🧠 What is the Ready Queue?
The Ready Queue holds all the processes that are loaded into RAM, ready to run, but waiting for the CPU.
These processes are not waiting for I/O or events — they are just paused, waiting their turn to execute.
📦 Key Characteristics of the Ready Queue
| Feature | Description |
|---|---|
| 📌 In-RAM Only | Contains processes that are already admitted into memory |
| ⏳ Actively Scheduled | These are the only processes the CPU scheduler can pick |
| ⚡ Preemptive Friendly | Processes here might have been preempted or newly created |
| 🧠 Stored as a Queue/List/Heap | Depends on scheduling algorithm (e.g., FIFO, Priority Queue, Red-Black Tree in Linux CFS) |
🔁 When Does a Process Enter the Ready Queue?
| Event | Cause |
|---|---|
| ✅ New Process Admitted | After creation and memory allocation |
| 🔁 Preemption | After time slice expires |
| ⌛ I/O Completion | Process was blocked, now unblocked |
| 💡 Wake-up Call | Timer/event/signal triggers process back to ready |
🧩 What Happens Inside the Ready Queue?
-
The CPU scheduler (short-term scheduler) picks one process from the Ready Queue.
-
It does a context switch: loads the selected process’s registers, stack, program counter, etc.
-
The selected process enters the Running state.
🔍 Real Example in Linux
You can use:
ps -eo pid,stat,commIf the process state is R, it’s either Running or Ready (the OS doesn’t always distinguish these precisely in user space).
🎯 Visualization
[ Ready Queue ]
┌────┬────┬────┬────┐
│ P1 │ P4 │ P6 │ P7 │ ← Waiting for CPU
└────┴────┴────┴────┘
↓ (CPU Scheduler picks one)
[ Running Process ]
⚙️ Real Life Analogy
Think of the Ready Queue as people standing in line to use a treadmill (CPU).
Everyone’s ready — shoes on, warmed up — just waiting for their turn.
🧠 Interview-Ready Definition:
The Ready Queue is a queue maintained by the OS containing all processes that are ready and waiting to execute but are currently not running. The CPU scheduler selects from this queue to allocate CPU time efficiently based on the scheduling algorithm.