Round-robin (RR) is one of the algorithms employed byprocess andnetwork schedulers incomputing.[1][2]As the term is generally used,time slices (also known as time quanta)[3] are assigned to each process in equal portions and in circular order, handling all processes withoutpriority (also known ascyclic executive). Round-robin scheduling is simple, easy to implement, andstarvation-free. Round-robin scheduling can be applied to other scheduling problems, such as data packet scheduling in computer networks. It is anoperating system concept.
The name of the algorithm comes from theround-robin principle known from other fields, where each person takes an equal share of something in turn.
Toschedule processes fairly, a round-robin scheduler generally employstime-sharing, giving each job a time slot orquantum[4] (its allowance of CPU time), and interrupting the job if it is not completed by then. The job is resumed next time a time slot is assigned to that process. If the process terminates or changes its state to waiting during its attributed time quantum, the scheduler selects the first process in the ready queue to execute. In the absence of time-sharing, or if the quanta were large relative to the sizes of the jobs, a process that produced large jobs would be favored over other processes.
Round-robin algorithm is a pre-emptive algorithm as the scheduler forces the process out of the CPU once the time quota expires.
For example, if the time slot is 100 milliseconds, andjob1 takes a total time of 250 ms to complete, the round-robin scheduler will suspend the job after 100 ms and give other jobs their time on the CPU. Once the other jobs have had their equal share (100 ms each),job1 will get another allocation ofCPU time and the cycle will repeat. This process continues until the job finishes and needs no more time on the CPU.
Consider the following table with the arrival time and execute time of the process with the quantum time of 100 ms to understand the round-robin scheduling:
| Process name | Arrival time | Execute time |
|---|---|---|
| P0 | 0 | 250 |
| P1 | 50 | 170 |
| P2 | 130 | 75 |
| P3 | 190 | 100 |
| P4 | 210 | 130 |
| P5 | 350 | 50 |
Another approach is to divide all processes into an equal number of timing quanta such that the quantum size is proportional to the size of the process. Hence, all processes end at the same time.
Inbest-effortpacket switching and otherstatistical multiplexing, round-robin scheduling can be used as an alternative tofirst-come first-served queuing.
A multiplexer, switch, or router that provides round-robin scheduling has a separate queue for every data flow, where a data flow may be identified by its source and destination address. The algorithm allows every active data flow that has data packets in the queue to take turns in transferring packets on a shared channel in a periodically repeated order. The scheduling iswork-conserving, meaning that if one flow is out of packets, the next data flow will take its place. Hence, the scheduling tries to prevent link resources from going unused.
Round-robin scheduling results inmax-min fairness if the data packets are equally sized, since the data flow that has waited the longest time is given scheduling priority. It may not be desirable if the size of the data packets varies widely from one job to another. A user that produces large packets would be favored over other users. In that casefair queuing would be preferable.
If guaranteed or differentiated quality of service is offered, and not only best-effort communication,deficit round-robin (DRR) scheduling,weighted round-robin (WRR) scheduling, orweighted fair queuing (WFQ) may be considered.
Inmultiple-access networks, where several terminals are connected to a shared physical medium, round-robin scheduling may be provided bytoken passingchannel access schemes such asToken Ring, or bypolling or resource reservation from a central control station.
In a centralized wireless packet radio network, where many stations share one frequency channel, a scheduling algorithm in a central base station may reserve time slots for the mobile stations in a round-robin fashion and provide fairness. However, iflink adaptation is used, it will take a much longer time to transmit a certain amount of data to "expensive" users than to others since the channel conditions differ. It would be more efficient to wait with the transmission until the channel conditions are improved, or at least to give scheduling priority to less expensive users. Round-robin scheduling does not utilize this. Higher throughput andsystem spectrum efficiency may be achieved by channel-dependent scheduling, for example aproportionally fair algorithm, ormaximum throughput scheduling. Note that the latter is characterized by undesirablescheduling starvation. This type of scheduling is one of the very basic algorithms for Operating Systems in computers which can be implemented through a circular queue data structure.
5.3.4 Round Robin Scheduling
| Single queueing nodes | |
|---|---|
| Arrival processes | |
| Queueing networks | |
| Service policies | |
| Key concepts | |
| Limit theorems | |
| Extensions | |
| Information systems | |
