1.\" Copyright (c) 1997 2.\" Steve Passe <fsmp@FreeBSD.org>. 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. The name of the developer may NOT be used to endorse or promote products 10.\" derived from this software without specific prior written permission. 11.\" 12.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 13.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 14.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 15.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 16.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 17.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 18.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 19.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 20.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 21.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 22.\" SUCH DAMAGE. 23.\" 24.\" $FreeBSD$ 25.\" 26.Dd January 4, 2019 27.Dt SMP 4 28.Os 29.Sh NAME 30.Nm SMP 31.Nd description of the FreeBSD Symmetric Multi-Processor kernel 32.Sh SYNOPSIS 33.Cd options SMP 34.Sh DESCRIPTION 35The 36.Nm 37kernel implements symmetric multi-processor support. 38.Pp 39.Nm 40support can be disabled by setting the loader tunable 41.Va kern.smp.disabled 42to 1. 43.Pp 44The number of CPUs detected by the system is available in 45the read-only sysctl variable 46.Va hw.ncpu . 47.Pp 48The number of online threads per CPU core is available in the read-only sysctl 49variable 50.Va kern.smp.threads_per_core . 51The number of physical CPU cores detected by the system is available in the 52read-only sysctl variable 53.Va kern.smp.cores . 54.Pp 55.Fx 56allows specific CPUs on a multi-processor system to be disabled. 57This can be done using the 58.Va hint.lapic.X.disabled 59tunable, where X is the APIC ID of a CPU. 60Setting this tunable to 1 will result in the corresponding CPU being 61disabled. 62.Pp 63.Fx 64supports simultaneous multithreading on x86 and powerpc platforms. 65On x86, the logical CPUs can be disabled by setting the 66.Va machdep.hyperthreading_allowed 67tunable to zero. 68.Pp 69The 70.Xr sched_ule 4 71scheduler implements CPU topology detection and adjusts the scheduling 72algorithms to make better use of modern multi-core CPUs. 73The sysctl variable 74.Va kern.sched.topology_spec 75reflects the detected CPU hardware in a parsable XML format. 76The top level XML tag is <groups>, which encloses one or more <group> tags 77containing data about individual CPU groups. 78A CPU group contains CPUs that are detected to be "close" together, usually 79by being cores in a single multi-core processor. 80Attributes available in a <group> tag are "level", corresponding to the 81nesting level of the CPU group and "cache-level", corresponding to the 82level of CPU caches shared by the CPUs in the group. 83The <group> tag contains the <cpu> and <flags> tags. 84The <cpu> tag describes CPUs in the group. 85Its attributes are "count", corresponding to the number of CPUs in the 86group and "mask", corresponding to the integer binary mask in which 87each bit position set to 1 signifies a CPU belonging to the group. 88The contents (CDATA) of the <cpu> tag is the comma-delimited list 89of CPU indexes (derived from the "mask" attribute). 90The <flags> tag contains special tags (if any) describing the relation 91of the CPUs in the group. 92The possible flags are currently "HTT" and "SMT", corresponding to 93the various implementations of hardware multithreading. 94An example topology_spec output for a system consisting of 95two quad-core processors is: 96.Bd -literal 97<groups> 98 <group level="1" cache-level="0"> 99 <cpu count="8" mask="0xff">0, 1, 2, 3, 4, 5, 6, 7</cpu> 100 <flags></flags> 101 <children> 102 <group level="2" cache-level="0"> 103 <cpu count="4" mask="0xf">0, 1, 2, 3</cpu> 104 <flags></flags> 105 </group> 106 <group level="2" cache-level="0"> 107 <cpu count="4" mask="0xf0">4, 5, 6, 7</cpu> 108 <flags></flags> 109 </group> 110 </children> 111 </group> 112</groups> 113.Ed 114.Pp 115This information is used internally by the kernel to schedule related 116tasks on CPUs that are closely grouped together. 117.Sh COMPATIBILITY 118Support for multi-processor systems is present for all Tier-1 and Tier-2 119architectures on 120.Fx . 121Currently, this includes x86, powerpc, arm, and sparc64. 122Support is enabled using 123.Cd options SMP . 124It is permissible to use the SMP kernel configuration on non-SMP hardware. 125.Sh I386 NOTES 126For i386 systems, the 127.Nm 128kernel supports motherboards that follow the Intel MP specification, 129version 1.4. 130In addition to 131.Cd options SMP , 132i386 also requires 133.Cd device apic . 134The 135.Xr mptable 1 136command may be used to view the status of multi-processor support. 137.Sh SEE ALSO 138.Xr cpuset 1 , 139.Xr mptable 1 , 140.Xr sched_4bsd 4 , 141.Xr sched_ule 4 , 142.Xr loader 8 , 143.Xr sysctl 8 , 144.Xr condvar 9 , 145.Xr msleep 9 , 146.Xr mtx_pool 9 , 147.Xr mutex 9 , 148.Xr rwlock 9 , 149.Xr sema 9 , 150.Xr sx 9 151.Sh HISTORY 152The 153.Nm 154kernel's early history is not (properly) recorded. 155It was developed 156in a separate CVS branch until April 26, 1997, at which point it was 157merged into 3.0-current. 158By this date 3.0-current had already been 159merged with Lite2 kernel code. 160.Pp 161.Fx 5.0 162introduced support for a host of new synchronization primitives, and 163a move towards fine-grained kernel locking rather than reliance on 164a Giant kernel lock. 165The SMPng Project relied heavily on the support of BSDi, who provided 166reference source code from the fine-grained SMP implementation found 167in 168.Bsx . 169.Pp 170.Fx 5.0 171also introduced support for SMP on the sparc64 architecture. 172.Sh AUTHORS 173.An Steve Passe Aq Mt fsmp@FreeBSD.org 174.Sh CAVEATS 175The 176.Va kern.smp.threads_per_core 177and 178.Va kern.smp.cores 179sysctl variables are provided as a best-effort guess. 180If an architecture or platform adds SMT and 181.Fx 182has not yet implemented detection, the reported values may be inaccurate. 183In this case, 184.Va kern.smp.threads_per_core 185will report 186.Dv 1 187and 188.Va kern.smp.cores 189will report the same value as 190.Va hw.ncpu . 191