Deficit round robin
Deficit Round Robin (DRR), also Deficit Weighted Round Robin (DWRR), is a scheduling algorithm for the
Details
In DRR, a scheduler handling N flows[a] is configured with one quantum for each flow. This global idea is that, at each round, the flow can send at most bytes, and the remaining, if any, is reported to the next round. In this way, the minimum rate that flow will achieve over a long term is ; where is the link rate.
Algorithm
The DRR scans all non-empty queues in sequence. When a non-empty queue is selected, its deficit counter is incremented by its quantum value. Then, the value of the deficit counter is a maximal number of bytes that can be sent at this turn: if the deficit counter is greater than the packet's size at the head of the queue (HoQ), this packet can be sent, and the value of the counter is decremented by the packet size. Then, the size of the next packet is compared to the counter value, etc. Once the queue is empty or the value of the counter is insufficient, the scheduler will skip to the next queue. If the queue is empty, the value of the deficit counter is reset to 0.
Variables and Constants const integer N // Nb of queues const integer Q[1..N] // Per queue quantum integer DC[1..N] // Per queue deficit counter queue queue[1..N] // The queues
Scheduling Loop while true do for i in 1..N do if not queue[i].empty() then DC[i]:= DC[i] + Q[i] while( not queue[i].empty() and DC[i] ≥ queue[i].head().size() ) do DC[i] := DC[i] − queue[i].head().size() send( queue[i].head() ) queue[i].dequeue() end while if queue[i].empty() then DC[i] := 0 end if end if end for end while
Performances: fairness, complexity, and latency
Like other GPS-like scheduling algorithm, the choice of the weights is left to the network administrator.
Like WFQ, DRR offers a minimal rate to each flow whatever the size of the packets is. In weighted round robin scheduling, the fraction of bandwidth used depend on the packet's sizes.
Compared with WFQ scheduler that has complexity of O(log(n)) (n is the number of active
Implementations
An implementation of the deficit round robin algorithm was written by Patrick McHardy for the Linux kernel[4] and published under the GNU General Public License.
In Cisco and Juniper routers, modified versions of DRR are implemented: since the latency of DRR can be larger for some class of traffic, these modified versions give higher priority to some queues, whereas the others are served with the standard DRR algorithm.[5][6]
See also
- Scheduling algorithm
- Fair Queuing
- Generalized processor sharing
- Weighted Fair Queuing
- Weighted round robin
- Fairness measure
Notes
- ^ Flows may also be called queues, classes or sessions
References
- ISSN 0146-4833.
- S2CID 62158653.
- S2CID 235294011.
- ^ "DRR Linux kernel network scheduler module". kernel.org. Retrieved 2013-09-07.
- ^ Lenzini, Luciano; Mingozzi, Enzo; Stea, Giovanni (2007). "Performance Analysis of Modified Deficit Round Robin Schedulers". IOS Journal of High Speed Networks. 16 (4): 399–422.
- ^ Lenzini, Luciano; Mingozzi, Enzo; Stea, Giovanni (2006). Performance Analysis of Modified Deficit Round Robin Schedulers (Technical report). Dipartimento di Ingegneria della Informazione, University of Pisa.