Operating System Fundamentals: Scheduling Algorithms and Deadlock Management

Posted on Nov 9, 2025 in Telecommunication Sciences and Technologies

CPU Scheduling: Core Concepts and Algorithms

CPU scheduling is a critical component of Operating System design. It involves allocating the CPU’s processing time to different processes or threads, ensuring efficient use of system resources and improving system performance.

What is CPU Scheduling?

CPU scheduling determines which process should be executed next by the CPU, allocating CPU time to processes.

Key Goals of CPU Scheduling

1. Maximize CPU Utilization: Keep the CPU busy as much as possible.
2. Minimize Response Time: Reduce the time it takes for a process to respond to user input.
3. Minimize Turnaround Time: Reduce the time it takes for a process to complete from arrival.
4. Maximize Throughput: Increase the number of processes completed per unit time.

Types of Scheduling

1. Preemptive Scheduling: The OS can interrupt a running process and allocate CPU time to another process.
2. Non-Preemptive Scheduling: A process runs until it completes execution or voluntarily yields the CPU.

Common Scheduling Algorithms

1. First-Come-First-Served (FCFS): Processes are executed in the order they arrive.
2. Shortest Job First (SJF): The process with the shortest execution time is executed first.
3. Priority Scheduling: Processes are assigned priorities, and the highest-priority process is executed first.
4. Round Robin (RR): Each process is allocated a fixed time slice (time quantum) before the next process is executed.
5. Multilevel Feedback Queue: Processes are divided into multiple queues based on priority and burst time.

Scheduling Criteria

1. Throughput: The number of processes completed per unit time.
2. Turnaround Time: The total time taken for a process to complete from its arrival.
3. Waiting Time: The time a process spends waiting in the ready queue.
4. Response Time: The time taken for a process to respond to an input.

Importance of CPU Scheduling

• Efficient Resource Utilization: CPU scheduling ensures efficient use of CPU resources.
• Improved System Performance: CPU scheduling optimizes system performance by allocating CPU time effectively.
• Enhanced User Experience: CPU scheduling enables multiple applications to run concurrently, improving user experience.

Deeper Dive into Scheduling Mechanisms

Levels of Scheduling

1. Long-term Scheduling: Determines which processes are admitted to the system (job scheduler).
2. Medium-term Scheduling: Determines which processes are swapped in or out of memory.
3. Short-term Scheduling: Determines which process is executed next by the CPU (CPU scheduler).

Advanced Scheduling Algorithms

1. Multilevel Queue: Processes are divided into multiple queues based on priority.
2. Multilevel Feedback Queue: Processes are divided into multiple queues based on priority and burst time, allowing processes to move between queues.

Multiple Processor Scheduling

1. Symmetric Multiprocessing (SMP): Multiple processors share a common memory and I/O devices.
2. Asymmetric Multiprocessing: Each processor has its own memory and I/O devices.
3. Scheduling Algorithms: Modified versions of single-processor scheduling algorithms, such as distributed scheduling, are used.

Algorithm Evaluation Methods

1. Performance Metrics: Analyzing throughput, turnaround time, waiting time, and response time.
2. Simulation: Modeling system behavior to evaluate scheduling algorithms under various loads.
3. Queueing Theory: Using mathematical models to analyze system performance characteristics.

Process Synchronization

Synchronization is crucial in operating systems, ensuring that multiple processes can access shared resources safely and efficiently.

The Critical Section Problem

1. Critical Section: A shared resource or code segment that only one process can access at a time.
2. Mutual Exclusion: Ensuring only one process can access the critical section at any given moment.
3. Synchronization: Coordinating processes to access shared resources without conflicts.

Semaphores

1. Semaphore Definition: A variable or data structure that controls access to a shared resource.
2. Types: Binary semaphores (values 0 or 1) and counting semaphores (integer value).
3. Operations: Wait (P) and Signal (V) operations to acquire and release semaphores, respectively.

Classical Problems of Synchronization

1. Producer-Consumer Problem: Coordinating producers and consumers accessing a shared buffer.
2. Reader-Writer Problem: Coordinating readers and writers accessing a shared resource.
3. Dining Philosophers Problem: Coordinating philosophers accessing shared resources (forks).

Monitors

1. Monitor Definition: A high-level synchronization construct that provides mutual exclusion and condition variables.
2. Condition Variables: Allow processes to wait or signal based on specific conditions within the monitor.
3. Benefits: Simplifies synchronization implementation and reduces common programming errors.

Importance of Synchronization

• Data Consistency: Ensures shared data is accessed and modified consistently.
• System Stability: Prevents system crashes and deadlocks caused by synchronization issues.
• Efficient Resource Utilization: Enables multiple processes to share resources efficiently.

Deadlock Management in Operating Systems

Deadlock management is crucial in operating systems, ensuring that systems remain stable and efficient.

Deadlock Characterization

A deadlock is a situation where two or more processes are blocked indefinitely, each waiting for the other to release a resource. This creates a cycle of dependency, where none of the processes can proceed, resulting in a system deadlock.

Necessary Conditions for Deadlock

1. Mutual Exclusion: Two or more processes require exclusive access to a common resource.
2. Hold and Wait: Processes hold onto resources while waiting for other resources.
3. No Preemption: Resources cannot be forcibly taken away from a process.
4. Circular Wait: Processes wait for each other to release resources, creating a cycle.

Deadlock Example

Suppose two processes, P1 and P2, require access to resources R1 and R2:

• P1 holds R1 and waits for R2.
• P2 holds R2 and waits for R1.

In this scenario, both processes are deadlocked, each waiting for the other to release a resource.

Methods for Handling Deadlocks

1. Deadlock Prevention: Ensuring that one of the necessary conditions is never met.
2. Deadlock Avoidance: Carefully allocating resources to ensure the system never enters an unsafe state.
3. Deadlock Detection: Detecting deadlocks and taking action to recover.
4. Ignore Deadlocks: Ignoring deadlocks if they are rare and system restarts can resolve them (often used in personal computing operating systems).

Deadlock Prevention Strategies

1. Mutual Exclusion: Allow shared access to resources or use synchronization mechanisms where possible.
2. Hold and Wait: Ensure processes request all resources at once or release resources before requesting new ones.
3. No Preemption: Allow preemption of resources or use priority-based scheduling.
4. Circular Wait: Order resources in a linear sequence to prevent circular waits.

Deadlock Avoidance Techniques

1. Safe State: Ensuring the system is always in a safe state, where deadlocks are impossible.
2. Banker’s Algorithm: A resource allocation algorithm that checks for safe states before granting resource requests.

Deadlock Detection and Recovery

1. Detection Algorithms: Using algorithms, such as cycle detection in resource allocation graphs, to identify deadlocked processes.
2. Recovery: Recovering from deadlocks by aborting processes, releasing resources, or rolling back processes to a safe state.

Importance of Deadlock Management

• System Stability: Prevents system crashes and instability due to deadlocks.
• Resource Utilization: Ensures efficient use of resources by preventing processes from being indefinitely blocked.
• System Reliability: Improves overall system reliability by handling resource conflicts effectively.