CS381 Homework - Suraj @ LUMS

Summer 2005-2006
Total Marks: 40
CS 381
Homework
Due Date: Thursday, July 06, 2006 @ 5 pm
1. [10]
You are given the following preemptive priority scheduling algorithm based on
dynamically changing priorities. Larger priority numbers imply higher priority. When a
process is waiting for the CPU (in the ready queue, its priority increases at rate A (P' = P
+ A). When it is running, its priority changes at rate B (P' = P + B). The parameters A
and B can be set to give many different scheduling algorithms.
1. What is the algorithm that results from B > A > 0
2. What is the algorithm that results from A < B < 0
2. [10]
A student majoring in anthropology and minoring in computer science has embarked on a
research project to see if African baboons can be taught about deadlocks. He locates a
deep canyon and fastens a rope across it, so the baboons can cross hand-over-hand.
Several baboons can cross at the same time, provided that they are all going in the same
direction. If eastward moving and westward moving baboons ever get onto the rope at the
same time, a deadlock will result (the baboons will get stuck in the middle) because it is
impossible for one baboon to climb over another one while suspended over the canyon. If
a baboon wants to cross the canyon, it must check to see that no other baboon is currently
crossing in the opposite direction. Give the pseudo code that avoids deadlock using
semaphores. Do not worry about a series of eastward moving baboons holding up the
westward moving baboons indefinitely (starvation).
3. [5]
Explain why spinlocks are not appropriate for single processor systems but are often used
in multiprocessor systems.
4. [5]
Explain the differences in degree to which the following scheduling algorithm
discriminate against or in favor of short processes.
• FCFS
• RR
• Multilevel feedback queue

Summer 2005-2006
5. [10]
Consider a system consisting of m resources of the same type being shared by n
processes. Resources can only be requested and released by processes one at a time (i.e.
they are non-sharable type). Show that the system is deadlock free if the following two
conditions hold:
• The maximum need of each process is between 1 and m resources
• The sum of all maximum needs is less than m+n
Hint: Show that the there exists a process P i whose Need i is zero. (I.e. it has at least one
resource it can release and hence the system cannot be in a deadlock state.)
Submission Instructions:
o Instructions about how and where to submit your homework will be posted on the
course website.