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tc-drr(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  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.



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