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     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.

   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 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.

   Enabling polling
     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

   MIB Variables
     The operation of polling is controlled by the following sysctl(8) MIB variables:

             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.

             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.

             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.

             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.

             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.

             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.

             How many active devices have registered for polling.

             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 <>.