TC(8) Linux TC(8)
NAME
drr - deficit round robin scheduler
SYNOPSIS
tc qdisc ... add drr [ quantum bytes ]
DESCRIPTION
The Deficit Round Robin Scheduler is a classful queuing discipline as a more flexible replacement for Stochas‐
tic Fairness Queuing.
Unlike SFQ, there are no built-in queues -- you need to add classes and then set up filters to classify pack‐
ets 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.
ALGORITHM
Each class is assigned a deficit counter, initialized to quantum.
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 quantum and the scheduler moves on to
the next class in the active list.
PARAMETERS
quantum
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.
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 flow filter:
tc qdisc add dev .. drr
for i in .. 1024;do
NOTES
This implementation does not drop packets from the longest queue on overrun, as limits are handled by the
individual child qdiscs.
SEE ALSO
tc(8), tc-htb(8), tc-sfq(8)
AUTHOR
sched_drr was written by Patrick McHardy.
iproute2 January 2010 TC(8)
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