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 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.
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 .. drrfor i in .. 1024;dotc class add dev .. classid $handle:$(print %x $i)tc qdisc add dev .. fifo limit 16donetc filter add .. protocol ip .. $handle flow hash keyssrc,dst,proto,proto-src,proto-dst divisor 1024 perturb 10
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iproute2 January 2010 TC(8)