FQ(8)                    (10 Sept 2015)                     FQ(8)

          FQ - Fair Queue traffic policing

          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 ]

          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 appro-
          priate 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

          FQ is non-work-conserving.

          TCP pacing is good for flows having idle times, as the con-
          gestion 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.

          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.

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          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 specifi-
          cally meant to allow using IW10 without added delay.
          Default is 10 * interface MTU, i.e. 15140 for 'standard'

          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 allo-
          cated 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 con-
          gestion control algorithms that require marking at very
          shallow queueing thresholds.

          #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

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          tc(8), socket(7)

          FQ was written by Eric Dumazet.

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