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POLLING(4)             FreeBSD 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
     devices 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
     responsivity 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 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 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, the system might be
     persistently processing interrupts without 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
     remaining 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 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.

     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
     increase 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 11.0-PRERELEASE        February 15, 2002       FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | PRINCIPLES OF OPERATION | SUPPORTED DEVICES | HISTORY

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