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 ] [ pacing | nopacing ]
DESCRIPTION
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 SO_MAX_PACING_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 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 low‐
ered, 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
ignored 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 col‐
lision sorting. Default: 1024.
[no]pacing
Enable or disable flow pacing. Default is enabled.
EXAMPLES
AUTHORS
FQ was written by Eric Dumazet.
iproute2 10 Sept 2015 FQ(8)
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