FQ(8)                                                                  Linux                                                                 FQ(8)

NAME

       FQ - Fair Queue traffic policing

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 con-
       sidered 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.

iproute2                                                           10 Sept 2015                                                              FQ(8)