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.
ALGORITHM
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 drop'.
PARAMETERS
limit
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).
For bfifo, it defaults to the txqueuelen multiplied by the interface MTU.
OUTPUT
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)
