FQ(8) Linux FQ(8)
Fair Queuing (FQ) - Traffic Pacing
tc qdisc ... fq [ limit PACKETS ] [ flow_limit PACKETS ] [ quantum
BYTES ] [ initial_quantum BYTES ] [ maxrate RATE ] [ buckets NUMBER ]
[ pacing | nopacing ]
FQ (Fair Queue) is a classless packet scheduler meant to be mostly used
for locally generated traffic. It is designed to achieve per flow pac-
ing. FQ does flow separation, and is able to respect pacing require-
ments set by TCP stack. All packets belonging to a socket are consid-
ered as a 'flow'. For non local packets (router workload), packet
rxhash is used as fallback.
An application can specify a maximum pacing rate using the SO_MAX_PAC-
ING_RATE setsockopt call. This packet scheduler adds delay between
packets to respect rate limitation set by TCP stack.
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 win-
dow 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
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:
Enable or disable flow pacing. Default is enabled.
#tc qdisc add dev eth0 root fq
#tc -s -d qdisc
qdisc fq 8003: dev eth0 root refcnt 2 limit 10000p flow_limit 100p
buckets 1024 quantum 3028 initial_quantum 15140
Sent 503727981 bytes 1146972 pkt (dropped 0, overlimits 0 requeues
backlog 0b 0p requeues 54452
1289 flows (1289 inactive, 0 throttled)
0 gc, 31 highprio, 27411 throttled
FQ was written by Eric Dumazet.
iproute2 10 Sept 2015 FQ(8)
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