.\" Copyright (c) 1997
.\"	Steve Passe <fsmp@FreeBSD.ORG>.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. The name of the developer may NOT be used to endorse or promote products
.\"    derived from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd December 17, 2004
.Dt SMP 4
.Os
.Sh NAME
.Nm SMP
.Nd description of the FreeBSD Symmetric Multi-Processor kernel
.Sh SYNOPSIS
.Cd options SMP
.Sh DESCRIPTION
The
.Nm
kernel implements symmetric multi-processor support.
.Sh COMPATIBILITY
Support for multi-processor systems is present for all Tier-1
architectures on
.Fx .
Currently, this includes amd64, i386, ia64, and sparc64.
Support is enabled using
.Cd options SMP .
It is permissible to use the SMP kernel configuration on non-SMP equipped
motherboards.
.Sh I386 NOTES
For i386 systems, the
.Nm
kernel supports motherboards that follow the Intel MP specification,
version 1.4.
In addition to
.Cd options SMP ,
i386 also requires
.Cd device apic .
The
.Xr mptable 1
command may be used to view the status of multi-processor support.
.Pp
The number of CPUs detected by the system is available in
the read-only sysctl variable
.Va hw.ncpu .
.Pp
.Fx
allows specific CPUs on a multi-processor system to be disabled.
The sysctl variable
.Va machdep.hlt_cpus
is an integer bitmask denoting CPUs to halt, counting from 0.
Setting a bit to 1 will result in the corresponding CPU being
disabled.
.Pp
.Fx
supports hyperthreading on Intel CPU's on the i386 platform.
Since using logical CPUs can cause performance penalties under certain loads,
the logical CPUs can be disabled by setting the
.Va machdep.hlt_logical_cpus
sysctl to one.
.Sh SEE ALSO
.Xr mptable 1 ,
.Xr sysctl 8 ,
.Xr condvar 9 ,
.Xr msleep 9 ,
.Xr mtx_pool 9 ,
.Xr mutex 9 ,
.Xr sema 9 ,
.Xr sx 9
.Sh HISTORY
The
.Nm
kernel's early history is not (properly) recorded.
It was developed
in a separate CVS branch until April 26, 1997, at which point it was
merged into 3.0-current.
By this date 3.0-current had already been
merged with Lite2 kernel code.
.Pp
.Fx 5.0
introduced support for a host of new synchronization primitives, and
a move towards fine-grained kernel locking rather than reliance on
a Giant kernel lock.
The SMPng Project relied heavily on the support of BSDi, who provided
reference source code from the fine-grained SMP implementation found
in
.Bsx .
.Pp
.Fx 5.0
also introduced support for SMP on the ia64 and sparc64 architectures.
.Sh AUTHORS
.An Steve Passe Aq fsmp@FreeBSD.org