🧠 What is a Short-Term Scheduler?
The Short-Term Scheduler selects one process from the Ready Queue and allocates the CPU to it.
It operates very frequently, often at every context switch, I/O event, or timer interrupt.
🔁 How It Fits Into the Process Lifecycle
[ Ready Queue ] --(Short-Term Scheduler)--> [ CPU ]
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Maintains high CPU utilization
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Picks from only processes in memory
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Must be extremely fast and efficient
📦 Characteristics of the Short-Term Scheduler
| Characteristic | Description |
|---|---|
| ⚡ Frequency | Very high (milliseconds) |
| ⏱️ Speed | Must be very fast (microseconds) |
| 🧠 Scope | Chooses only from the Ready Queue |
| 🔁 Repeated Execution | Runs at every context switch or timer interrupt |
| 📊 Algorithm Based | Implements policies like Round Robin, Priority, etc. |
🎯 Goals of Short-Term Scheduling
| Goal | Description |
|---|---|
| ✅ Maximize CPU usage | Keep the CPU busy |
| 🚀 Optimize performance | Reduce response/wait times |
| ⚖️ Ensure fairness | Prevent starvation |
| 🧠 Enforce priorities | Let important processes run sooner |
🔀 Trigger Events for the Short-Term Scheduler
| Event | Example |
|---|---|
| 🕰️ Time slice expired | Round Robin timeout |
| 🔁 I/O completes | Process returns from disk read |
| 🪫 Process blocks | Waiting for I/O |
| 🏁 Process terminates | CPU becomes free |
| 🆕 New process admitted | Higher priority may preempt |
🧩 Scheduling Algorithms Used by the Short-Term Scheduler
| Type | Examples |
|---|---|
| 🧮 Non-Preemptive | FCFS, SJF |
| ⚡ Preemptive | Round Robin, Priority, SRTF, Multilevel Queue |
⚙️ Real World Analogy
Imagine a CPU as a single chair, and a line of students waiting to present.
The short-term scheduler decides:
Who gets to sit next
How long they stay
Whether someone can interrupt mid-way (preemption)
📚 Real OS Example
In Linux, the Short-Term Scheduler is implemented as the CFS (Completely Fair Scheduler).
It uses a Red-Black Tree to balance load and fairness among processes.
🧠 Interview-Ready Definition:
The Short-Term Scheduler, also known as the CPU Scheduler, is responsible for selecting a process from the ready queue and allocating the CPU to it. It executes frequently and quickly, using a scheduling algorithm to optimize performance, fairness, and responsiveness.