VE482 — Introduction to Operating Systems Assignment 5 solved


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Ex. 1 — Simple questions
1. A system has two processes and three identical resources. Each process needs a maximum of two
resources. Can a deadlock occur? Explain.
2. A computer has six tape drives, with n processes competing for them. Each process may need
two drives. For which values of n is the system deadlock free?
3. A real-time system has four periodic events with periods of 50, 100, 200, and 250 msec each.
Suppose the four events require 35, 20, 10, and x msec of CPU time, respectively. What is the
largest value x for which the system is schedulable?
4. Round-robin schedulers normally maintain a list of all runnable processes, with each process occurring exactly once in the list. What would happen if a process occurred more than once in the
list? Would there be any reason for allowing this?
5. Can a measure of whether a process is likely to be CPU bound or I/O bound be detected by
analyzing the source code. How to determine it at runtime?
Ex. 2 — Deadlocks
Assuming three resources consider the following snapshot of a system.
Process Allocated Maximum Available
P1 010 753 332
P2 200 322
P3 302 902
P4 211 222
P5 002 433
1. Determine the content of the Request matrix.
2. Is the system in a safe state?
3. Can all the processes be completed without the system being in an unsafe state at any stage?
Ex. 3 — Research
Write about a page on the topic of viruses, worms and Trojans; In particular explain what they are, how
they are created, and how to avoid them. Do not forget to reference your sources of information.
Ex. 4 — Programming
Implement the Banker’s algorithm.
Ex. 5 — Minix 3
How is scheduling handled in Minix 3? Provide clear explanations on how to find the information just by
exploring the source code of Minix kernel.
Ex. 6 — The reader-writer problem
In the reader-writer problem, some data could be accessed for reading but also sometimes for writing.
When processes want to read the data they get a read lock and a write lock for writing. Multiple processes
could get a read lock at the same time while a write lock should prevent anybody else from reading or
writing the data until the write lock is released.
To solve the problem we decide to use a global variable count together with two semaphores: count lock
for locking the count variable, and db lock for locking the database. To get a write lock we can proceed
as follows:
1 void write“˙lock() –
2 down(db“˙lock);
3 ˝
1 void write“˙unlock() –
2 up(db“˙lock);
3 ˝
1. Explain how to get a read lock, and write the corresponding pseudocode.
2. Describe what is happening if many readers request a lock.
To overcome the previous problem we will block any new reader when a writer becomes available.
3. Explain how to implement this idea using another semaphore called read lock.
4. Is this solution giving any unfair priority to the writer or the reader? Can the problem be considered
as solved?