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

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

       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:

       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 packets)

       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
        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 
       If you have a bug report for this manual page, send it to
       netdev@vger.kernel.org, shemminger@osdl.org.  This page was obtained
       from the project's upstream Git repository
       ⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
       2020-08-13.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-06-24.)  If you discover any
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       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

iproute2                        10 Sept 2015                           FQ(8)