|.TH TC 8 "January 2010" "iproute2" "Linux"
|drr \- deficit round robin scheduler
|.B tc qdisc ... add drr
|.B [ quantum
|The Deficit Round Robin Scheduler is a classful queuing discipline as
|a more flexible replacement for Stochastic Fairness Queuing.
|Unlike SFQ, there are no built-in queues \-\- you need to add classes
|and then set up filters to classify packets accordingly.
|This can be useful e.g. for using RED qdiscs with different settings for particular
|traffic. There is no default class \-\- if a packet cannot be classified,
|it is dropped.
|Each class is assigned a deficit counter, initialized to
|DRR maintains an (internal) ''active'' list of classes whose qdiscs are
|non-empty. This list is used for dequeuing. A packet is dequeued from
|the class at the head of the list if the packet size is smaller or equal
|to the deficit counter. If the counter is too small, it is increased by
|and the scheduler moves on to the next class in the active list.
|Amount of bytes a flow is allowed to dequeue before the scheduler moves to
|the next class. Defaults to the MTU of the interface. The minimum value is 1.
|.SH EXAMPLE & USAGE
|To attach to device eth0, using the interface MTU as its quantum:
|# tc qdisc add dev eth0 handle 1 root drr
|Adding two classes:
|# tc class add dev eth0 parent 1: classid 1:1 drr
|# tc class add dev eth0 parent 1: classid 1:2 drr
|You also need to add at least one filter to classify packets.
|# tc filter add dev eth0 protocol .. classid 1:1
|Like SFQ, DRR is only useful when it owns the queue \-\- it is a pure scheduler and does
|not delay packets. Attaching non-work-conserving qdiscs like tbf to it does not make
|sense \-\- other qdiscs in the active list will also become inactive until the dequeue
|operation succeeds. Embed DRR within another qdisc like HTB or HFSC to ensure it owns the queue.
|You can mimic SFQ behavior by assigning packets to the attached classes using the
|.B tc qdisc add dev .. drr
|.B for i in .. 1024;do
|.B "\ttc class add dev .. classid $handle:$(print %x $i)"
|.B "\ttc qdisc add dev .. fifo limit 16"
|.B tc filter add .. protocol ip .. $handle flow hash keys src,dst,proto,proto-src,proto-dst divisor 1024 perturb 10
|M. Shreedhar and George Varghese "Efficient Fair
|Queuing using Deficit Round Robin", Proc. SIGCOMM 95.
|This implementation does not drop packets from the longest queue on overrun,
|as limits are handled by the individual child qdiscs.
|.SH SEE ALSO
|.BR tc (8),
|.BR tc-htb (8),
|.BR tc-sfq (8)
|sched_drr was written by Patrick McHardy.