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SMP(4)                   BSD Kernel Interfaces Manual                   SMP(4)

     SMP -- description of the FreeBSD Symmetric MultiProcessor kernel

     options SMP

     The SMP kernel implements symmetric multiprocessor support.

     Support for multi-processor systems is present for all Tier-1 architec-
     tures 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.

     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 corre-
     sponding 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),
     sema(9), sx(9)

     The SMP kernel's early history is not (properly) recorded.  It was devel-
     oped 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 primi-
     tives, 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.

     Steve Passe <fsmpATFreeBSD.org>

BSD                              March 1, 2005                             BSD