bionic (4) polling.4freebsd.gz

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 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
     done

   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 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.

SUPPORTED DEVICES

     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), ixgb(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.

HISTORY

     Device polling first appeared in FreeBSD 4.6 and FreeBSD 5.0.

AUTHORS

     Device polling was written by Luigi Rizzo <luigi@iet.unipi.it>.