Resource Allocation Techniques for Processes

1. Introduction to Resource Allocation

Resource allocation in an operating system is about managing how processes get and use system resources. The goal is to maximize system efficiency, prevent problems like deadlocks, and ensure fair access to resources.

2. Types of Resources

Operating systems manage various resources:

  • CPU Cycles: The processing time available from the Central Processing Unit.

  • Memory Space: RAM needed for processes to run.

  • I/O Devices: Hardware like printers, scanners, disk drives, etc.

  • Files: Access to data files.

  • Locks/Semaphores: Software mechanisms used for synchronization.

Resources can be:

  • Preemptable: Can be taken away from a process without causing issues (e.g., CPU time).

  • Non-preemptable: Cannot be taken away without causing failure or major problems (e.g., a printer printing a document).

3. Issues in Resource Management

The primary problem in resource management is deadlock, where a set of processes are blocked indefinitely because each process is waiting for a resource held by another process in the set.

3.1. Necessary Conditions for Deadlock

For a deadlock to occur, all four of these conditions must be present simultaneously:

  1. Mutual Exclusion: At least one resource must be held in a non-shareable mode (only one process can use it at a time).

  2. Hold and Wait: A process is currently holding at least one resource and is waiting to acquire additional resources held by other processes.

  3. No Preemption: Resources cannot be forcibly taken away from a process; they can only be released voluntarily by the process holding them.

  4. Circular Wait: A group of processes exists where each process is waiting for a resource held by the next process in the cycle.

4. Techniques for Handling Deadlocks

Operating systems use several strategies to deal with deadlocks:

4.1. Deadlock Prevention

This approach ensures that at least one of the four necessary conditions for deadlock cannot occur.

  • Eliminating Mutual Exclusion: Not always practical, as some resources are inherently non-shareable (e.g., a printer).

  • Eliminating Hold and Wait: Processes must request and be allocated all their needed resources before starting execution, or release all current resources before requesting new ones.

  • Eliminating No Preemption: If a process requests a resource that is not available, all resources it currently holds are preempted (taken away) and added to the list of resources being waited for. The process restarts only when it can get all its old and new resources.

  • Eliminating Circular Wait: Impose a total ordering of all resource types. Processes must request resources in increasing order of enumeration. For example, if resource R1 is type 1 and R2 is type 2, a process holding R2 cannot request R1, forcing a non-circular request pattern.

4.2. Deadlock Avoidance

This method requires the OS to have some information about future resource requests. It dynamically checks each resource-allocation state to ensure that a circular-wait condition can never arise. A system state is considered "safe" if it's possible to allocate resources to each process in some sequence without causing a deadlock.

  • Banker's Algorithm: A well-known deadlock avoidance algorithm. It requires a process to declare its maximum number of resources of each type that it may need. When a process requests resources, the OS checks if granting the request would leave the system in a safe state. If not, the request is denied, and the process waits.

4.3. Deadlock Detection and Recovery

This approach allows deadlocks to occur, detects them, and then recovers by breaking the deadlock.

  • Detection: The system periodically runs an algorithm to detect if a deadlock has occurred (e.g., by checking for cycles in a resource-allocation graph).

  • Recovery: Once a deadlock is detected, the system can recover by:

    • Process Termination: Abort all processes involved in the deadlock, or abort one process at a time until the deadlock is resolved.

    • Resource Preemption: Take resources away from one or more deadlocked processes and give them to others. This might require rolling back the preempted process to an earlier state.

4.4. Ignoring Deadlocks

In some operating systems (like most versions of Windows/macOS), deadlocks are simply ignored. This is because deadlocks are rare, and the cost of prevention, avoidance, or detection/recovery might outweigh the benefits, especially in personal computing environments where a system restart is a tolerable solution.