Long-Term Scheduling in Operating Systems: Definition, Role, and Examples

In an Operating System (OS), process scheduling plays a vital role in ensuring efficient CPU utilization and system performance. Scheduling is generally classified into three levels:

1. Long-term scheduling

2. Medium-term scheduling

3. Short-term scheduling

Among these, Long-Term Scheduling (LTS) is the first decision-making stage that controls which jobs enter the system for processing.

What is Long-Term Scheduling?

Long-Term Scheduling is the process of selecting jobs from the job pool (in secondary storage) and loading them into main memory for execution.

• It decides how many jobs should be admitted into the system.

• Helps in maintaining a balanced mix of I/O-bound and CPU-bound processes.

• Directly impacts system throughput, degree of multiprogramming, and system load.

👉 It is also called Job Scheduling.

Working of Long-Term Scheduling

1. Jobs are submitted by users and placed in a job queue (on disk).

2. The long-term scheduler (job scheduler) selects a set of jobs based on scheduling policies.

3. Selected jobs are moved into main memory, where they become ready processes.

4. These processes are later handled by the short-term scheduler for CPU allocation.

Objectives of Long-Term Scheduling

• Maintain a good degree of multiprogramming (number of jobs in memory).

• Ensure CPU utilization is high.

• Prevent I/O or CPU bottlenecks by balancing process types.

• Control the system load (too many jobs → thrashing; too few jobs → underutilization).

Characteristics of Long-Term Scheduling

• Frequency – Invoked less frequently than short-term scheduling.

• Decision basis – Works on a job pool (disk queue).

• Granularity – Deals with entire jobs, not CPU bursts.

• Impact – Strong influence on system performance and responsiveness.

Example of Long-Term Scheduling

Suppose 10 jobs are waiting in the disk queue:

• 6 jobs are CPU-bound (require more CPU time).

• 4 jobs are I/O-bound (frequently perform I/O operations).

If the scheduler only selects CPU-bound jobs, the I/O devices will remain idle. If only I/O-bound jobs are selected, the CPU will remain idle.

👉 Therefore, the long-term scheduler picks a balanced mix (e.g., 3 CPU-bound + 2 I/O-bound jobs).

Long-Term vs Medium-Term vs Short-Term Scheduling

Advantages of Long-Term Scheduling

1. Prevents system overload.

2. Maintains balance between CPU-bound and I/O-bound jobs.

3. Improves overall throughput.

4. Reduces risk of starvation of certain jobs.

Disadvantages of Long-Term Scheduling

1. Less flexible – Once jobs are admitted, they cannot be easily removed.

2. Not suitable for interactive/time-sharing systems (used mainly in batch systems).

3. Poor long-term scheduling decisions → system inefficiency.

Important Exam Pointers

• Long-term scheduling = job scheduling (admission control).

• Controls degree of multiprogramming.

• Frequency: least frequent compared to other scheduling types.

• Balances between CPU-bound and I/O-bound processes.

• Found in batch processing systems, rarely in time-sharing OS.

Conclusion

Long-Term Scheduling is the first stage of process scheduling that decides which jobs will enter main memory for execution. By carefully balancing I/O-bound and CPU-bound jobs, it ensures efficient CPU utilization and avoids system overload.

For competitive exams, focus on:

• Definition & role of LTS.

• Comparison with short-term & medium-term scheduling.

• Impact on degree of multiprogramming.

• Examples of job selection policies.