CSE120, Fall 2005, Homework 3
Due: November 10, in class

1. [Silberschatz] [10 pts] What is the cause of thrashing?  How does the 
system detect thrashing?  Once it detects thrashing, what can the system 
do to eliminate this problem?

2. [Silberschatz] [10 pts] Why are segmentation and paging sometimes 
combined into one scheme?

3. [10 pts] Typically, why must the TLB be flushed on a context switch?  
What feature must the hardware provide in the TLB to obviate flushing on 
context switch?  Why?

4. [McKusick] [10 pts] When a process exits, all its pages may not be 
placed immediately on the memory free list.  Explain this behavior.

5. [McKusick] [15 pts] What is the relationship between paging and 
swapping on a demand-paged virtual-memory system?  Explain whether it is 
desirable to provide both mechanisms in the same system.

6. [15 pts] Unix provides a flag for the mmap() system call, 
MAP_PRIVATE, which maps a file as copy-on-write.  Explain how this might 
work by describing how the OS would manage the page tables and virtual 
memory of a process which calls (1) mmap() with the MAP_PRIVATE flag, 
then (2) reads some memory from the mmap()ed page, then (3) reads the 
same memory again, and then (4) writes to the memory.  (Suggestion: type 
'man mmap' into a Unix shell to get the manual page for mmap().)

7. [Tanenbaum] [15 pts] A group of operating system designers for the 
Frugal Computer Company are thinking about ways of reducing the amount 
of backing store needed in their new operating system.  The head guru 
has just sugested not bothering to save the program text in the swap 
area at all, but just page it in directly from the binary file whenever 
it is needed.  Are there any problems with this approach?

8. [15 pts] Suppose we devise a new page replacement algorithm, MRU, 
which is like LRU page replacement except that it always replaces the 
most recently used page rather than the least recently used page.  Does 
MRU suffer from Belady's anomaly?  Why or why not?  Describe a sequence 
of page accesses (be sure to specify the number of frames) for which MRU 
performs better than LRU.