Overview
Round Robin Scheduling rotates tasks through a queue so each task gets a fair chance to run.
FoundationSource checked
Round Robin Scheduling
Gives each ready task a fixed time slice in cyclic order.
Quick Facts
Compact metadata for implementation and discovery.
- Category
- Scheduling
- Difficulty
- Beginner
- Family
- Preemptive Scheduling
- Tier
- Foundation
- Status
- Verified
- Review
- Source Checked
- Sources
- 1
- Formulas
- 1
- Relationships
- 20
- Code example
- Ready
- Visualization
- Ready
- Aliases
- RR Scheduling
Core Concepts
task orderingprocess schedulingqueue frontierpreemptive schedulingready queuetask priorityresource allocation
Quick facts
Complexity
Runtime and memory behavior at a glance.
Best
O(n)
Average
O(n + c)
Worst
O(n + c)
Space
O(n)
When to Use
- Use it when ready tasks should receive time slices in cyclic order.
- Use it for simple fair sharing among similar tasks.
- Use it when response time matters more than minimizing total completion time.
When Not to Use
- Avoid it when tasks have strict deadlines or priorities that must dominate fairness.
- Avoid quantum sizes that are too small relative to context-switch overhead.
- Avoid it when workload-aware scheduling can substantially improve latency or throughput.
What It Does
- Cycles through ready tasks
- Limits each run to a time quantum
Use Cases
- CPU time sharing
- Fair task scheduling
- Interactive systems
Inputs
- Ready task queue
- Time quantum
Outputs
- Execution order
- Completion times
Key Formulas (1)
Time slice
runtime = min(timequantum, remainingtime)
run_time = min(time_quantum, remaining_time)Limits each ready task to the configured quantum before rotating.
Pseudocode
- 1.Put ready tasks in a queue
- 2.Run the front task for one time quantum
- 3.Finish it if its work is complete
- 4.Otherwise put it at the back of the queue
- 5.Repeat until all tasks finish
Canonical Example
- Input
- tasks A=5ms, B=3ms, quantum=2ms
- Output
- execution slices A, B, A, B, A
- Explanation
- Each unfinished task returns to the back of the queue after its time slice.
Implementation Notes
- Keep ready tasks in a FIFO queue.
- Run each task for at most one quantum before requeuing it if unfinished.
- Track arrival, completion, and remaining time separately.
Common Pitfalls
- Choosing a time quantum that causes excessive context switching.
- Treating all tasks equally when priorities or deadlines are required.
- Failing to remove completed tasks from the rotation.
Interactive Mechanism Walkthrough
Graph Frontier & Edge RelaxationShow tasks rotating through a queue with fixed time slices.
Step 1 of 5: Put ready tasks in a queue
task-queueNotes
Round robin improves fairness but does not account for task length unless combined with other policies.
Sources (1)
What each reference supports for this entry.
Operating System Concepts
BookOperating-systems reference for Round Robin Scheduling CPU scheduling policy, ready-queue ordering, and preemption behavior.
Explore Next
Good next jumps from the relationship graph, ranked for useful discovery.
Has variantsWeighted Round Robin
Scheduling · Core
Weighted round robin extends round robin by assigning different service weights to queues.
Compare with thisAlternativesCompletely Fair Scheduler
Scheduling · Core
CFS models proportional fair CPU sharing instead of fixed time-slice round-robin dispatch.
Compare with thisAlternativesHighest Response Ratio Next
Scheduling · Core
HRRN ranks ready jobs by response ratio rather than cycling through fixed time quanta.
Compare with thisAlternativesHighest Response Ratio Next Scheduling
Scheduling · Core
HRRN is a non-preemptive response-ratio policy, while round robin is a preemptive time-slicing policy.
Compare with thisRelationships
View full relationship pageAlternative toShortest Job First
Round robin shares CPU time by time slices, while shortest-job-first prioritizes short bursts.
Related Algorithms
Tags
SchedulingOperating SystemsQueue