tc-fq(8) — Linux manual page


FQ(8)                             Linux                            FQ(8)

NAME         top

       FQ - Fair Queue traffic policing

SYNOPSIS         top

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

DESCRIPTION         top

       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 hash 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 on each
       socket. Note that after linux-4.20, linux adopted EDT (Earliest
       Departure Time) and TCP directly sets the appropriate Departure
       Time for each skb.

       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         top

       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.

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

       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.

       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.

       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.

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

       For packets not owned by a socket, fq is able to mask a part of
       skb->hash and reduce number of buckets associated with the
       traffic. This is a DDOS prevention mechanism, and the default is
       1023 (meaning no more than 1024 flows are allocated for these

       Enable or disable flow pacing. Default is enabled.

       sets a threshold above which all packets are marked with ECN
       Congestion Experienced. This is useful for DCTCP-style congestion
       control algorithms that require marking at very shallow queueing

EXAMPLES         top

       #tc qdisc add dev eth0 root fq ce_threshold 4ms
       #tc -s -d qdisc show dev eth0
       qdisc fq 8001: dev eth0 root refcnt 2 limit 10000p flow_limit
       100p buckets 1024 orphan_mask 1023 quantum 3028b initial_quantum
       15140b low_rate_threshold 550Kbit refill_delay 40.0ms
       ce_threshold 4.0ms
        Sent 72149092 bytes 48062 pkt (dropped 2176, overlimits 0
       requeues 0)
        backlog 1937920b 1280p requeues 0
         flows 34 (inactive 17 throttled 0)
         gc 0 highprio 0 throttled 0 ce_mark 47622 flows_plimit 2176

SEE ALSO         top

       tc(8), socket(7)

AUTHORS         top

       FQ was written by Eric Dumazet.

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling
       TCP/IP networking and traffic) project.  Information about the
       project can be found at 
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iproute2                      10 Sept 2015                         FQ(8)

Pages that refer to this page: tc(8)