CS439: Principles of Computer Systems
Homework 6, Part 1
Due: 8:45a Friday, March 7, 2014 Part 1 of the
homeworks must be submitted electronically. Please refer to the
homework turnin instructions.
Assume you have a virtual memory system that uses paging. Is the
system vulnerable to internal and/or external fragmentation? Explain.
Sam P. Hacker is a Head Guru in a software company known for
operating systems with very sorry quality. Hacker suggested a trick to
reduce the pressure on the swap space. Instead of swapping out pages
that belong to code texts into the swap area, the operating system
could just take away the memory frames. If the code page is needed
later, it could be paged directly from its binary file. Hacker's
argument is that the operating system will save time by not storing
the code page into the swap space, and will save space in the swap
area. The code text exists on disk anyway, and could be fetched from
there. Would you approve of this design? Why or why not?
What causes a page fault? What is the end result for the running
Belady's anomaly: Intuitively, it seems that the more frames the
memory has, the fewer page faults a program will get.
Surprisingly enough, this is not always true. Belady (1969)
discovered an example in which FIFO page replacement causes more
faults with four page frames than with three. This strange
situation has become known as Belady's anomaly. To illustrate, a
program with five virtual pages numbered from 0 to 4 references
its pages in the order:
0 1 2 3 0 1 4 0 1 2 3 4
- Using FIFO replacement and assuming demand paging, compute
the number of page faults with 3 frames. Repeat for 4 frames.
Compute the number of page faults under LRU, the clock
algorithm, and the optimal algorithm. What do you notice?
Question 9.19, page 852, B&O.