🧠 What is a Long-Term Scheduler?
The Long-Term Scheduler decides which processes from the job queue should be brought into memory (RAM) and moved to the ready queue for execution.
It controls the admission of new processes into the system and helps manage the overall load.
🏗️ Where Does It Fit?
[ Job Queue ] --(Long-Term Scheduler)--> [ Ready Queue ] --(Short-Term Scheduler)--> [ CPU ]
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Job Queue: All submitted jobs waiting for system resources.
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Ready Queue: Admitted jobs loaded into RAM, waiting for CPU time.
🎯 Goals of the Long-Term Scheduler
| Goal | Description |
|---|---|
| ✅ Control Degree of Multiprogramming | Keep enough processes in memory to use CPU efficiently |
| ⚖️ Balance CPU-bound vs I/O-bound processes | Avoid bottlenecks by mixing types |
| 🚫 Prevent Overload | Avoid memory thrashing or CPU starvation |
🔄 Characteristics
| Property | Value |
|---|---|
| 📌 Frequency | Low (runs only when a job is submitted or when a process finishes) |
| 💰 Cost | High (allocates memory, loads program, creates PCB) |
| 🧠 Intelligence | Can be smart (e.g., choose based on priority, resource needs) |
| 🚧 Involved In | Admission control, not direct execution |
🧩 Example Use Case
You have:
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100 jobs in the job queue
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Only 10 can be in memory at once
The long-term scheduler:
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Admits 5 CPU-bound jobs
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Then mixes in 5 I/O-bound jobs
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Keeps the system balanced and responsive
⚠️ What Happens If It’s Missing?
Without a Long-Term Scheduler:
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The OS might admit too many processes
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This can lead to:
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Thrashing (excessive paging/swapping)
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High context switching overhead
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Poor response time
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In modern systems, this function is implicitly handled by memory managers and workload balancers.
🧠 Interview-Ready Definition:
The Long-Term Scheduler (Job Scheduler) is responsible for admitting processes from the job queue into the ready queue, ensuring a balanced mix of process types and controlling system load. It runs infrequently but makes heavyweight decisions that affect overall system stability and performance.