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 issome work to do, up to this percent of the CPU cycles is reservedto userland tasks, the remaining fraction being available forpolling 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