FQ_CoDel(8) Linux FQ_CoDel(8)
NAME
CoDel - Fair Queuing (FQ) with Controlled Delay (CoDel)
SYNOPSIS
tc qdisc ... fq_codel [ limit PACKETS ] [ flows NUMBER ] [ target TIME
] [ interval TIME ] [ quantum BYTES ] [ ecn | noecn ] [ ce_threshold
TIME ] [ ce_threshold_selector VALUE/MASK ] [ memory_limit BYTES ]
DESCRIPTION
FQ_Codel (Fair Queuing Controlled Delay) is queuing discipline that
combines Fair Queuing with the CoDel AQM scheme. FQ_Codel uses a sto-
chastic model to classify incoming packets into different flows and is
used to provide a fair share of the bandwidth to all the flows using
the queue. Each such flow is managed by the CoDel queuing discipline.
Reordering within a flow is avoided since Codel internally uses a FIFO
queue.
PARAMETERS
limit
has the same semantics as codel and is the hard limit on the real queue
size. When this limit is reached, incoming packets are dropped. De-
fault is 10240 packets.
memory_limit
sets a limit on the total number of bytes that can be queued in this
FQ-CoDel instance. The lower of the packet limit of the limit parameter
and the memory limit will be enforced. Default is 32 MB.
flows
is the number of flows into which the incoming packets are classified.
Due to the stochastic nature of hashing, multiple flows may end up be-
ing 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. Default value is 1024.
target
has the same semantics as codel and is the acceptable minimum stand-
ing/persistent queue delay. This minimum delay is identified by track-
ing the local minimum queue delay that packets experience. Default
value is 5ms.
interval
has the same semantics as codel and is used to ensure that the measured
minimum delay does not become too stale. The minimum delay must be ex-
perienced in the last epoch of length interval. It should be set on
the order of the worst-case RTT through the bottleneck to give end-
points sufficient time to react. Default value is 100ms.
quantum
is the number of bytes used as 'deficit' in the fair queuing algorithm.
Default is set to 1514 bytes which corresponds to the Ethernet MTU plus
the hardware header length of 14 bytes.
ecn | noecn
has the same semantics as codel 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. Unlike codel, ecn is turned on by default.
ce_threshold
sets a threshold above which all packets are marked with ECN Congestion
Experienced. This is useful for DCTCP-style congestion control algo-
rithms that require marking at very shallow queueing thresholds.
ce_threshold_selector
sets a filter so that the ce_threshold feature is applied to only a
subset of the traffic seen by the qdisc. If set, the MASK value will be
applied as a bitwise AND to the diffserv/ECN byte of the IP header, and
only if the result of this masking equals VALUE, will the ce_threshold
logic be applied to the packet.
drop_batch
sets the maximum number of packets to drop when limit or memory_limit
is exceeded. Default value is 64.
EXAMPLES
#tc qdisc add dev eth0 root fq_codel
#tc -s qdisc show
qdisc fq_codel 8002: dev eth0 root refcnt 2 limit 10240p flows 1024
quantum 1514
target 5.0ms interval 100.0ms ecn
Sent 428514 bytes 2269 pkt (dropped 0, overlimits 0 requeues 0)
backlog 0b 0p requeues 0
maxpacket 256 drop_overlimit 0 new_flow_count 0 ecn_mark 0
new_flows_len 0 old_flows_len 0
#tc qdisc add dev eth0 root fq_codel limit 2000 target 3ms interval
40ms noecn
#tc -s qdisc show
qdisc fq_codel 8003: dev eth0 root refcnt 2 limit 2000p flows 1024
quantum 1514 target 3.0ms interval 40.0ms
Sent 2588985006 bytes 1783629 pkt (dropped 0, overlimits 0 requeues
34869)
backlog 0b 0p requeues 34869
maxpacket 65226 drop_overlimit 0 new_flow_count 73 ecn_mark 0
new_flows_len 1 old_flows_len 3
SEE ALSO
tc(8), tc-codel(8), tc-red(8)
AUTHORS
FQ_CoDel was implemented by Eric Dumazet. This manpage was written by
Vijay Subramanian. Please report corrections to the Linux Networking
mailing list <netdev@vger.kernel.org>.
iproute2 4 June 2012 FQ_CoDel(8)
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