Efficient Packet Scheduling with Tight Bandwidth Distribution Guarantees

One of the key issues in providing tight bandwidthand delay guarantees with high speed systems is the cost of packetscheduling. Through timestamp rounding and flow groupingseveral schedulers have been designed that provide near-optimalbandwidth distribution with O(1) time complexity. However, inone of the two lowest complexity proposals in the literature, thecost of each packet enqueue/dequeue is still proportional to thenumber of groups in which flows are partitioned. In the otherproposal, this cost is independent of the number of groups, buta number of operations proportional to the length of the packetmust be executed during each transmission.In this paper we present Quick Fair Queueing (QFQ), a newmember of this class of schedulers that provides the same near-optimal guarantees, yet has a constant cost also with respect tothe number of groups and the packet length. The peculiarityof QFQ is to partition groups so that enqueue/dequeue can beaccomplished in a number of steps independent of the numberof groups, without requiring extra operations during packettransmissions.To validate the effectiveness of QFQ, we implemented it inthe Linux kernel. QFQ proved able to execute each packetenqueue/dequeue in 340 instructions worst case, irrespectiveof number of flows and arrival patterns. Furthermore, theinstructions used by QFQ are extremely well suited to a hardwareimplementation.