FQ-PIE(8) Linux FQ-PIE(8)
NAME
FQ-PIE - Flow Queue Proportional Integral controller Enhanced
SYNOPSIS
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
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 al-
gorithm.
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 algo-
rithm 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-tu-
ple 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 ini-
tiated 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 neg-
ative number of credits (i.e., it has already dequeued at least a quan-
tum 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 pro-
ceeds 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 se-
lected a queue from which to dequeue a packet, the PIE algorithm is in-
voked 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 de-
queue 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 se-
lected queue and returns the packet as the result of the dequeue opera-
tion.
PARAMETERS
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 classi-
fied. 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 de-
fault target delay is 15ms.
tupdate
It is the time interval at which the system drop probability is calcu-
lated. 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 de-
fault 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 pa-
rameter 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 byte-
mode 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.
EXAMPLES
# 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
SEE ALSO
tc(8), tc-pie(8), tc-fq_codel(8)
SOURCES
RFC 8033: https://tools.ietf.org/html/rfc8033
AUTHORS
FQ-PIE was implemented by Mohit P. Tahiliani. Please report corrections
to the Linux Networking mailing list <netdev@vger.kernel.org>.
iproute2 23 January 2020 FQ-PIE(8)
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