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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 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.


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
            tc class add dev .. classid $handle:$(print %x $i)
            tc qdisc add dev .. fifo limit 16
       done

       tc  filter  add .. protocol ip .. $handle flow hash keys src,dst,proto,proto-src,proto-dst divisor 1024 perturb
       10



SOURCE
       o      M. Shreedhar and George Varghese "Efficient Fair Queuing using Deficit Round Robin", Proc. SIGCOMM 95.


NOTES
       This implementation does not drop packets from the longest queue on overrun, as limits are handled by the indi-
       vidual 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)