UNIVERSITY OF WISCONSIN-MADISON
Computer Sciences Department
Operating Systems Spring 2009
Instructions: There are six questions on this exam; you must answer all of the following questions.
Question 1: Think about Synch-ronization
This question is about synchronization in multi-threaded programs.
Composability is always a problem for monitor-based synchronization systems (e.g. Mesa). Suppose procedure P1
of monitor M1 calls procedure P2 of monitor M2 and P2 executes a "wait"; what would the monitor system do at this
point? What are the potential problems?
1.
Deadlock is a potentially nasty problem one encounters in multi-threaded programs. Please list the necessary
conditions for a deadlock to occur.
2.
Can the following approaches guarantee that deadlock will be avoided or recovered from? Please explain, using
examples to support your answers.
Never do context switch inside a critical sectionA.
Whenever a deadlock occurs, roll back the program to an earlier checkpoint outside any critical section and
re-execute the program
B.
3.
Question 2: NFS - Not-another File System question!
This question is about NFS version 2, the classic stateless distributed file systems protocol from Sun.
An application on a client issues a write to a file. That write is buffered in client memory for some time. Why do NFS
clients do this? What are the advantages and disadvantages?
1.
The client at some point decides to issue the write to the server. When does the client usually do this? Why?2.
The write reaches the server, and the server writes it to disk immediately, before acknowledging the request. Why
does the server do this? What if the server ack'd the request and then wrote the data to disk?
3.
The ack from the server gets lost, and the client resends the write protocol request. What happens in this case?
(and why does it work?) What could the server do to improve performance in this situation?
4.
Question 3: Kernel.org(anization)
Many important operating system kernels were designed in the context of a uniprocessor or a system with a small number
of processors.
List and describe three impacts that the number of processors have on operating system kernel design.1.
Now that you have done part 1, let's get more specific. For parts 2 and 3 of this question, answer the questions about any
two of these important systems:
THE
Mach
exokernel
Does the existence of a large number of processor cores improve the design, providing additional benefits beyond
those on a single processor? If so, under what circumstances?
2.
Do additional processors expose flaws in the design? If so, under what circumstances.3.
