Short-Term, Long-Term, and Mid-Term Scheduling | Operating system |

Types of Scheduling in Operating Systems: Short-Term, Long-Term, and Mid-Term Scheduling

Scheduling is the heart of an operating system (OS), ensuring efficient resource allocation, smooth multitasking, and optimized system performance. In essence, scheduling determines how processes are prioritized and executed by the CPU.

This article dives into the three primary types of scheduling in operating systems: Short-Term Scheduling, Long-Term Scheduling, and Mid-Term Scheduling. With detailed explanations, examples, and scenarios, let’s unravel how these scheduling techniques contribute to seamless process management.

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Overview of Scheduling

At its core, scheduling is the process of deciding which tasks (or processes) should execute at a given time. This decision-making involves selecting processes from queues, allocating CPU time, and managing system resources effectively.

There are three main levels of scheduling based on when decisions are made and which processes are handled:

1. Long-Term Scheduling: Manages which processes enter the system.
2. Mid-Term Scheduling: Temporarily removes or swaps processes to optimize resource usage.
3. Short-Term Scheduling: Decides which process gets the CPU next.

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1. Long-Term Scheduling: Controlling System Workload

Definition:

Long-term scheduling, also known as job scheduling, determines which processes are admitted to the system for processing. It regulates the degree of multiprogramming (the number of processes in memory simultaneously).

How It Works:

• When a user submits a process, it first enters the job queue.
• The long-term scheduler selects jobs from this queue and moves them to the ready queue for execution.
• The scheduler ensures a balance between CPU-bound and I/O-bound processes to maximize efficiency.

Example:

Imagine a library where visitors (processes) queue up to enter. The librarian (long-term scheduler) decides how many visitors can be admitted based on available seating (system resources).

Characteristics:

• Frequency: Runs infrequently since it only deals with newly arrived processes.
• Responsibility: Controls the overall system workload.
• Focus: Balancing system resources and preventing overload.

Real-Life Scenario:

In a batch processing system, users submit multiple jobs. The long-term scheduler decides which jobs to start based on system load and job priority.

Advantages:

• Prevents system overloading.
• Ensures a balanced mix of processes for efficient CPU and I/O utilization.

Disadvantages:

• May introduce delays if the queue grows too long.
• Limited flexibility in real-time environments.

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2. Mid-Term Scheduling: Optimizing Resource Allocation

Definition:

Mid-term scheduling, also known as swap scheduling, temporarily removes processes from memory (swaps them out) and stores them on secondary storage (disk). This process, known as swapping, helps optimize system performance by managing memory usage.

How It Works:

• When the system is overloaded or a process is idle (e.g., waiting for I/O), the mid-term scheduler removes it from memory.
• The swapped-out process is reloaded later when system resources are available or when the process is ready to execute again.

Example:

In a factory, some workers (processes) are sent on a break (swapped out) during peak workload to free up space for urgent tasks. They return when the workload stabilizes.

Characteristics:

• Frequency: Runs more frequently than long-term scheduling but less often than short-term scheduling.
• Responsibility: Balances memory usage and avoids resource contention.
• Focus: Managing active processes to optimize memory and CPU utilization.

Real-Life Scenario:

In a time-sharing system, when memory is full, inactive processes are swapped to the disk to make room for active ones.

Advantages:

• Improves system performance under heavy loads.
• Frees up memory for critical processes.

Disadvantages:

• Disk I/O for swapping adds overhead.
• May lead to delays if a swapped-out process is urgently required.

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3. Short-Term Scheduling: Allocating CPU Time

Definition:

Short-term scheduling, also known as CPU scheduling, decides which process in the ready queue will execute next. This decision happens frequently and is critical for responsive and efficient CPU usage.

How It Works:

• Processes waiting in the ready queue are prioritized based on a scheduling algorithm (e.g., Round Robin, FCFS, or Priority Scheduling).
• The short-term scheduler selects a process and assigns the CPU for execution.

Example:

In a call center, the next customer (process) is selected from a queue based on specific rules (e.g., priority or arrival time).

Characteristics:

• Frequency: Runs very frequently (milliseconds).
• Responsibility: Ensures the CPU remains busy by minimizing idle time.
• Focus: Maximizing throughput and minimizing response time.

Real-Life Scenario:

In a real-time system, the short-term scheduler must prioritize critical tasks, such as handling sensor data or controlling machinery.

Advantages:

• Ensures efficient CPU utilization.
• Provides quick responses to interactive users.

Disadvantages:

• Frequent context switching may introduce overhead.
• Performance depends heavily on the scheduling algorithm used.

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Comparison of Scheduling Types

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Scheduling Algorithms

Common Algorithms Used in Scheduling Levels:

1. Long-Term Scheduling:

• First-Come, First-Served (FCFS)
• Priority-based selection.

1. Mid-Term Scheduling:

• Based on memory and I/O requirements.

1. Short-Term Scheduling:

• Round Robin
• Shortest Job Next (SJN)
• Priority Scheduling
• Multilevel Queue Scheduling

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Real-World Analogy

A Restaurant Kitchen:

• Long-Term Scheduling:
• Decides how many orders (jobs) the kitchen can handle.
• Mid-Term Scheduling:
• If the kitchen gets overwhelmed, some dishes (processes) are set aside temporarily.
• Short-Term Scheduling:
• The chef (CPU) decides which dish to prepare next, ensuring the kitchen operates efficiently.

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Conclusion

Scheduling is an essential function of operating systems, enabling efficient resource utilization and smooth multitasking. The Long-Term Scheduler regulates the workload, the Mid-Term Scheduler optimizes resource usage, and the Short-Term Scheduler ensures quick, responsive CPU allocation.

Each type of scheduling plays a distinct role, and understanding these concepts is crucial for anyone studying OS design or performance optimization.

Would you like to dive deeper into specific scheduling algorithms or examples? Let me know!