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