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polling - 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, polling 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
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
drivers). 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.
Enabling polling 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) command.
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 sysctl(8) MIB
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 polling.
Device polling requires explicit modifications to the device drivers. As
of this writing, the bge(4), dc(4), em(4), fwe(4), fwip(4), fxp(4),
ixgb(4), nfe(4), nge(4), re(4), rl(4), sf(4), sis(4), ste(4), stge(4),
vge(4), vr(4), and xl(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 function, *_poll(), 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 and FreeBSD 5.0.
Device polling was written by Luigi Rizzo 〈firstname.lastname@example.org〉.