POLLING(4) BSD Kernel Interfaces Manual POLLING(4)NAMEpolling — 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 sched‐
uling of a CPU between various tasks (user processes, software inter‐
rupts, 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 DragonFly
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 on clock inter‐
rupts. 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. An interface does
not have to be “up” in order to turn on its polling feature.
Loader Tunables
The following tunables can be set from loader.conf(5):
kern.polling.enable
If set to non-zero, polling is enabled. Default is enabled.
kern.polling.cpumask
A bitmask that controls which CPUs support device polling.
Default is 0xffffffff.
MIB Variables
The operation of polling is controlled by the following per CPU sysctl(8)
MIB variables (X is the CPU number):
kern.polling.X.enable
If set to non-zero, polling is enabled. Default is enabled.
kern.polling.X.pollhz
The polling frequency, whose range is 1 to 30000. Default is
2000.
kern.polling.cpumask
A read only bitmask of the CPUs that support device polling.
kern.polling.defcpu
The default CPU used to run device polling (read only).
kern.polling.X.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.X.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.X.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.X.burst_max
Upper bound for kern.polling.burst. Note that when polling is
enabled, each interface can receive at most (pollhz * 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
pollhz=1000.
kern.polling.X.reg_frac
Controls how often (every reg_frac / pollhz 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.
kern.polling.X.handlers
How many active devices have registered for polling.
kern.polling.X.short_ticks
kern.polling.X.lost_polls
kern.polling.X.pending_polls
kern.polling.X.residual_burst
kern.polling.X.phase
kern.polling.X.suspect
kern.polling.X.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), fxp(4),
jme(4), nfe(4), nge(4), re(4), rl(4), sis(4), stge(4), vge(4), vr(4),
wi(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.)
In order to reduce the latency in processing packets, it is advisable to
set the sysctl(8) variable kern.polling.X.pollhz to at least 1000.
HISTORY
Device polling first appeared in FreeBSD 4.6. It was rewritten in
DragonFly 1.3.
AUTHORS
The device polling code was rewritten by Matt Dillon based on the origi‐
nal code by Luigi Rizzo ⟨luigi@iet.unipi.it⟩. Sepherosa Ziehau made the
polling frequency settable at runtime and added per CPU polling.
BSD October 2, 2007 BSD
