polling(4)

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 rely
ing 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 han
dle devices,

with a number of advantages in terms of system responsive
ness 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 in
terrupt 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 pro
grammed with realtime 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 ap
propriate

times, i.e., on clock interrupts and within the idle loop.
This way, the

context switch overhead is removed. Furthermore, the oper
ating 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 in
terrupts 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 processing. Default is 50.
kern.polling.burst
Maximum number of packets grabbed from each network
interface in

each timer tick. This number is dynamically adjust
ed by the kernel, 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 reg
istered 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 handling of idle priority kernel threads) to disable
this.
kern.polling.reg_frac
Controls how often (every reg_frac / HZ seconds) the
status registers of the device are checked for error condi
tions 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 ca
pable interfaces. If set to 0, polling is disabled on all in
terfaces.
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.

SUPPORTED DEVICES

Device polling requires explicit modifications to the device

drivers. As

of this writing, the bce(4), 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), 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 <lui

gi@iet.unipi.it>.
BSD April 13, 2006
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