Betriebssysteme/hw3/simu4/QUESTIONS.md

Questions 18-Paging-Linear-Translate

In this simulation task, you will use a simple program, which is known as paging-linear-translate.py, to see if you understand how simple virtual-to-physical address translation works with linear page tables. See the README for details.

Questions

1. Before doing any translations, let’s use the simulator to study how linear page tables change size given different parameters. Compute the size of linear page tables as different parameters change. Some suggested inputs are below; by using the -v flag, you can see how many page-table entries are filled.

First, to understand how linear page table size changes as the address space grows:

      paging-linear-translate.py -P 1k -a 1m -p 512m -v -n 0
      paging-linear-translate.py -P 1k -a 2m -p 512m -v -n 0
      paging-linear-translate.py -P 1k -a 4m -p 512m -v -n 0

Then, to understand how linear page table size changes as page size grows:

      paging-linear-translate.py -P 1k -a 1m -p 512m -v -n 0
      paging-linear-translate.py -P 2k -a 1m -p 512m -v -n 0
      paging-linear-translate.py -P 4k -a 1m -p 512m -v -n 0

Before running any of these, try to think about the expected trends.

1.1. How can the page table size governed in equations? Give the equations to show, which parameters govern the page table size.

1.2. How should page-table size change as the address space grows? As the page size grows?

1.3. Why shouldn’t we just use really big pages in general?

1. Now let’s do some translations. Start with some small examples, and change the number of pages that are allocated to the address space with the -u flag. For example:

      paging-linear-translate.py -P 1k -a 16k -p 32k -v -u 0
      paging-linear-translate.py -P 1k -a 16k -p 32k -v -u 25
      paging-linear-translate.py -P 1k -a 16k -p 32k -v -u 50
      paging-linear-translate.py -P 1k -a 16k -p 32k -v -u 75
      paging-linear-translate.py -P 1k -a 16k -p 32k -v -u 100

What happens as you increase the percentage of pages that are allocated in each address space?

1. Now let’s try some different random seeds, and some different (and sometimes quite crazy) address-space parameters, for variety:

      paging-linear-translate.py -P 8  -a 32   -p 1024 -v -s 1
      paging-linear-translate.py -P 8k -a 32k  -p 1m   -v -s 2
      paging-linear-translate.py -P 1m -a 256m -p 512m -v -s 3

Which of these parameter combinations are unrealistic? Why?

1. Use the program to try out some other problems. Can you find the limits of where the program doesn’t work anymore? For example, what happens if the address-space size is bigger than physical memory?