FQ - Fair Queue traffic policing
tc qdisc ... fq
PACKETS ] [ flow_limit
PACKETS ] [
BYTES ] [ initial_quantum
BYTES ] [ maxrate
] [ buckets
NUMBER ] [ pacing
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 fallback.
An application can specify a maximum pacing rate using the
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 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
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 '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
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 15140
Sent 503727981 bytes 1146972 pkt (dropped 0, overlimits 0 requeues 54452)
backlog 0b 0p requeues 54452
1289 flows (1289 inactive, 0 throttled)
0 gc, 31 highprio, 27411 throttled
FQ was written by Eric Dumazet.