Techniques Used to Resolve Starvation in Operating Systems

Starvation is a resource management problem in operating systems where a process does not get the resources it needs for a long time. The most effective technique used to resolve starvation involves prioritizing waiting processes to ensure they eventually receive system resources.

Quick Summary

Starvation, a scheduling problem where low-priority processes are indefinitely denied resources, can be resolved with techniques like aging, which boosts a process's priority over time. Other solutions include fair scheduling algorithms like Round Robin and specialized mechanisms such as Priority Inheritance to prevent high-priority tasks from waiting on lower-priority ones.

Key Points

• Aging: A primary technique to resolve starvation by gradually increasing the priority of a process the longer it waits for resources.

• Round Robin: A scheduling algorithm that naturally prevents starvation by giving every process a small, equal time slice in a cyclic order.

• Priority Inheritance: A method used in real-time systems to temporarily boost the priority of a low-priority task holding a resource needed by a high-priority task, preventing priority inversion.

• Fair Scheduling: Employing algorithms that ensure all processes, regardless of priority, eventually get a turn, preventing indefinite postponement.

• Resource Management: Implementing policies like setting resource quotas or using a dedicated manager to fairly distribute resources and prevent monopoly.

• Multi-level Feedback Queues: Using a scheduling algorithm that moves processes between queues of different priorities, promoting lower-priority processes over time.

Understanding Starvation and its Causes

In a multi-process operating system, scheduling algorithms determine which process gets access to the CPU and other resources. Starvation, also known as indefinite blocking or indefinite postponement, occurs when a process is repeatedly passed over and prevented from gaining access to the resources it needs, even though the resources are being constantly allocated to other processes. This is most common in systems that use a priority-based scheduling algorithm, where a continuous stream of higher-priority tasks can perpetually deny resources to a lower-priority task.

Common Causes of Starvation

• Pure Priority-Based Scheduling: In a system where processes are scheduled strictly by priority, a low-priority task might never get a chance to run if there is a constant arrival of higher-priority tasks.
• Random Selection: If the scheduler uses a random selection process to choose the next task, it is statistically possible, though unlikely, for a specific process to be repeatedly overlooked.
• Resource Hoarding: One or more processes can monopolize a resource for a long time, preventing other processes from accessing it and leading to starvation.

Key Techniques to Resolve Starvation

To combat the unfairness of starvation, operating systems employ several techniques to ensure that every process eventually gets a fair share of resources. The primary method is a mechanism known as aging, but other strategies are also effective.

Aging

Aging is a fundamental technique designed to prevent starvation in priority-based scheduling systems. The core concept is to gradually increase the priority of a process the longer it waits in the system. This ensures that a task that has been waiting indefinitely will eventually have its priority elevated high enough to be selected for execution.

How Aging Works:

1. Initial Priority: A new process enters the ready queue with a set priority.
2. Incrementing Priority: The scheduler periodically checks the waiting time of all processes. For processes that have been waiting for a certain period (e.g., every 15 minutes), their priority is incremented.
3. Guaranteed Execution: Over time, even the lowest-priority process will see its priority rise, eventually reaching a level high enough to get executed.

Round Robin Scheduling

Round Robin (RR) is a scheduling algorithm that provides a straightforward way to avoid starvation by giving every process a fair turn. In this method, a small, fixed time slice (time quantum) is allocated to each process in a cyclic order. If a process does not complete its execution within its time quantum, it is preempted and moved to the back of the ready queue.

Why Round Robin Prevents Starvation:

• Cyclic Allocation: Since the CPU is allocated to each process in a circular fashion, every process gets a chance to execute, eliminating the possibility of indefinite blocking.
• No Priority Bias: The algorithm does not rely on process priorities for scheduling decisions, thus removing the main cause of starvation in priority-based systems.

Priority Inheritance

Priority inheritance is a specific solution used primarily in real-time operating systems to handle a problem called priority inversion, which can be a precursor to starvation. Priority inversion occurs when a high-priority task is blocked by a low-priority task that holds a required resource.

How Priority Inheritance Works:

1. High-Priority Block: A high-priority task (H) needs a resource currently held by a low-priority task (L).
2. Temporary Priority Elevation: The priority of the low-priority task (L) is temporarily raised to that of the high-priority task (H).
3. Resource Release: Task L completes its critical section at this elevated priority, releasing the resource quickly.
4. Priority Restoration: After releasing the resource, task L's priority is restored to its original level, and task H can now acquire the resource and proceed.

Comparison of Starvation Resolution Techniques

Aspect	Aging	Round Robin	Priority Inheritance
Primary Goal	Prevents indefinite waiting by boosting priority over time.	Ensures fairness by giving equal time slices to all processes.	Solves priority inversion issues in real-time systems.
Mechanism	Dynamically increases waiting processes' priorities.	Cycles through processes, allocating a fixed time quantum.	Temporarily raises a low-priority task's priority to that of a waiting high-priority task.
Best For	Systems with priority-based scheduling to prevent long-term starvation.	General-purpose computing to provide a fair turnaround time for all tasks.	Real-time systems to prevent high-priority tasks from being blocked by low-priority tasks.
Implementation Complexity	Moderate, requires a mechanism to track waiting time.	Simple, requires a queue and a timer.	Complex, requires careful management of task priorities and resource locks.
System Impact	Balances fairness with priority scheme, adding some overhead.	Ensures fairness but can increase context-switching overhead with a small time quantum.	Minimizes priority inversion effects but can add overhead and complexity.

Other Solutions and Best Practices

In addition to the main techniques, several other strategies contribute to preventing starvation and improving overall system fairness and responsiveness. For example, using a multi-level feedback queue scheduling algorithm can help balance system throughput with low-priority process execution by moving processes between queues of different priorities based on their behavior and wait times.

Another approach is to design fair resource allocation policies that prevent any single process from monopolizing a resource. This could involve setting quotas or limits on how long a process can hold a resource. Additionally, avoiding random resource selection and using a structured approach to resource management is crucial for preventing unfair scheduling outcomes.

Conclusion

Resolving starvation is a key concern for any robust operating system, ensuring fairness and preventing system degradation. Techniques such as aging, round robin scheduling, and priority inheritance offer effective strategies for mitigating starvation, each suited to different system requirements. Aging directly addresses the issue by elevating the priority of long-waiting processes, while round robin provides an inherently fair allocation of CPU time. Priority inheritance, though more complex, is a targeted solution for real-time systems where priority inversion could otherwise cripple critical tasks. By implementing a combination of these and other fair resource management practices, developers can create more reliable and responsive systems that ensure all processes receive the attention they need.

Related Resources

For deeper insights into priority inheritance protocols and their implementation, explore this resource on real-time scheduling concepts: Priority inheritance protocols: An approach to real-time synchronization.

Frequently Asked Questions

What is starvation in the context of an operating system?

Starvation is a situation where a process is repeatedly delayed in gaining access to a required resource because other, typically higher-priority, processes are constantly being served instead.

How does aging specifically prevent starvation?

Aging prevents starvation by incrementally raising the priority of a process the longer it waits in the ready queue. This guarantees that even a low-priority process will eventually have its priority increased enough to be scheduled for execution.

Can Round Robin scheduling cause starvation?

No, Round Robin scheduling inherently prevents starvation. Because it allocates an equal time slice to each process in a rotating sequence, every process is guaranteed to get a turn and is never indefinitely blocked.

How is starvation different from deadlock?

Starvation involves an indefinite waiting period for a process, but the overall system may continue functioning. Deadlock is a more serious issue where a group of processes are all blocked, waiting for resources held by each other, causing the entire system to halt.

What is priority inversion and how does it relate to starvation?

Priority inversion is a scenario where a high-priority task is forced to wait for a low-priority task that holds a necessary resource. This can be a form of starvation, as the high-priority task is indefinitely blocked. Priority inheritance is a specific technique to solve this problem.

In what type of system is priority inheritance most critical?

Priority inheritance is most critical in real-time operating systems where tasks have strict timing deadlines. It is crucial for preventing high-priority, time-sensitive tasks from being delayed by lower-priority tasks, which could lead to system failure.

Are there other scheduling algorithms that prevent starvation?

Yes, other fair scheduling policies can prevent starvation. For example, using a multi-level feedback queue system allows processes to move between priority levels based on their behavior, ensuring that a low-priority process won't be permanently stuck.