Question

Suppose each nonshareable resource in a computer system is classified as a level 1 , level 2 , or level 3 resource. Moreover, suppose each process in the system is required to request the resources it needs according to this classification. That is, it must request all the required level 1 resources at once before requesting any level 2 resources. Once it receives the level 1 resources, it can request all the required level 2 resources, and so on. Can deadlock occur in such a system? Why or why not?

Short answer

Deadlock cannot occur because the defined resource request order prevents circular wait and hold and wait conditions.

Step-by-step solution

Understanding the Resource Request Order

The problem states that each process can only request resources in increasing order of levels, specifically from level 1 to level 3. This means a process must acquire all level 1 resources it needs before it moves on to request level 2 resources, and similarly, it must have all level 2 resources before requesting level 3 resources.

Condition for Deadlock

Deadlock occurs when four conditions are true simultaneously: mutual exclusion, hold and wait, no preemption, and circular wait. In this system, mutual exclusion and no preemption are usually assumed to hold as resources are nonshareable and cannot be forcibly taken from processes.

Hold and Wait Condition

For hold and wait to occur, a process must be holding some resources while waiting for others. In this scenario, since a process must acquire all resources of a lower level before requesting higher levels, it will not hold lower-level resources while waiting for higher-level ones to satisfy the hold and wait condition.

Circular Wait Condition

The request strategy stipulated prevents circular wait. Since processes request resources in a predetermined global order (level 1, then level 2, then level 3), it's impossible to have a sequence of processes each holding a resource needed by the next one, thereby forming a circle of dependency.

Conclusion

Given the enforced order of acquiring resources based on levels, both the hold and wait condition and the circular wait condition are effectively prevented. Therefore, deadlock cannot occur in this system.

Key concepts

Resource Allocation Order

In a computer system, resources such as CPUs, memory, and printers, are often in high demand. To manage these resources efficiently, a defined order of request known as a resource allocation order is often utilized. This order ensures that processes request resources systematically, which can help prevent potential deadlocks. Consider a system where resources are classified into different levels such as level 1, level 2, and level 3. A resource allocation order involves processes requesting all required resources at one level before moving to the next one.

• A process must request all needed level 1 resources before any level 2 resources.
• Likewise, it must acquire all level 2 resources before requesting any level 3 resources.

This orderliness helps in avoiding the overlap in requests, which can lead to deadlocks. By sticking to an established resource allocation order, it ensures that each process waits for its turn to access the resources it requires, thereby fostering smooth system operation.

Deadlock Conditions

Deadlock is a critical situation in computer systems where processes halt indefinitely due to blocked resources. There are four conditions, if all exist simultaneously, which result in a deadlock: mutual exclusion, hold and wait, no preemption, and circular wait.

• Mutual Exclusion: Resource can't be shared among multiple processes simultaneously.
• Hold and Wait: A process holding one resource is waiting to acquire additional resources that are currently being held by other processes.
• No Preemption: Once a process acquires a resource, it cannot be forcibly taken away.
• Circular Wait: A closed loop of processes exists where each process holds at least one resource needed by the next process in the loop.

These conditions are crucial to understand, as preventing one or more can break the deadlock. In systems with a structured approach for resource request and acquisition, the likelihood of deadlock is reduced.

Hold and Wait Condition

The hold and wait condition is a scenario where a process is in possession of some resources while waiting for others that are held by other processes. This situation can lead to deadlock because a process cannot move forward without the additional resources, and it also blocks other processes from using its currently held resources. In a structured system following resource allocation order, hold and wait is less likely to occur.

• Processes are required to request all resources at a particular level before moving to the next level.
• This means they do not hold resources while waiting for others at a different level.

By ensuring a rigid order of resource requests, hold and wait conditions are effectively minimized, making the system less prone to deadlocks.

Circular Wait Condition

Circular wait occurs when there is a closed chain of processes, where each process holds at least one resource the next process in the sequence requires. This creates a cycle of dependency, where no process can proceed until others release required resources, leading to a deadlock. In systems with a predetermined resource allocation order, circular wait is avoided because resource requests are made in a global order.

• Each process moves in a linear fashion from lower-level to higher-level resources.
• Due to this linear progression, there cannot be a cycle of resource holding, as requested resources follow a single chain of dependency.

Thus, the occurrence of circular waits is prevented, ensuring system processes continue without interruption or deadlock.