ETF
Section: Linux (8)
Updated: 05 Jul 2018
Page Index
NAME
ETF - Earliest TxTime First (ETF) Qdisc
SYNOPSIS
tc qdisc ... dev
dev
parent
classid
[ handle
major:
] etf clockid
clockid
[ delta
delta_nsecs
] [ deadline_mode ]
[ offload ]
DESCRIPTION
The ETF (Earliest TxTime First) qdisc allows applications to control
the instant when a packet should be dequeued from the traffic control
layer into the netdevice. If
offload
is configured and supported by the network interface card, the it will
also control when packets leave the network controller.
ETF achieves that by buffering packets until a configurable time
before their transmission time (i.e. txtime, or deadline), which can
be configured through the
delta
option.
The qdisc uses a rb-tree internally so packets are always 'ordered' by
their txtime and will be dequeued following the (next) earliest txtime
first.
It relies on the SO_TXTIME socket option and the SCM_TXTIME CMSG in
each packet field to configure the behavior of time dependent sockets:
the clockid to be used as a reference, if the expected mode of txtime
for that socket is deadline or strict mode, and if packet drops should
be reported on the socket's error queue. See
socket(7)
for more information.
The etf qdisc will drop any packets with a txtime in the past, or if a
packet expires while waiting for being dequeued.
This queueing discipline is intended to be used by TSN (Time Sensitive
Networking) applications, and it exposes a traffic shaping functionality
that is commonly documented as "Launch Time" or "Time-Based Scheduling"
by vendors and the documentation of network interface controllers.
ETF is meant to be installed under another qdisc that maps packet flows
to traffic classes, one example is
mqprio(8).
PARAMETERS
- clockid
-
Specifies the clock to be used by qdisc's internal timer for measuring
time and scheduling events. The qdisc expects that packets passing
through it to be using this same
clockid
as the reference of their txtime timestamps. It will drop packets
coming from sockets that do not comply with that.
For more information about time and clocks on Linux, please refer
to
time(7)
and
clock_gettime(3).
- delta
-
After enqueueing or dequeueing a packet, the qdisc will schedule its
next wake-up time for the next txtime minus this delta value.
This means
delta
can be used as a fudge factor for the scheduler latency of a system.
This value must be specified in nanoseconds.
The default value is 0 nanoseconds.
- deadline_mode
-
When
deadline_mode
is set, the qdisc will handle txtime with a different semantics,
changed from a 'strict' transmission time to a deadline.
In practice, this means during the dequeue flow
etf(8)
will set the txtime of the packet being dequeued to 'now'.
The default is for this option to be disabled.
- offload
-
When
offload
is set,
etf(8)
will try to configure the network interface so time-based transmission
arbitration is enabled in the controller. This feature is commonly
referred to as "Launch Time" or "Time-Based Scheduling" by the
documentation of network interface controllers.
The default is for this option to be disabled.
- skip_sock_check
-
etf(8)
currently drops any packet which does not have a socket associated with it or
if the socket does not have SO_TXTIME socket option set. But, this will not
work if the launchtime is set by another entity inside the kernel (e.g. some
other Qdisc). Setting the skip_sock_check will skip checking for a socket
associated with the packet.
EXAMPLES
ETF is used to enforce a Quality of Service. It controls when each
packets should be dequeued and transmitted, and can be used for
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 queueing discipline by ETF in traffic class
number 0, issue:
# tc qdisc replace dev eth0 parent 100:1 etf \
clockid CLOCK_TAI delta 300000 offload
With the options above, etf will be configured to use CLOCK_TAI as
its clockid_t, will schedule packets for 300 us before their txtime,
and will enable the functionality on that in the network interface
card. Deadline mode will not be configured for this mode.
AUTHORS
Jesus Sanchez-Palencia <
jesus.sanchez-palencia@intel.com>
Vinicius Costa Gomes <
vinicius.gomes@intel.com>