NAME | SYNOPSIS | DESCRIPTION | PARAMETERS | EXAMPLES | SEE ALSO | AUTHORS | COLOPHON

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

   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.

   orphan_mask
       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)

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

   ce_threshold
       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
       thresholds.

EXAMPLES         top

       #tc qdisc add dev eth0 root est 1sec 4sec fq ce_threshold 4ms
       #tc -s -d qdisc sh dev eth0
       qdisc fq 800e: root refcnt 9 limit 10000p flow_limit 1000p buckets
       1024 orphan_mask 1023 quantum 3028 initial_quantum 15140
       low_rate_threshold 550Kbit refill_delay 40.0ms ce_threshold 4.0ms
        Sent 533368436185 bytes 352296695 pkt (dropped 0, overlimits 0
       requeues 1339864)
        rate 39220Mbit 3238202pps backlog 12417828b 358p requeues 1339864
         1052 flows (852 inactive, 0 throttled)
         112 gc, 0 highprio, 212 throttled, 21501 ns latency, 470241 ce_mark

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 
       ⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
       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
       2019-07-28.  (At that time, the date of the most recent commit that
       was found in the repository was 2019-07-26.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

iproute2                        10 Sept 2015                           FQ(8)

Pages that refer to this page: tc(8)