1.\" Copyright (c) 2002 Luigi Rizzo 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD: src/share/man/man4/polling.4,v 1.27 2007/04/06 14:25:14 brueffer Exp $ 26.\" $DragonFly: src/share/man/man4/polling.4,v 1.13 2007/11/03 07:35:52 swildner Exp $ 27.\" 28.Dd November 16, 2012 29.Dt POLLING 4 30.Os 31.Sh NAME 32.Nm polling 33.Nd network device driver polling support 34.Sh SYNOPSIS 35.Cd "options IFPOLL_ENABLE" 36.Sh DESCRIPTION 37Device polling 38.Nm ( 39for brevity) refers to a technique that 40lets the operating system periodically poll devices, instead of 41relying on the devices to generate interrupts when they need attention. 42This might seem inefficient and counterintuitive, but when done 43properly, 44.Nm 45gives more control to the operating system on 46when and how to handle devices, with a number of advantages in terms 47of system responsiveness and performance. 48.Pp 49In particular, 50.Nm 51reduces the overhead for context 52switches which is incurred when servicing interrupts, and 53gives more control on the scheduling of a CPU between various 54tasks (user processes, software interrupts, device handling) 55which ultimately reduces the chances of livelock in the system. 56.Ss Principles of Operation 57In the normal, interrupt-based mode, devices generate an interrupt 58whenever they need attention. 59This in turn causes a 60context switch and the execution of an interrupt handler 61which performs whatever processing is needed by the device. 62The duration of the interrupt handler is potentially unbounded 63unless the device driver has been programmed with real-time 64concerns in mind (which is generally not the case for 65.Dx 66drivers). 67Furthermore, under heavy traffic load, the system might be 68persistently processing interrupts without being able to 69complete other work, either in the kernel or in userland. 70.Pp 71Device polling disables interrupts by polling devices on clock 72interrupts. 73This way, the context switch overhead is removed. 74Furthermore, 75the operating system can control accurately how much work to spend 76in handling device events, and thus prevent livelock by reserving 77some amount of CPU to other tasks. 78.Pp 79Enabling 80.Nm 81also changes the way software network interrupts 82are scheduled, so there is never the risk of livelock because 83packets are not processed to completion. 84.Ss Enabling polling 85Currently only network interface drivers support the 86.Nm 87feature. 88It is turned on and off with help of 89.Xr ifconfig 8 90command. 91An interface does not have to be 92.Dq up 93in order to turn on its 94.Nm 95feature. 96.Ss Loader Tunables 97The following tunables can be set from 98.Xr loader.conf 5 99.Em ( X 100is the CPU number): 101.Bl -tag -width indent -compact 102.It Va net.ifpoll.burst_max 103Default value for 104.Va net.ifpoll.X.rx.burst_max 105sysctl nodes. 106.Pp 107.It Va net.ifpoll.each_burst 108Default value for 109.Va net.ifpoll.X.rx.each_burst 110sysctl nodes. 111.Pp 112.It Va net.ifpoll.pollhz 113Default value for 114.Va net.ifpoll.X.pollhz 115sysctl nodes. 116.Pp 117.It Va net.ifpoll.status_frac 118Default value for 119.Va net.ifpoll.0.status_frac 120sysctl node. 121.Pp 122.It Va net.ifpoll.tx_frac 123Default value for 124.Va net.ifpoll.X.tx_frac 125sysctl nodes. 126.El 127.Ss MIB Variables 128The operation of 129.Nm 130is controlled by the following per CPU 131.Xr sysctl 8 132MIB variables 133.Em ( X 134is the CPU number): 135.Pp 136.Bl -tag -width indent -compact 137.It Va net.ifpoll.X.pollhz 138The polling frequency, whose range is 1 to 30000. 139Default is 4000. 140.Pp 141.It Va net.ifpoll.X.rx.user_frac 142When 143.Nm 144is enabled, and provided that there is some work to do, 145up to this percent of the CPU cycles is reserved to userland tasks, 146the remaining fraction being available for 147.Nm 148processing. 149Default is 50. 150.Pp 151.It Va net.ifpoll.X.rx.burst 152Maximum number of packets grabbed from each network interface in 153each timer tick. 154This number is dynamically adjusted by the kernel, 155according to the programmed 156.Va user_frac , burst_max , 157CPU speed, and system load. 158.Pp 159.It Va net.ifpoll.X.rx.each_burst 160The burst above is split into smaller chunks of this number of 161packets, going round-robin among all interfaces registered for 162.Nm . 163This prevents the case that a large burst from a single interface 164can saturate the IP interrupt queue. 165Default is 15. 166.Pp 167.It Va net.ifpoll.X.rx.burst_max 168Upper bound for 169.Va net.ifpoll.X.rx.burst . 170Note that when 171.Nm 172is enabled, each interface can receive at most 173.Pq Va pollhz No * Va burst_max 174packets per second unless there are spare CPU cycles available for 175.Nm 176in the idle loop. 177This number should be tuned to match the expected load. 178Default is 375 which is adequate for 1000Mbit network and pollhz=4000. 179.Pp 180.It Va net.ifpoll.X.rx.handlers 181How many active devices have registered for packet reception 182.Nm . 183.Pp 184.It Va net.ifpoll.X.tx_frac 185Controls how often (every 186.Va tx_frac No / Va pollhz 187seconds) the tranmission queue is checked for packet transmission 188done events. 189Increasing this value reduces the time spent on checking packets 190transmission done events thus reduces bus load, 191but it also increases chance 192that the transmission queue getting saturated. 193Default is 1. 194.Pp 195.It Va net.ifpoll.X.tx.handlers 196How many active devices have registered for packet transmission 197.Nm . 198.Pp 199.It Va net.ifpoll.0.status_frac 200Controls how often (every 201.Va status_frac No / Va pollhz 202seconds) the status registers of the device are checked for error 203conditions and the like. 204Increasing this value reduces the load on the bus, 205but also delays the error detection. 206Default is 80. 207.Pp 208.It Va net.ifpoll.0.status.handlers 209How many active devices have registered for status 210.Nm . 211.Pp 212.It Va net.ifpoll.X.rx.short_ticks 213.It Va net.ifpoll.X.rx.lost_polls 214.It Va net.ifpoll.X.rx.pending_polls 215.It Va net.ifpoll.X.rx.residual_burst 216.It Va net.ifpoll.X.rx.phase 217.It Va net.ifpoll.X.rx.suspect 218.It Va net.ifpoll.X.rx.stalled 219.It Va net.ifpoll.X.tx.short_ticks 220.It Va net.ifpoll.X.tx.lost_polls 221.It Va net.ifpoll.X.tx.pending_polls 222.It Va net.ifpoll.X.tx.residual_burst 223.It Va net.ifpoll.X.tx.phase 224.It Va net.ifpoll.X.tx.suspect 225.It Va net.ifpoll.X.tx.stalled 226Debugging variables. 227.El 228.Sh SUPPORTED DEVICES 229Device polling requires explicit modifications to the device drivers. 230As of this writing, the 231.Xr bce 4 , 232.Xr bge 4 , 233.Xr bnx 4 , 234.Xr dc 4 , 235.Xr em 4 , 236.Xr emx 4 , 237.Xr fwe 4 , 238.Xr fxp 4 , 239.Xr igb 4 , 240.Xr jme 4 , 241.Xr nfe 4 , 242.Xr nge 4 , 243.Xr re 4 , 244.Xr rl 4 , 245.Xr sis 4 , 246.Xr stge 4 , 247.Xr vge 4 , 248.Xr vr 4 , 249and 250.Xr xl 4 251devices are supported, 252with others in the works. 253The 254.Xr emx 4 , 255.Xr igb 4 , 256and 257.Xr jme 4 258support multiple reception queues based 259.Nm . 260The modifications are rather straightforward, consisting in 261the extraction of the inner part of the interrupt service routine 262and writing a callback function, 263.Fn *_npoll , 264which is invoked 265to probe the device for events and process them. 266(See the 267conditionally compiled sections of the devices mentioned above 268for more details.) 269.Pp 270In order to reduce the latency in processing packets, 271it is advisable to set the 272.Xr sysctl 8 273variable 274.Va net.ifpoll.X.pollhz 275to at least 1000. 276.Sh HISTORY 277Device polling first appeared in 278.Fx 4.6 . 279It was rewritten in 280.Dx 1.3 . 281.Sh AUTHORS 282.An -nosplit 283The device polling code was rewritten by 284.An Matt Dillon 285based on the original code by 286.An Luigi Rizzo Aq luigi@iet.unipi.it . 287.An Sepherosa Ziehau 288made the polling frequency settable at runtime, 289added per CPU polling 290and added multiple reception queue polling support. 291