SMP4
Description of the freebsd symmetric multi-processor kernel
Usage
options SMP
Overview
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 permissible to use the SMP kernel configuration on non-SMP equipped motherboards.
I386 notes
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.