кредитный шейпер (CBS) Qdisc (Credit Based Shaper (CBS) Qdisc)
Имя (Name)
CBS - Credit Based Shaper (CBS) Qdisc
Синопсис (Synopsis)
tc qdisc ... dev dev parent classid [ handle major: ] cbs
idleslope idleslope sendslope sendslope hicredit hicredit
locredit locredit [ offload 0|1 ]
Описание (Description)
The CBS (Credit Based Shaper) qdisc implements the shaping
algorithm defined by the IEEE 802.1Q-2014 Section 8.6.8.2, which
applies a well defined rate limiting method to the traffic.
This queueing discipline is intended to be used by TSN (Time
Sensitive Networking) applications, the CBS parameters are
derived directly by what is described by the Annex L of the IEEE
802.1Q-2014 Specification. The algorithm and how it affects the
latency are detailed there.
CBS is meant to be installed under another qdisc that maps packet
flows to traffic classes, one example is mqprio(8).
Параметры (Parameters)
idleslope
Idleslope is the rate of credits that is accumulated (in
kilobits per second) when there is at least one packet
waiting for transmission. Packets are transmitted when
the current value of credits is equal or greater than
zero. When there is no packet to be transmitted the amount
of credits is set to zero. This is the main tunable of the
CBS algorithm and represents the bandwidth that will be
consumed. Note that when calculating idleslope, the
entire packet size must be considered, including headers
from all layers (i.e. MAC framing and any overhead from
the physical layer), as described by IEEE 802.1Q-2014
section 34.4.
As an example, for an ethernet frame carrying 284 bytes of
payload, and with no VLAN tags, you must add 14 bytes for
the Ethernet headers, 4 bytes for the Frame check sequence
(CRC), and 20 bytes for the L1 overhead: 12 bytes of
interpacket gap, 7 bytes of preamble and 1 byte of start
of frame delimiter. That results in 322 bytes for the
total packet size, which is then used for calculating the
idleslope.
sendslope
Sendslope is the rate of credits that is depleted (it
should be a negative number of kilobits per second) when a
transmission is occurring. It can be calculated as
follows, (IEEE 802.1Q-2014 Section 8.6.8.2 item g):
sendslope = idleslope - port_transmit_rate
hicredit
Hicredit defines the maximum amount of credits (in bytes)
that can be accumulated. Hicredit depends on the
characteristics of interfering traffic,
'max_interference_size' is the maximum size of any burst
of traffic that can delay the transmission of a frame that
is available for transmission for this traffic class,
(IEEE 802.1Q-2014 Annex L, Equation L-3):
hicredit = max_interference_size * (idleslope /
port_transmit_rate)
locredit
Locredit is the minimum amount of credits that can be
reached. It is a function of the traffic flowing through
this qdisc (IEEE 802.1Q-2014 Annex L, Equation L-2):
locredit = max_frame_size * (sendslope /
port_transmit_rate)
offload
When offload is 1, cbs(8) will try to configure the
network interface so the CBS algorithm runs in the
controller. The default is 0.
Примеры (Examples)
CBS is used to enforce a Quality of Service by limiting the data
rate of a traffic class, to separate packets into traffic classes
the user may choose mqprio(8), and configure it like this:
# tc qdisc add dev eth0 handle 100: parent root mqprio num_tc 3 \
map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
queues 1@0 1@1 2@2 \
hw 0
To replace the current queuing disciple by CBS in the current
queueing discipline connected to traffic class number 0, issue:
# tc qdisc replace dev eth0 parent 100:4 cbs \
locredit -1470 hicredit 30 sendslope -980000 idleslope 20000
These values are obtained from the following parameters,
idleslope is 20mbit/s, the transmission rate is 1Gbit/s and the
maximum interfering frame size is 1500 bytes.
