(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 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 rxhash is used as
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 by TCP stack.
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.
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
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
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 bytes.
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
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.
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
Sent 503727981 bytes 1146972 pkt (dropped 0, overlimits 0
backlog 0b 0p requeues 54452
1289 flows (1289 inactive, 0 throttled)
0 gc, 31 highprio, 27411 throttled
FQ was written
by Eric Dumazet.