1FQ(8) Linux FQ(8)
2
6 FQ - Fair Queue traffic policing
7
9 tc qdisc ... fq [ limit PACKETS ] [ flow_limit PACKETS ] [ quantum
10 BYTES ] [ initial_quantum BYTES ] [ maxrate RATE ] [ buckets NUMBER ] [
11 orphan_mask NUMBER ] [ pacing | nopacing ] [ ce_threshold TIME ]
12
15 FQ (Fair Queue) is a classless packet scheduler meant to be mostly used
16 for locally generated traffic. It is designed to achieve per flow pac‐
17 ing. FQ does flow separation, and is able to respect pacing require‐
18 ments set by TCP stack. All packets belonging to a socket are consid‐
19 ered as a 'flow'. For non local packets (router workload), packet hash
20 is used as fallback.
21 An application can specify a maximum pacing rate using the SO_MAX_PAC‐
23 ING_RATE setsockopt call. This packet scheduler adds delay between
24 packets to respect rate limitation set on each socket. Note that after
25 linux-4.20, linux adopted EDT (Earliest Departure Time) and TCP
26 directly sets the appropriate Departure Time for each skb.
27 Dequeueing happens in a round-robin fashion. A special FIFO queue is
29 reserved for high priority packets ( TC_PRIO_CONTROL priority), such
30 packets are always dequeued first.
31 FQ is non-work-conserving.
33 TCP pacing is good for flows having idle times, as the congestion win‐
35 dow permits TCP stack to queue a possibly large number of packets.
36 This removes the 'slow start after idle' choice, badly hitting large
37 BDP flows and applications delivering chunks of data such as video
38 streams.
39
42 limit
43 Hard limit on the real queue size. When this limit is reached, new
44 packets are dropped. If the value is lowered, packets are dropped so
45 that the new limit is met. Default is 10000 packets.
46 flow_limit
48 Hard limit on the maximum number of packets queued per flow. Default
49 value is 100.
50 quantum
52 The credit per dequeue RR round, i.e. the amount of bytes a flow is
53 allowed to dequeue at once. A larger value means a longer time period
54 before the next flow will be served. Default is 2 * interface MTU
55 bytes.
56 initial_quantum
58 The initial sending rate credit, i.e. the amount of bytes a new flow is
59 allowed to dequeue initially. This is specifically meant to allow
60 using IW10 without added delay. Default is 10 * interface MTU, i.e.
61 15140 for 'standard' ethernet.
62 maxrate
64 Maximum sending rate of a flow. Default is unlimited. Application
65 specific setting via SO_MAX_PACING_RATE is ignored only if it is larger
66 than this value.
67 buckets
69 The size of the hash table used for flow lookups. Each bucket is
70 assigned a red-black tree for efficient collision sorting. Default:
71 1024.
72 orphan_mask
74 For packets not owned by a socket, fq is able to mask a part of
75 skb->hash and reduce number of buckets associated with the traffic.
76 This is a DDOS prevention mechanism, and the default is 1023 (meaning
77 no more than 1024 flows are allocated for these packets)
78 [no]pacing
80 Enable or disable flow pacing. Default is enabled.
81 ce_threshold
83 sets a threshold above which all packets are marked with ECN Congestion
84 Experienced. This is useful for DCTCP-style congestion control algo‐
85 rithms that require marking at very shallow queueing thresholds.
86
89 #tc qdisc add dev eth0 root fq ce_threshold 4ms
90 #tc -s -d qdisc show dev eth0
91 qdisc fq 8001: dev eth0 root refcnt 2 limit 10000p flow_limit 100p
92 buckets 1024 orphan_mask 1023 quantum 3028b initial_quantum 15140b
93 low_rate_threshold 550Kbit refill_delay 40.0ms ce_threshold 4.0ms
94 Sent 72149092 bytes 48062 pkt (dropped 2176, overlimits 0 requeues 0)
95 backlog 1937920b 1280p requeues 0
96 flows 34 (inactive 17 throttled 0)
97 gc 0 highprio 0 throttled 0 ce_mark 47622 flows_plimit 2176
98
100 tc(8), socket(7)
101
103 FQ was written by Eric Dumazet.
104iproute2 10 Sept 2015 FQ(8)