man > tc-drr(8)

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 re‐
       placement 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 classi‐
       fied, 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 fil‐
       ter:

       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  di‐
       visor 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)