smp(4)

NAME

SMP - description of the FreeBSD Symmetric Multi-Processor

kernel

SYNOPSIS

options SMP

DESCRIPTION

The SMP kernel implements symmetric multi-processor support.

COMPATIBILITY

Support for multi-processor systems is present for all

Tier-1 architectures on FreeBSD. Currently, this includes alpha, amd64,

i386, ia64, and
sparc64. Support is enabled using options SMP. It is per

missible to use
the SMP kernel configuration on non-SMP equipped mother

boards.

I386 NOTES

For i386 systems, the SMP kernel supports motherboards that

follow the
Intel MP specification, version 1.4. In addition to options

SMP

also requires device apic. The mptable(1) command may be

used to view
the status of multi-processor support.

The number of CPUs detected by the system is available in

the read-only
sysctl variable hw.ncpu.

FreeBSD allows specific CPUs on a multi-processor system to

be disabled.
The sysctl variable 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.

FreeBSD supports hyperthreading on Intel CPU's on the i386

platform.
Since using logical CPUs can cause performance penalties un

der certain
loads, the logical CPUs can be disabled by setting the
machdep.hlt_logical_cpus sysctl to one.

SEE ALSO

mptable(1), sysctl(8), condvar(9), msleep(9), mtx_pool(9),

mutex(9),
sema(9), sx(9)

HISTORY

The SMP 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 al

ready been
merged with Lite2 kernel code.

FreeBSD 5.0 introduced support for a host of new synchro

nization 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 BSD/OS.

FreeBSD 5.0 also introduced support for SMP on the alpha,

ia64, and
sparc64 architectures.

AUTHORS

Steve Passe <fsmp@FreeBSD.org>

BSD December 17, 2004

BSD