FQ-PIE(8) — Linux manual page


FQ-PIE(8)                           Linux                          FQ-PIE(8)

NAME         top

       FQ-PIE - Flow Queue Proportional Integral controller Enhanced

SYNOPSIS         top

       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 ]

DESCRIPTION         top

       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.

ALGORITHM         top

       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.

PARAMETERS         top

       It is the limit on the queue size in packets. Incoming packets are
       dropped when the limit is reached. The default value is 10240

       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

       It is the queue delay which the PIE algorithm tries to maintain. The
       default target delay is 15ms.

       It is the time interval at which the system drop probability is
       calculated.  The default is 15ms.

       alpha and beta are parameters chosen to control the drop probability.
       These should be in the range between 0 and 32.

       quantum signifies the number of bytes that may be dequeued from a
       queue before switching to the next queue in the deficit round robin

       It is the maximum total memory allowed for packets of all flows. The
       default is 32Mb.

       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.

       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.

       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.

       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.

EXAMPLES         top

       # 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
        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

       # 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

SEE ALSO         top

       tc(8), tc-pie(8), tc-fq_codel(8)

SOURCES         top

       RFC 8033: https://tools.ietf.org/html/rfc8033

AUTHORS         top

       FQ-PIE was implemented by Mohit P. Tahiliani. Please report
       corrections to the Linux Networking mailing list

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling TCP/IP
       networking and traffic) project.  Information about the project can
       be found at 
       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
       2020-11-01.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-10-28.)  If you discover any
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       (which is not part of the original manual page), send a mail to

iproute2                       23 January 2020                     FQ-PIE(8)