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NAME
polling - device polling support
SYNOPSIS
options DEVICE_POLLING
DESCRIPTION
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 perfor‐
mance.
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 when‐
ever 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 poten‐
tially 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.
Enabling polling
Currently only network interface drivers support the polling feature. It
is turned on and off with help of ifconfig(8) command.
MIB Variables
The operation of polling is controlled by the following sysctl(8) MIB
variables:
kern.polling.user_frac
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 pro‐
cessing. Default is 50.
kern.polling.burst
Maximum number of packets grabbed from each network interface in
each timer tick. This number is dynamically adjusted by the ker‐
nel, according to the programmed user_frac, burst_max, CPU speed,
and system load.
kern.polling.each_burst
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.
kern.polling.burst_max
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
HZ=1000.
kern.polling.idle_poll
Controls if polling is enabled in the idle loop. There are no
reasons (other than power saving or bugs in the scheduler’s han‐
dling of idle priority kernel threads) to disable this.
kern.polling.reg_frac
Controls how often (every reg_frac / HZ seconds) the status reg‐
isters 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.
kern.polling.handlers
How many active devices have registered for polling.
kern.polling.enable
Legacy MIB, that was used to enable or disable polling globally.
Currently if set to 1, polling is enabled on all capable inter‐
faces. If set to 0, polling is disabled on all interfaces.
kern.polling.short_ticks
kern.polling.lost_polls
kern.polling.pending_polls
kern.polling.residual_burst
kern.polling.phase
kern.polling.suspect
kern.polling.stalled
Debugging variables.
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), 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.
HISTORY
Device polling first appeared in FreeBSD 4.6 and FreeBSD 5.0.
AUTHORS
Device polling was written by Luigi Rizzo 〈luigi@iet.unipi.it〉.