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.

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 Programming

Implement the Bankers algorithm.

Ex. 4 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. 5 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. 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?