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SMP - description of the FreeBSD Symmetric Multi-Processor kernel
The SMP kernel implements symmetric multi-processor support.
Support for multi-processor systems is present for all Tier-1
architectures on FreeBSD. Currently, this includes amd64, i386, ia64,
and sparc64. Support is enabled using options SMP. It is permissible to
use the SMP kernel configuration on non-SMP equipped motherboards.
For i386 systems, the SMP kernel supports motherboards that follow the
Intel MP specification, version 1.4. In addition to options SMP, i386
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 under certain
loads, the logical CPUs can be disabled by setting the
machdep.hlt_logical_cpus sysctl to one.
mptable(1), sysctl(8), condvar(9), msleep(9), mtx_pool(9), mutex(9),
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 already
been merged with Lite2 kernel code.
FreeBSD 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 BSD/OS.
FreeBSD 5.0 also introduced support for SMP on the ia64 and sparc64
Steve Passe 〈fsmp@FreeBSD.org〉