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.\" 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 5, 2002
.Dt SMP 4
.Os
.Sh NAME
.Nm SMP
.Nd description of the FreeBSD Symmetric MultiProcessor kernel
.Sh SYNOPSIS
.Cd options SMP
.Sh DESCRIPTION
The
.Nm
kernel implements symmetric multiprocessor support.
.Sh COMPATIBILITY
Support for multi-processor systems is present for all Tier-1
architectures on
.Fx .
Currently, this includes alpha, i386, ia64, and sparc64.
Support is enabled using
.Cd options SMP .
On most platforms it is permissible to use the SMP
kernel configuration on non-SMP equipped motherboards.
The only exception to this rule is the i386 platform.
.Pp
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 options APIC_IO .
The
.Xr mptable 1
command may be used to view the status of multi-processor support.
.Pp
.Fx
supports hyperthreading on Intel CPU's on the i386 platform.
By default, logical CPUs are not used to execute user processes due to
performance penalties under common loads.
To allow the logical CPUs to execute user processes,
turn off the
.Va machdep.hlt_logical_cpus
sysctl by setting its value to zero.
.Sh SEE ALSO
.Xr mptable 1 ,
.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 alpha, ia64, and sparc64 architectures.
.Sh AUTHORS
.An Steve Passe Aq fsmp@FreeBSD.org
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