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

BSD