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 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.

       SMP support can be disabled by setting the loader tunable kern.smp.disabled to 1.

       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 sparc64 architecture.

AUTHORS

       Steve Passe <fsmp@FreeBSD.org>

Debian                                           January 6, 2018                                          SMP(4)