

  just language of the people living in England. The modem English language grew from about the year

  1400. Now English is the most widely spoken language in the world. It is one of the working languages at

   International meetings, business, science and in other fields.

       The differences between American English and British English are not very great. Written English is

  more or less the same in both Britain and America, though there are some spelling differences. For

  example, centre, licence, colour and travelled are spelt center, license, color and traveled in American

  English. In America, "cock, rubber, ill, holiday, lorry and post" are called "rooster, eraser, sick, vacation,

  truck, mail". But people from the two countries can still understand each other easily.

       There are only a very small number of differences in grammar between American English and British

  English. Some words may have two kinds of pronunciation. You may wonder "Which is correct?" The

  answer is that it doesn't matter. But you should speak either American English or British English. Don't try

   to speak both at the same time!

1. When did the modem English language begin to grow?

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2. Is English the only working language at International meetings?

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3. What is the word "color" spelt in British English?

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4. What is the word "ill" called in American English?

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5. How many differences are there in grammar between American English and British English?

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