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NAME | SYNOPSIS | DESCRIPTION | ALGORITHM | PARAMETERS | EXAMPLES | SEE ALSO | SOURCES | AUTHORS | COLOPHON |
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FQ-PIE(8) Linux FQ-PIE(8)
FQ-PIE - Flow Queue Proportional Integral controller Enhanced
tc qdisc ... fq_pie [ limit PACKETS ] [ flows NUMBER ]
[ target TIME ] [ tupdate TIME ]
[ alpha NUMBER ] [ beta NUMBER ]
[ quantum BYTES ] [ memory_limit BYTES ]
[ ecn_prob PERENTAGE ] [ [no]ecn ]
[ [no]bytemode ] [ [no_]dq_rate_estimator ]
FQ-PIE (Flow Queuing with Proportional Integral controller
Enhanced) is a queuing discipline that combines Flow Queuing with
the PIE AQM scheme. FQ-PIE uses a Jenkins hash function to
classify incoming packets into different flows and is used to
provide a fair share of the bandwidth to all the flows using the
qdisc. Each such flow is managed by the PIE algorithm.
The FQ-PIE algorithm consists of two logical parts: the scheduler
which selects which queue to dequeue a packet from, and the PIE
AQM which works on each of the queues. The major work of FQ-PIE is
mostly in the scheduling part. The interaction between the
scheduler and the PIE algorithm is straight forward.
During the enqueue stage, a hashing-based scheme is used, where
flows are hashed into a number of buckets with each bucket having
its own queue. The number of buckets is configurable, and
presently defaults to 1024 in the implementation. The flow
hashing is performed on the 5-tuple of source and destination IP
addresses, port numbers and IP protocol number. Once the packet
has been successfully classified into a queue, it is handed over
to the PIE algorithm for enqueuing. It is then added to the tail
of the selected queue, and the queue's byte count is updated by
the packet size. If the queue is not currently active (i.e., if it
is not in either the list of new or the list of old queues) , it
is added to the end of the list of new queues, and its number of
credits is initiated to the configured quantum. Otherwise, the
queue is left in its current queue list.
During the dequeue stage, the scheduler first looks at the list of
new queues; for the queue at the head of that list, if that queue
has a negative number of credits (i.e., it has already dequeued at
least a quantum of bytes), it is given an additional quantum of
credits, the queue is put onto the end of the list of old queues,
and the routine selects the next queue and starts again.
Otherwise, that queue is selected for dequeue again. If the list
of new queues is empty, the scheduler proceeds down the list of
old queues in the same fashion (checking the credits, and either
selecting the queue for dequeuing, or adding credits and putting
the queue back at the end of the list). After having selected a
queue from which to dequeue a packet, the PIE algorithm is invoked
on that queue.
Finally, if the PIE algorithm does not return a packet, then the
queue must be empty and the scheduler does one of two things:
If the queue selected for dequeue came from the list of new
queues, it is moved to the end of the list of old queues. If
instead it came from the list of old queues, that queue is removed
from the list, to be added back (as a new queue) the next time a
packet arrives that hashes to that queue. Then (since no packet
was available for dequeue), the whole dequeue process is restarted
from the beginning.
If, instead, the scheduler did get a packet back from the PIE
algorithm, it subtracts the size of the packet from the byte
credits for the selected queue and returns the packet as the
result of the dequeue operation.
limit
It is the limit on the queue size in packets. Incoming packets are
dropped when the limit is reached. The default value is 10240
packets.
flows
It is the number of flows into which the incoming packets are
classified. Due to the stochastic nature of hashing, multiple
flows may end up being hashed into the same slot. Newer flows have
priority over older ones. This parameter can be set only at load
time since memory has to be allocated for the hash table. The
default value is 1024.
target
It is the queue delay which the PIE algorithm tries to maintain.
The default target delay is 15ms.
tupdate
It is the time interval at which the system drop probability is
calculated. The default is 15ms.
alpha
beta
alpha and beta are parameters chosen to control the drop
probability. These should be in the range between 0 and 32.
quantum
quantum signifies the number of bytes that may be dequeued from a
queue before switching to the next queue in the deficit round
robin scheme.
memory_limit
It is the maximum total memory allowed for packets of all flows.
The default is 32Mb.
ecn_prob
It is the drop probability threshold below which packets will be
ECN marked instead of getting dropped. The default is 10%. Setting
this parameter requires ecn to be enabled.
[no]ecn
It has the same semantics as pie and can be used to mark packets
instead of dropping them. If ecn has been enabled, noecn can be
used to turn it off and vice-a-versa.
[no]bytemode
It is used to scale drop probability proportional to packet size
bytemode to turn on bytemode, nobytemode to turn off bytemode. By
default, bytemode is turned off.
[no_]dq_rate_estimator
dq_rate_estimator can be used to calculate queue delay using
Little's Law, no_dq_rate_estimator can be used to calculate queue
delay using timestamp. By default, dq_rate_estimator is turned
off.
# tc qdisc add dev eth0 root fq_pie
# tc -s qdisc show dev eth0
qdisc fq_pie 8001: root refcnt 2 limit 10240p flows 1024 target
15.0ms tupdate 16.0ms alpha 2 beta 20 quantum 1514b memory_limit
32Mb ecn_prob 10
Sent 159173586 bytes 105261 pkt (dropped 24, overlimits 0
requeues 0)
backlog 75700b 50p requeues 0
pkts_in 105311 overlimit 0 overmemory 0 dropped 24 ecn_mark 0
new_flow_count 7332 new_flows_len 0 old_flows_len 4 memory_used
108800
# tc qdisc add dev eth0 root fq_pie dq_rate_estimator
# tc -s qdisc show dev eth0
qdisc fq_pie 8001: root refcnt 2 limit 10240p flows 1024 target
15.0ms tupdate 16.0ms alpha 2 beta 20 quantum 1514b memory_limit
32Mb ecn_prob 10 dq_rate_estimator
Sent 8263620 bytes 5550 pkt (dropped 4, overlimits 0 requeues 0)
backlog 805448b 532p requeues 0
pkts_in 6082 overlimit 0 overmemory 0 dropped 4 ecn_mark 0
new_flow_count 94 new_flows_len 0 old_flows_len 8 memory_used
1157632
tc(8), tc-pie(8), tc-fq_codel(8)
RFC 8033: https://tools.ietf.org/html/rfc8033
FQ-PIE was implemented by Mohit P. Tahiliani. Please report
corrections to the Linux Networking mailing list
<netdev@vger.kernel.org>.
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
netdev@vger.kernel.org, shemminger@osdl.org. 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 man-pages@man7.org
iproute2 23 January 2020 FQ-PIE(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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