Short Term vs Long Term Scheduler: Difference and Comparison

The short-term scheduler, also known as the CPU scheduler, selects from among the processes that are ready to execute and allocates CPU time to them, managing the execution of processes in the immediate future. In contrast, the long-term scheduler, or admission scheduler, determines which processes are brought into the ready queue from the pool of all processes, balancing system load and ensuring efficient resource utilization over the long run.

Key Takeaways

1. A short-term or CPU scheduler allocates system resources to processes ready to execute. In contrast, a long-term scheduler, also known as a job scheduler, determines which processes should be admitted to the system.
2. Short-term scheduler focuses on maximizing CPU utilization and minimizing wait time for processes. In contrast, a long-term scheduler focuses on ensuring the system is manageable with only a few processes simultaneously.
3. Short-term and long-term schedulers are important in managing system resources and ensuring efficient performance.

Short Term vs Long Term Scheduler

Short term scheduler selects the process from the ready queue which needs to be executed. The process is sent by the short term scheduler to the CPU. CPU scheduler is another name of short term scheduler. Long term scheduler place the process in the ready queue. Job scheduler is another name of a long term scheduler.

Schedules work in tandem to facilitate process scheduling. Short-term and Long-term Schedulers are the system software that handles the process scheduling effectively.

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Comparison Table

Feature	Short Term Scheduler (CPU Scheduler)	Long Term Scheduler (Job Scheduler)
Function	Selects the next process from the ready queue to be allocated the CPU	Selects jobs from the job pool and admits them to the system, placing them in the ready queue
Queue Used	Ready Queue (processes in memory, ready to run)	Job Pool (all submitted jobs)
Focus	CPU utilization (maximize CPU usage by keeping it busy)	Multiprogramming level (control the number of processes in the system)
Selection Criteria	Priority, Round Robin, Shortest Job First (SJF), etc.	Various factors, such as priority, memory requirements, resource availability
Execution Speed	Very fast (needs to happen frequently)	Slower (doesn’t need to happen as often)
Impact on User Experience	Determines responsiveness of the system (how quickly processes get CPU time)	Affects throughput (number of jobs completed over time)
Example	First Come First Served (FCFS), Shortest Remaining Time First (SRTF), Priority Scheduling	Multilevel Queue Scheduling, Priority Based Admission Control
Present in	All operating systems	Not present in time-sharing systems (processes do not leave memory)

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What is Short Term Scheduler?

Functionality

The primary function of the short-term scheduler is to select the next process from the pool of ready processes and allocate CPU time to it. This decision-making process occurs frequently, with a time quantum ranging from milliseconds to microseconds. The scheduler must balance competing priorities such as process priority levels, I/O operations, and overall system load to optimize system performance.

Characteristics

1. Fast Execution: The short-term scheduler operates quickly, as it needs to make rapid decisions regarding process execution. It must efficiently manage the CPU to prevent idle time and maximize throughput.
2. Priority-Based Scheduling: Many short-term schedulers employ priority-based scheduling algorithms to determine which process to execute next. Higher-priority processes may receive preferential treatment to meet critical system requirements or user expectations.
3. Preemption: Preemption is a key feature of the short-term scheduler, allowing it to interrupt the execution of a lower-priority process to allocate CPU time to a higher-priority process. This ensures that important tasks are executed promptly, contributing to system responsiveness.

Importance

The short-term scheduler significantly influences the overall performance and responsiveness of the operating system. By efficiently managing CPU resources and prioritizing processes based on their importance, it helps maintain system stability and user satisfaction. Additionally, effective short-term scheduling contributes to optimal resource utilization, enhancing the efficiency of the computing environment.

 

What is Long Term Scheduler?

Functionality

The primary function of the long-term scheduler is to manage the influx of processes into the system, determining which processes should be admitted for execution based on various criteria such as system resources, priorities, and scheduling policies. This scheduler controls the degree of multiprogramming, influencing factors such as system throughput, response time, and overall system performance.

Characteristics

1. Job Selection: The long-term scheduler selects processes from the pool of incoming jobs and admits them to the system based on predefined criteria. These criteria may include factors such as process priority, resource requirements, and system load. By carefully selecting which processes to admit, the long-term scheduler aims to maintain a balance between system responsiveness and resource utilization.
2. Load Balancing: Long-term scheduling plays a crucial role in load balancing by distributing incoming processes evenly across available resources. It prevents system overload by regulating the rate at which new processes are admitted, ensuring that system resources are not overwhelmed and that all processes receive adequate attention.
3. Resource Management: The long-term scheduler coordinates with other system components to manage system resources effectively. It considers factors such as CPU availability, memory usage, and I/O device utilization when admitting new processes, optimizing resource allocation to enhance system performance and stability.

Importance

The long-term scheduler is essential for maintaining system stability, preventing resource contention, and maximizing resource utilization in the operating system. By controlling the influx of processes and regulating system load, it helps ensure fair access to resources and maintains overall system efficiency. Effective long-term scheduling contributes to improved system throughput, reduced response times, and enhanced user experience in multi-user and multitasking environments.

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Main Differences Between Short-Term and Long-Term Schedulers

1. Scope:
   • Short-Term Scheduler:
     • Operates at a rapid pace, making frequent decisions on which ready process to execute next.
     • Focuses on immediate allocation of CPU time to ready processes.
   • Long-Term Scheduler:
     • Operates at a higher level, determining which processes should be admitted into the system for execution.
     • Balances system load and ensures efficient resource utilization over the long run.
2. Timeframe:
   • Short-Term Scheduler:
     • Deals with decisions that are made frequently, with a time quantum ranging from milliseconds to microseconds.
     • Makes rapid decisions to manage CPU resources and maintain system responsiveness.
   • Long-Term Scheduler:
     • Deals with decisions that are made less frequently, when new processes enter the system.
     • Controls the influx of processes into the system, influencing factors such as system throughput and resource utilization.
3. Functionality:
   • Short-Term Scheduler:
     • Selects the next process from the pool of ready processes and allocates CPU time to it.
     • Primarily concerned with CPU resource management and process execution.
   • Long-Term Scheduler:
     • Selects which processes from the pool of incoming jobs should be admitted to the system for execution.
     • Balances system load, manages system resources, and regulates the rate at which new processes are admitted.

References

1. https://ieeexplore.ieee.org/abstract/document/5532867/
2. https://arxiv.org/abs/1111.5348

Last Updated : 04 March, 2024