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 flexible
       replacement for Stochastic Fairness Queuing.

       Unlike SFQ, there are no built-in queues -- you need to add classes and then set  up  fil-
       ters  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 suc-
       ceeds. 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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