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操作系统第八次作业

问题一：**A certain computer provides its users with a virtual-memory space of 2 32 2^{32} 232 **bytes. The computer has 2 18 2^{18} 218 bytes of physical memory. The virtual memory is implemented by paging, and the page size is 4,096 bytes. A user process generates the virtual address 11123456. Explain how the system establishes the corresponding physical location. Distinguish between software and hardware operations.

答：

  • 11123456转化为二进制为：
    0001 0001 0001 0010 0011 0100 0101 0110

​ 地址为32位

  • 已知页大小为 4096 = 2 12 4096=2^{12} 4096=212

​ 故32位地址后12位为页内偏移地址，前二十位为页号，且页表大小为 2 20 2^{20} 220

  • 物理内存为 2 18 2^{18} 218= 64 × 2 12 64\times2^{12} 64×212bytes

故共有64个帧

  • 11123456前二十位0001 0001 0001 0010 0011在页表中寻找对应的物理块，

后12位0100 0101 0110为物理帧内的偏移地址

实际的物理地址为物理块号首地址+偏移量（0100 0101 0110）

问题二： Consider a demand-paging system with the following time-measured utilizations:
CPU utilization20%
Paging disk97.7%
Other I/O devices5%

For each of the following, say whether it will (or is likely to) improve CPU utilization. Explain your answers.

a. Install a faster CPU.

b. Install a bigger paging disk.

c. Increase the degree of multiprogramming.

d. Decrease the degree of multiprogramming.

e. Install more main memory.

f. Install a faster hard dist or multiple controllers with multiple hard disks.

g. Add prepaging to the page-fetch algorithms.

h. Increase the page size.

答：

a、b、c不可以

d、e、f、g、h可以

d减少进程个数之后可以为余下的进程分配更多的页帧，进而减小缺页率、降低分页磁盘利用率的同时提高CPU利用率

e则是通过扩大内存帧的数量来为进程分配更多页帧

f则是通过提高页面置换的速度来使进程的等待时间更短，更快地进入就绪状态

g通过预先调页使得CPU不需要等待就可以获得数据，从而提高CPU利用率

h通过增加页的大小可以减少页面置换、降低缺页率进而提高CPU利用率

问题三： A page-replacement algorithm should minimize the number of page faults. We can achieve this minimization by distributing heavily used pages evenly over all of memory, rather than having them compete for a small number of page frames. We can associate with each page frame a counter of the number of pages associated with that frame. Then, to replace a page, we can search for the page frame with the smallest counter.

a. Define a page-replacement algorithm using this basic idea. Specifically address these problems:

​ 1. What the initial value of the counters is

​ 2. When counters are increased

​ 3. When counters are decreased

​ 4. How the page to be replaced is selected

b. How many page faults occur for your algorithm for the following reference string, with four page frames?

​ 1,2,3,4,5,3,4,1,6,7,8,7,8,9,7,8,9,5,4,5,4,2.

c. What is the minimum number of page faults for an optimal page-replacement strategy for the reference string in part b with four page frames?

答：

  • a
  • 1、计数器初值为0
  • 2、当一个新的页和此页帧相关联
  • 3、与该页帧相关联的一个页不再被需要时
  • 4、选择计数器最少的页帧，当计数器相同时使用FIFO
  • b:4+10=14
  • c:4+7=11

本文标签： 作业第八次操作系统

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