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.  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.  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 corresponding 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" together, usually by being cores in a  single  multi-core  processor.   Attributes
       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 attributes 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)  describing  the relation of the CPUs in the group.  The
       possible flags are currently "HTT" and "SMT", corresponding to the various  implementations  of  hardware
       multithreading.  An example topology_spec output for a system consisting 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 platforms.   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 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 architectures.

AUTHORS

       Steve Passe <fsmp@FreeBSD.org>

Debian                                             May 7, 2008                                            SMP(4)