POLLING(4) Kernel Interfaces Manual POLLING(4) NAME polling -- device polling support SYNOPSIS options DEVICE_POLLING options HZ=1000 DESCRIPTION "Device polling" (polling for brevity) refers to a technique to handle de- vices that does not rely on the latter to generate interrupts when they need attention, but rather lets the CPU poll devices to service their needs. 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 re- sponsivity and performance. In particular, polling reduces the overhead for context switches which is incurred when servicing interrupts, and gives more control on the schedul- ing 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 interrupt whenever they need attention. This in turn causes a context switch and the execution of a interrupt handler which performs whatever processing is needed by the device. The duration of the interrupt handler is potentially unbounded un- less 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, the system might be persistently processing interrupts with- out being able to complete other work, either in the kernel or in userland. Polling disables interrupts by polling devices at appropriate times, i.e. on clock interrupts, system calls 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. Polling is enabled with a sysctl variable kern.polling.enable whereas the percentage of CPU cycles reserved to userland processes is controlled by the sysctl variable kern.polling.user_frac whose range is 0 to 100 (50 is the default value). When polling is enabled, and provided that there is work to do, up to user_frac percent of the CPU cycles is reserved to userland tasks, the re- maining fraction being available for device processing. Enabling polling also changes the way network software interrupts are scheduled, so there is never the risk of livelock because packets are not processed to completion. There are other variables which control or monitor the behaviour of devices operating in polling mode, but they are unlikely to require modifications, and are documented in the source file src/sys/kern/kern_poll.c SUPPORTED DEVICES Polling requires explicit modifications to the device drivers. As of this writing, the dc, fxp, rl and sis devices are supported, with other in the works. The modifications are rather straightforward, consisting in the ex- traction 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 de- vices mentioned above for more details. Because in the worst case devices are only polled on clock interrupts, in order to reduce the latency in processing packets it is advisable to in- crease the frequency of the clock to at least 1000 HZ. HISTORY Device polling was introduced in February 2002 by Luigi Rizzo <luigi@iet.unipi.it>. FreeBSD 4.7 February 15, 2002 POLLING(4)
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