tc-drr(8) — Linux manual page


TC(8)                             Linux                            TC(8)

NAME         top

       drr - deficit round robin scheduler

SYNOPSIS         top

       tc qdisc ... add drr [ quantum bytes ]

DESCRIPTION         top

       The Deficit Round Robin Scheduler is a classful queuing
       discipline as a more flexible replacement for Stochastic Fairness

       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         top

       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         top

              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         top

       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

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

SOURCE         top

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

NOTES         top

       This implementation does not drop packets from the longest queue
       on overrun, as limits are handled by the individual child qdiscs.

SEE ALSO         top

       tc(8), tc-htb(8), tc-sfq(8)

AUTHOR         top

       sched_drr was written by Patrick McHardy.

COLOPHON         top

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iproute2                      January 2010                         TC(8)

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