Manual: tc-fq

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 ] [ orphan_mask NUMBER ] [ pacing | nopacing ] [ ce_threshold TIME ]

DESCRIPTION
       FQ  (Fair  Queue)  is  a classless packet scheduler meant to be mostly used for locally generated traffic.  It is de‐
       signed 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
   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 ig‐
       nored 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
       #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	ini‐
       tial_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
       tc(8), socket(7)

AUTHORS
       FQ was written by Eric Dumazet.

iproute2						10 Sept 2015						       FQ(8)