— device polling support
Device polling ( polling
for brevity) refers to a technique
that lets the operating system periodically poll devices, instead of relying
on the devices to generate interrupts when they need attention. This might
seem inefficient and counterintuitive, but when done properly,
gives more control to the operating system on when
and how to handle devices, with a number of advantages in terms of system
responsiveness and performance.
In particular, polling
reduces the overhead for context
switches which is incurred when servicing interrupts, and gives more control
on the scheduling of the CPU between various tasks (user processes, software
interrupts, device handling) which ultimately reduces the chances of livelock
in the system.
Principles of Operation¶
In the normal, interrupt-based mode, devices generate an interrupt whenever they
need attention. This in turn causes a context switch and the execution of an
interrupt handler which performs whatever processing is needed by the device.
The duration of the interrupt handler is potentially unbounded unless the
device driver has been programmed with real-time concerns in mind (which is
generally not the case for FreeBSD
Furthermore, under heavy traffic load, the system might be persistently
processing interrupts without being able to complete other work, either in the
kernel or in userland.
Device polling disables interrupts by polling devices at appropriate times,
i.e., on clock interrupts and within the idle loop. This way, the context
switch overhead is removed. Furthermore, the operating system can control
accurately how much work to spend in handling device events, and thus prevent
livelock by reserving some amount of CPU to other tasks.
also changes the way software network
interrupts are scheduled, so there is never the risk of livelock because
packets are not processed to completion.
Currently only network interface drivers support the polling
feature. It is turned on and off with help of ifconfig(8)
The historic kern.polling.enable
, which enabled polling
for all interfaces, can be replaced with the following code:
for i in `ifconfig -l` ;
do ifconfig $i polling; # use -polling to disable
The operation of polling
is controlled by the following
- When polling is enabled, and provided
that there is some work to do, up to this percent of the CPU cycles is
reserved to userland tasks, the remaining fraction being available for
polling processing. Default is 50.
- Maximum number of packets grabbed from each network
interface in each timer tick. This number is dynamically adjusted by the
kernel, according to the programmed user_frac,
burst_max, CPU speed, and system load.
- The burst above is split into smaller chunks of this number
of packets, going round-robin among all interfaces registered for
polling. This prevents the case that a large burst from
a single interface can saturate the IP interrupt queue
(net.inet.ip.intr_queue_maxlen). Default is 5.
- Upper bound for kern.polling.burst.
Note that when polling is enabled, each interface can
receive at most (HZ *
burst_max) packets per second unless there are spare
CPU cycles available for polling in the idle loop. This
number should be tuned to match the expected load (which can be quite high
with GigE cards). Default is 150 which is adequate for 100Mbit network and
- Controls if polling is enabled in the
idle loop. There are no reasons (other than power saving or bugs in the
scheduler's handling of idle priority kernel threads) to disable this.
- Controls how often (every reg_frac
/ HZ seconds) the status
registers of the device are checked for error conditions and the like.
Increasing this value reduces the load on the bus, but also delays the
error detection. Default is 20.
- How many active devices have registered for
- Debugging variables.
Device polling requires explicit modifications to the device drivers. As of this
writing, the bge(4)
devices are supported, with others in the works.
The modifications are rather straightforward, consisting in the extraction of
the inner part of the interrupt service routine and writing a callback
(), which is invoked to probe the device for
events and process them. (See the conditionally compiled sections of the
devices mentioned above for more details.)
As in the worst case the devices are only polled on clock interrupts, in order
to reduce the latency in processing packets, it is not advisable to decrease
the frequency of the clock below 1000 Hz.
Device polling first appeared in FreeBSD 4.6
Device polling was written by Luigi Rizzo