What Are the Process Scheduling Queues in an OS?
Learn the job, ready, and device queues in OS scheduling and how processes move between them, with an interview-ready example.
Expected Interview Answer
An operating system tracks processes through several distinct scheduling queues — the job queue, the ready queue, and one or more device (I/O) queues — and a process moves between them as different schedulers decide when it is admitted, when it runs, and when it waits for I/O.
The job queue holds every process that exists in the system, including ones still on disk waiting to be admitted into memory. The ready queue holds only processes that are in memory and waiting for their turn on the CPU; it is usually implemented as a linked list of process control blocks (PCBs), with the CPU scheduler picking the head according to the active algorithm. Device queues, one per I/O device, hold processes waiting for that specific device to become free after issuing a request. A process migrates continuously: admitted from the job queue into the ready queue, dispatched to the CPU, then either moved to a device queue on a blocking I/O call, moved back to the ready queue after an interrupt or time-slice expiry, or terminated and removed entirely. Understanding these queues explains why a process’s state (new, ready, running, waiting, terminated) is really just a label for which queue it currently sits in.
- Explains why process state models map directly onto physical queues
- Clarifies the difference between admission and CPU dispatch
- Shows why I/O-bound and CPU-bound processes queue differently
- Foundation for understanding multilevel queue scheduling
AI Mentor Explanation
The process scheduling queues are like a cricket academy pipeline: the job queue is every registered trainee waiting for a spot in a training batch, the ready queue is the group already on the ground padded up and waiting for their turn in the nets, and a device queue is a line of players waiting for the one bowling machine to free up. A trainee moves from registration to the ground to the machine queue and back to the ground line once done, exactly as a process migrates between queues as it is admitted, dispatched, and blocked on I/O.
Step-by-Step Explanation
Step 1
Admission
A new process enters the job queue, holding every process known to the system regardless of memory residency.
Step 2
Ready queue entry
The long-term scheduler admits the process into memory, placing its PCB at the tail of the ready queue.
Step 3
Dispatch or block
The CPU scheduler dispatches the head of the ready queue to run; if it issues a blocking I/O call, it moves to that device's queue.
Step 4
Requeue or terminate
After I/O completes or a time slice expires, the process returns to the ready queue, or exits and is removed from all queues on termination.
What Interviewer Expects
- Naming and distinguishing the job queue, ready queue, and device queues
- Explaining that process state maps to which queue the PCB currently sits in
- Describing the transitions between queues correctly
- Awareness that each I/O device typically has its own queue
Common Mistakes
- Treating the job queue and ready queue as the same thing
- Forgetting that each device usually has a separate queue, not one shared I/O queue
- Describing scheduling as a single queue with no transitions
- Confusing queue transitions with context switching itself
Best Answer (HR Friendly)
“Think of the OS as juggling several waiting lines: one line for everything that exists but is not yet loaded into memory, one line for things loaded and ready to use the CPU, and separate small lines for each device a task might be waiting on, like a disk or a printer. A task keeps hopping between these lines as it gets admitted, runs, waits for a device, and comes back, and that hopping is exactly what we mean by process scheduling.”
Code Example
struct pcb {
int pid;
enum { NEW, READY, RUNNING, WAITING, TERMINATED } state;
struct pcb *next;
};
struct pcb *job_queue = NULL; /* every process not yet admitted */
struct pcb *ready_queue = NULL; /* in-memory, waiting for the CPU */
struct pcb *disk_queue = NULL; /* waiting on the disk device */
void admit_process(struct pcb *p) {
remove_from(&job_queue, p);
p->state = READY;
enqueue(&ready_queue, p); /* moves job queue -> ready queue */
}
void request_disk_io(struct pcb *p) {
remove_from(&ready_queue, p);
p->state = WAITING;
enqueue(&disk_queue, p); /* moves ready queue -> device queue */
}Follow-up Questions
- What is the difference between the ready queue and a device queue?
- Which scheduler moves a process from the job queue to the ready queue?
- What happens to a process's queue membership when it calls read() on a file?
- How do multilevel queue schedulers use multiple ready queues?
MCQ Practice
1. Which queue holds every process in the system, including ones not yet in memory?
The job queue is the master list of all processes, including those still waiting to be admitted into memory.
2. A process that issues a blocking I/O call moves from the ready queue to?
Blocking on I/O moves the process to the queue for that specific device until the request completes.
3. What determines which process a device queue releases first?
Each device queue can apply its own ordering policy, independent of whatever algorithm the CPU scheduler uses.
Flash Cards
What is the job queue? — The set of all processes in the system, including those not yet resident in memory.
What is the ready queue? — In-memory processes waiting for their turn on the CPU.
What is a device queue? — Processes waiting for a specific I/O device to become available.
What triggers a move from ready queue to device queue? — The running process issuing a blocking I/O request.