pfifo - Packet limited First In, First Out queue
bfifo - Byte limited First In, First Out queue
tc qdisc ... add pfifo [ limit
tc qdisc ... add bfifo [ limit
The pfifo and bfifo qdiscs are unadorned First In, First Out queues. They are
the simplest queues possible and therefore have no overhead. pfifo
constrains the queue size as measured in packets. bfifo
does so as
measured in bytes.
Like all non-default qdiscs, they maintain statistics. This might be a reason to
prefer pfifo or bfifo over the default.
A list of packets is maintained, when a packet is enqueued it gets inserted at
the tail of a list. When a packet needs to be sent out to the network, it is
taken from the head of the list.
If the list is too long, no further packets are allowed on. This is called 'tail
- Maximum queue size. Specified in bytes for bfifo, in
packets for pfifo. For pfifo, defaults to the interface txqueuelen, as
specified with ifconfig(8) or ip(8). The range for this
parameter is [0, UINT32_MAX].
For bfifo, it defaults to the txqueuelen multiplied by the interface MTU.
The range for this parameter is [0, UINT32_MAX] bytes.
Note: The link layer header was considered when counting packets length.
The output of tc -s qdisc ls
contains the limit, either in packets or in
bytes, and the number of bytes and packets actually sent. An unsent and
dropped packet only appears between braces and is not counted as 'Sent'.
In this example, the queue length is 100 packets, 45894 bytes were sent over 681
packets. No packets were dropped, and as the pfifo queue does not slow down
packets, there were also no overlimits:
# tc -s qdisc ls dev eth0
qdisc pfifo 8001: dev eth0 limit 100p
Sent 45894 bytes 681 pkts (dropped 0, overlimits 0)
If a backlog occurs, this is displayed as well.
Alexey N. Kuznetsov, <email@example.com>
This manpage maintained by bert hubert <firstname.lastname@example.org>