tc-drr(8) - phpMan

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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 flexi-
       ble 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.


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 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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