NAME

       Fair Queuing (FQ) - Traffic Pacing

SYNOPSIS

       tc  qdisc  ...  fq [ limit PACKETS ] [ flow_limit PACKETS ] [ quantum BYTES ] [ initial_quantum BYTES ] [
       maxrate RATE ] [ buckets NUMBER ]  [ pacing | nopacing ]

DESCRIPTION

       FQ (Fair Queue) is a classless packet scheduler meant to be mostly used for  locally  generated  traffic.
       It  is  designed  to  achieve  per  flow  pacing.  FQ does flow separation, and is able to respect pacing
       requirements set by TCP stack.  All packets belonging to a socket are considered as a  'flow'.   For  non
       local packets (router workload), packet rxhash is used as fallback.

       An  application  can  specify  a  maximum pacing rate using the SO_MAX_PACING_RATE setsockopt call.  This
       packet scheduler adds delay between packets to respect rate limitation set by TCP stack.

       Dequeueing happens in a round-robin fashion.  A special FIFO queue is reserved for high priority  packets
       ( TC_PRIO_CONTROL priority), such packets are always dequeued first.

       FQ is non-work-conserving.

       TCP  pacing  is  good  for flows having idle times, as the congestion window permits TCP stack to queue a
       possibly large number of packets.  This removes the 'slow start after idle' choice, badly  hitting  large
       BDP flows and applications delivering chunks of data such as video streams.

PARAMETERS

   limit
       Hard  limit  on the real queue size. When this limit is reached, new packets are dropped. If the value is
       lowered, packets are dropped so that the new limit is met. Default is 10000 packets.

   flow_limit
       Hard limit on the maximum number of packets queued per flow.  Default value is 100.

   quantum
       The credit per dequeue RR round, i.e. the amount of bytes a flow is allowed to dequeue at once. A  larger
       value  means  a  longer  time  period  before the next flow will be served.  Default is 2 * interface MTU
       bytes.

   initial_quantum
       The initial sending rate credit, i.e. the amount of bytes a new flow is  allowed  to  dequeue  initially.
       This  is specifically meant to allow using IW10 without added delay.  Default is 10 * interface MTU, i.e.
       15140 for 'standard' ethernet.

   maxrate
       Maximum  sending  rate  of  a  flow.   Default  is   unlimited.    Application   specific   setting   via
       SO_MAX_PACING_RATE is ignored only if it is larger than this value.

   buckets
       The  size of the hash table used for flow lookups. Each bucket is assigned a red-black tree for efficient
       collision sorting.  Default: 1024.

   [no]pacing
       Enable or disable flow pacing. Default is enabled.

EXAMPLES

       #tc qdisc add dev eth0 root fq
       #tc -s -d qdisc
       qdisc fq 8003:  dev  eth0  root  refcnt  2  limit  10000p  flow_limit  100p  buckets  1024  quantum  3028
       initial_quantum 15140
        Sent 503727981 bytes 1146972 pkt (dropped 0, overlimits 0 requeues 54452)
        backlog 0b 0p requeues 54452
         1289 flows (1289 inactive, 0 throttled)
         0 gc, 31 highprio, 27411 throttled

SEE ALSO

       tc(8), socket(7)

AUTHORS

       FQ was written by Eric Dumazet.