In CPU scheduling, what are short-term, medium-term, and long-term scheduling and how do they differ in their roles?

Introduction / Context:
Operating systems must decide not only how to share the CPU among ready processes, but also which jobs to admit into the system and which processes to keep in memory. These decisions are made by different levels of scheduling: long term, medium term, and short term. Understanding the distinct roles of each level helps explain how multiprogramming is controlled and how system responsiveness and throughput are balanced.

Given Data / Assumptions:

• The system maintains a job pool of processes waiting to enter memory.
• Some processes may be swapped out to secondary storage when memory is tight.
• Ready processes compete for the CPU on a short time scale.
• The question asks how short term, medium term, and long term scheduling differ in their roles.

Concept / Approach:
Long term scheduling, also called job scheduling, decides which jobs from the job pool are admitted into the system to become ready processes. It controls the degree of multiprogramming. Medium term scheduling manages swapping by deciding which processes to remove from memory (suspend) and later bring back, balancing memory usage and CPU utilization. Short term scheduling, or CPU scheduling, runs very frequently and selects which ready process in memory should execute next on the CPU. The correct explanation must assign these distinct tasks to the three levels of scheduling.

Step-by-Step Solution:
Step 1: Define long-term scheduling as the mechanism that controls how many jobs are allowed into the system from the job pool, affecting the overall mix of CPU bound and I or O bound jobs.Step 2: Define medium-term scheduling as the decision to swap processes out of main memory and later swap them back in, often to relieve memory pressure or adjust the degree of multiprogramming.Step 3: Define short-term scheduling as the frequent selection of one ready process to run on the CPU next, based on policies such as round robin, priority, or shortest job next.Step 4: Recognize that these three levels operate on different time scales: long term is infrequent, medium term is occasional, and short term is very frequent.Step 5: Conclude that the correct option is the one that matches these established roles.

Verification / Alternative check:
Operating systems references present diagrams showing the flow of jobs from an external job queue into ready, running, blocked, and suspended states. Long term scheduling selects jobs from the external queue to become ready. Medium term scheduling moves processes between ready or blocked states and suspended variants when swapping occurs. Short term scheduling selects among ready processes which one gets the CPU. These diagrams and explanations confirm the respective roles of the three scheduling levels.

Why Other Options Are Wrong:
Option B is incorrect because disk I or O and printer access are specific resources managed by separate subsystems; they are not the general functions of the three scheduling levels. Option C is wrong because the scheduling levels are conceptually distinct and operate at different frequencies with different responsibilities. Option D is incorrect because short-term scheduling typically runs more frequently than long-term scheduling; it is invoked on every timer tick or when processes change state, while long-term scheduling occurs much less often.

Common Pitfalls:
Students sometimes focus only on CPU scheduling and overlook the existence of long term and medium term scheduling. Another pitfall is to confuse medium term scheduling with memory allocation alone; while related, medium term scheduling specifically refers to swapping decisions. Remembering that long term decides who gets into the system, medium term decides who stays in memory, and short term decides who runs next on the CPU provides a clear mental model.

Final Answer:
Short term, medium term, and long term scheduling have distinct roles: short-term scheduling chooses which ready process runs on the CPU next, medium-term scheduling swaps processes out of and into memory, and long-term scheduling decides which new jobs are admitted to the system.