polling —
device
polling support
options DEVICE_POLLING
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 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.
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
done
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 adjusted 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 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 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 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.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),
igb(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
<
[email protected]>.