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SMP(4)			     Kernel Interfaces Manual			  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 amd64, i386, ia64, and sparc64.  Sup-
     port 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 In-
     tel 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 sta-
     tus 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.
     This can be done using the hint.lapic.X.disabled tunable, where  X  is  the
     APIC ID of a CPU.	Setting this tunable to 1 will result in the correspond-
     ing CPU being disabled.

     The  sched_ule(4)	scheduler  implements CPU topology detection and adjusts
     the scheduling algorithms to make better use  of  modern  multi-core  CPUs.
     The  sysctl  variable  kern.sched.topology_spec  reflects	the detected CPU
     hardware in a parsable XML format.  The top  level  XML  tag  is  <groups>,
     which  encloses  one  or more <group> tags containing data about individual
     CPU groups.  A CPU group contains CPUs that are detected to be "close"  to-
     gether,  usually  by being cores in a single multi-core processor.  Attrib-
     utes available in a <group> tag are "level", corresponding to  the  nesting
     level of the CPU group and "cache-level", corresponding to the level of CPU
     caches shared by the CPUs in the group.  The <group> tag contains the <cpu>
     and  <flags> tags.  The <cpu> tag describes CPUs in the group.  Its attrib-
     utes are "count", corresponding to the number of  CPUs  in  the  group  and
     "mask", corresponding to the integer binary mask in which each bit position
     set  to  1 signifies a CPU belonging to the group.  The contents (CDATA) of
     the <cpu> tag is the comma-delimited list of CPU indexes (derived from  the
     "mask" attribute).  The <flags> tag contains special tags (if any) describ-
     ing  the  relation  of  the CPUs in the group.  The possible flags are cur-
     rently "HTT" and "SMT", corresponding to  the  various  implementations  of
     hardware multithreading.  An example topology_spec output for a system con-
     sisting of two quad-core processors is:

     <groups>
       <group level="1" cache-level="0">
	 <cpu count="8" mask="0xff">0, 1, 2, 3, 4, 5, 6, 7</cpu>
	 <flags></flags>
	 <children>
	   <group level="2" cache-level="0">
	     <cpu count="4" mask="0xf">0, 1, 2, 3</cpu>
	     <flags></flags>
	   </group>
	   <group level="2" cache-level="0">
	     <cpu count="4" mask="0xf0">4, 5, 6, 7</cpu>
	     <flags></flags>
	   </group>
	 </children>
       </group>
     </groups>

     This information is used internally by the kernel to schedule related tasks
     on CPUs that are closely grouped together.

     FreeBSD  supports hyperthreading on Intel CPU's on the i386 and AMD64 plat-
     forms.  Because using logical CPUs can cause  performance	penalties  under
     certain   loads,	the   logical  CPUs  can  be  disabled	by  setting  the
     machdep.hyperthreading_allowed tunable to zero.

SEE ALSO
     cpuset(1), mptable(1), sched_4bsd(4), sched_ule(4),  loader(8),  sysctl(8),
     condvar(9), msleep(9), mtx_pool(9), mutex(9), rwlock(9), sema(9), sx(9)

HISTORY
     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  imple-
     mentation found in BSD/OS.

     FreeBSD  5.0 also introduced support for SMP on the ia64 and sparc64 archi-
     tectures.

AUTHORS
     Steve Passe <fsmp@FreeBSD.org>

FreeBSD 9.1			   May 7, 2008				  SMP(4)
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