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NAME | SYNOPSIS | DESCRIPTION | PARAMETERS | EXAMPLES | SEE ALSO | AUTHORS | COLOPHON |
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FQ(8) Linux FQ(8)
FQ - Fair Queue traffic policing
tc qdisc ... fq [ limit PACKETS ] [ flow_limit PACKETS ] [ quantum
BYTES ] [ initial_quantum BYTES ] [ maxrate RATE ] [ buckets
NUMBER ] [ orphan_mask NUMBER ] [ pacing | nopacing ] [
ce_threshold TIME ]
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 hash 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 on each
socket. Note that after linux-4.20, linux adopted EDT (Earliest
Departure Time) and TCP directly sets the appropriate Departure
Time for each skb.
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.
limit
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.
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 collision sorting.
Default: 1024.
orphan_mask
For packets not owned by a socket, fq is able to mask a part of
skb->hash and reduce number of buckets associated with the
traffic. This is a DDOS prevention mechanism, and the default is
1023 (meaning no more than 1024 flows are allocated for these
packets)
[no]pacing
Enable or disable flow pacing. Default is enabled.
ce_threshold
sets a threshold above which all packets are marked with ECN
Congestion Experienced. This is useful for DCTCP-style congestion
control algorithms that require marking at very shallow queueing
thresholds.
#tc qdisc add dev eth0 root fq ce_threshold 4ms
#tc -s -d qdisc show dev eth0
qdisc fq 8001: dev eth0 root refcnt 2 limit 10000p flow_limit 100p
buckets 1024 orphan_mask 1023 quantum 3028b initial_quantum 15140b
low_rate_threshold 550Kbit refill_delay 40.0ms ce_threshold 4.0ms
Sent 72149092 bytes 48062 pkt (dropped 2176, overlimits 0
requeues 0)
backlog 1937920b 1280p requeues 0
flows 34 (inactive 17 throttled 0)
gc 0 highprio 0 throttled 0 ce_mark 47622 flows_plimit 2176
tc(8), socket(7)
FQ was written by Eric Dumazet.
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
[email protected], [email protected]. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-08.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to [email protected]
iproute2 10 Sept 2015 FQ(8)
Pages that refer to this page: tc(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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