device polling support
Device 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
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.
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.
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
feature. It is turned on and off
with help of
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
is controlled by the
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
- Upper bound for kern.polling.burst. Note
polling is enabled, each
interface can receive at most (HZ
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
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 function,
(), 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