алгоритм управления активной очередью с контролируемой задержкой (Controlled-Delay Active Queue Management algorithm)
Имя (Name)
CoDel - Controlled-Delay Active Queue Management algorithm
Синопсис (Synopsis)
tc qdisc ... codel [ limit PACKETS ] [ target TIME ] [ interval
TIME ] [ ecn | noecn ] [ ce_threshold TIME ]
Описание (Description)
CoDel (pronounced "coddle") is an adaptive "no-knobs" active
queue management algorithm (AQM) scheme that was developed to
address the shortcomings of RED and its variants. It was
developed with the following goals in mind:
o It should be parameterless.
o It should keep delays low while permitting bursts of traffic.
o It should control delay.
o It should adapt dynamically to changing link rates with no
impact on utilization.
o It should be simple and efficient and should scale from simple
to complex routers.
Алгоритм (Algorithm)
CoDel comes with three major innovations. Instead of using queue
size or queue average, it uses the local minimum queue as a
measure of the standing/persistent queue. Second, it uses a
single state-tracking variable of the minimum delay to see where
it is relative to the standing queue delay. Third, instead of
measuring queue size in bytes or packets, it is measured in
packet-sojourn time in the queue.
CoDel measures the minimum local queue delay (i.e. standing queue
delay) and compares it to the value of the given acceptable queue
delay target. As long as the minimum queue delay is less than
target or the buffer contains fewer than MTU worth of bytes,
packets are not dropped. Codel enters a dropping mode when the
minimum queue delay has exceeded target for a time greater than
interval. In this mode, packets are dropped at different drop
times which is set by a control law. The control law ensures that
the packet drops cause a linear change in the throughput. Once
the minimum delay goes below target, packets are no longer
dropped.
Additional details can be found in the paper cited below.
Параметры (Parameters)
limit
hard limit on the real queue size. When this limit is reached,
incoming packets are dropped. If the value is lowered, packets
are dropped so that the new limit is met. Default is 1000
packets.
target
is the acceptable minimum standing/persistent queue delay. This
minimum delay is identified by tracking the local minimum queue
delay that packets experience. Default and recommended value is
5ms.
interval
is used to ensure that the measured minimum delay does not become
too stale. The minimum delay must be experienced in the last
epoch of length interval. It should be set on the order of the
worst-case RTT through the bottleneck to give endpoints
sufficient time to react. Default value is 100ms.
ecn | noecn
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.
By default, ecn is turned off.
ce_threshold
sets a threshold above which all packets are marked with ECN
Congestion Experienced. This is useful for DCTCP-style congestion
control algorithms that require marking at very shallow queueing
thresholds.
Примеры (Examples)
# tc qdisc add dev eth0 root codel
# tc -s qdisc show
qdisc codel 801b: dev eth0 root refcnt 2 limit 1000p target
5.0ms interval 100.0ms
Sent 245801662 bytes 275853 pkt (dropped 0, overlimits 0
requeues 24)
backlog 0b 0p requeues 24
count 0 lastcount 0 ldelay 2us drop_next 0us
maxpacket 7306 ecn_mark 0 drop_overlimit 0
# tc qdisc add dev eth0 root codel limit 100 target 4ms interval
30ms ecn
# tc -s qdisc show
qdisc codel 801c: dev eth0 root refcnt 2 limit 100p target
4.0ms interval 30.0ms ecn
Sent 237573074 bytes 268561 pkt (dropped 0, overlimits 0
requeues 5)
backlog 0b 0p requeues 5
count 0 lastcount 0 ldelay 76us drop_next 0us
maxpacket 2962 ecn_mark 0 drop_overlimit 0
Смотри также (See also)
tc(8), tc-red(8)
Источники (Sources)
o Kathleen Nichols and Van Jacobson, "Controlling Queue Delay",
ACM Queue, http://queue.acm.org/detail.cfm?id=2209336
