221 lines
6.9 KiB
Groff
221 lines
6.9 KiB
Groff
.\" Copyright (c) 2002 Luigi Rizzo
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 26, 2020
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.Dt POLLING 4
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.Os
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.Sh NAME
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.Nm polling
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.Nd device polling support
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.Sh SYNOPSIS
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.Cd "options DEVICE_POLLING"
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.Sh DESCRIPTION
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Device polling
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.Nm (
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for brevity) refers to a technique that
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lets the operating system periodically poll devices, instead of
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relying on the devices to generate interrupts when they need attention.
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This might seem inefficient and counterintuitive, but when done
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properly,
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.Nm
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gives more control to the operating system on
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when and how to handle devices, with a number of advantages in terms
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of system responsiveness and performance.
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.Pp
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In particular,
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.Nm
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reduces the overhead for context
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switches which is incurred when servicing interrupts, and
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gives more control on the scheduling of the CPU between various
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tasks (user processes, software interrupts, device handling)
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which ultimately reduces the chances of livelock in the system.
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.Ss Principles of Operation
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In the normal, interrupt-based mode, devices generate an interrupt
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whenever they need attention.
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This in turn causes a
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context switch and the execution of an interrupt handler
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which performs whatever processing is needed by the device.
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The duration of the interrupt handler is potentially unbounded
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unless the device driver has been programmed with real-time
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concerns in mind (which is generally not the case for
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.Fx
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drivers).
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Furthermore, under heavy traffic load, the system might be
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persistently processing interrupts without being able to
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complete other work, either in the kernel or in userland.
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.Pp
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Device polling disables interrupts by polling devices at appropriate
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times, i.e., on clock interrupts and within the idle loop.
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This way, the context switch overhead is removed.
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Furthermore,
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the operating system can control accurately how much work to spend
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in handling device events, and thus prevent livelock by reserving
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some amount of CPU to other tasks.
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.Pp
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Enabling
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.Nm
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also changes the way software network interrupts
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are scheduled, so there is never the risk of livelock because
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packets are not processed to completion.
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.Ss Enabling polling
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Currently only network interface drivers support the
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.Nm
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feature.
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It is turned on and off with help of
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.Xr ifconfig 8
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command.
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.Pp
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The historic
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.Va kern.polling.enable ,
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which enabled polling for all interfaces, can be replaced with the following
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code:
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.Bd -literal
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for i in `ifconfig -l` ;
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do ifconfig $i polling; # use -polling to disable
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done
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.Ed
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.Ss MIB Variables
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The operation of
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.Nm
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is controlled by the following
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.Xr sysctl 8
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MIB variables:
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.Pp
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.Bl -tag -width indent -compact
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.It Va kern.polling.user_frac
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When
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.Nm
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is enabled, and provided that there is some work to do,
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up to this percent of the CPU cycles is reserved to userland tasks,
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the remaining fraction being available for
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.Nm
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processing.
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Default is 50.
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.Pp
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.It Va kern.polling.burst
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Maximum number of packets grabbed from each network interface in
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each timer tick.
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This number is dynamically adjusted by the kernel,
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according to the programmed
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.Va user_frac , burst_max ,
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CPU speed, and system load.
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.Pp
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.It Va kern.polling.each_burst
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The burst above is split into smaller chunks of this number of
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packets, going round-robin among all interfaces registered for
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.Nm .
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This prevents the case that a large burst from a single interface
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can saturate the IP interrupt queue
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.Pq Va net.inet.ip.intr_queue_maxlen .
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Default is 5.
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.Pp
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.It Va kern.polling.burst_max
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Upper bound for
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.Va kern.polling.burst .
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Note that when
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.Nm
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is enabled, each interface can receive at most
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.Pq Va HZ No * Va burst_max
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packets per second unless there are spare CPU cycles available for
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.Nm
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in the idle loop.
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This number should be tuned to match the expected load
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(which can be quite high with GigE cards).
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Default is 150 which is adequate for 100Mbit network and HZ=1000.
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.Pp
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.It Va kern.polling.idle_poll
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Controls if
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.Nm
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is enabled in the idle loop.
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There are no reasons (other than power saving or bugs in the scheduler's
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handling of idle priority kernel threads) to disable this.
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.Pp
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.It Va kern.polling.reg_frac
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Controls how often (every
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.Va reg_frac No / Va HZ
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seconds) the status registers of the device are checked for error
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conditions and the like.
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Increasing this value reduces the load on the bus, but also delays
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the error detection.
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Default is 20.
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.Pp
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.It Va kern.polling.handlers
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How many active devices have registered for
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.Nm .
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.Pp
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.It Va kern.polling.short_ticks
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.It Va kern.polling.lost_polls
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.It Va kern.polling.pending_polls
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.It Va kern.polling.residual_burst
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.It Va kern.polling.phase
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.It Va kern.polling.suspect
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.It Va kern.polling.stalled
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Debugging variables.
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.El
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.Sh SUPPORTED DEVICES
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Device polling requires explicit modifications to the device drivers.
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As of this writing, the
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.Xr bge 4 ,
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.Xr dc 4 ,
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.Xr em 4 ,
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.Xr fwe 4 ,
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.Xr fwip 4 ,
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.Xr fxp 4 ,
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.Xr igb 4 ,
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.Xr nfe 4 ,
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.Xr nge 4 ,
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.Xr re 4 ,
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.Xr rl 4 ,
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.Xr sis 4 ,
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.Xr ste 4 ,
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.Xr stge 4 ,
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.Xr vge 4 ,
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.Xr vr 4 ,
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and
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.Xr xl 4
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devices are supported, with others in the works.
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The modifications are rather straightforward, consisting in
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the extraction of the inner part of the interrupt service routine
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and writing a callback function,
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.Fn *_poll ,
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which is invoked
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to probe the device for events and process them.
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(See the
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conditionally compiled sections of the devices mentioned above
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for more details.)
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.Pp
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As in the worst case the devices are only polled on clock interrupts,
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in order to reduce the latency in processing packets, it is not advisable
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to decrease the frequency of the clock below 1000 Hz.
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.Sh HISTORY
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Device polling first appeared in
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.Fx 4.6
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and
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.Fx 5.0 .
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.Sh AUTHORS
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Device polling was written by
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.An Luigi Rizzo Aq Mt luigi@iet.unipi.it .
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