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     SMP — description of the FreeBSD Symmetric Multi-Processor kernel


     options SMP


     The SMP kernel implements symmetric 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

     The number of online threads per CPU core is available in the read-only sysctl variable
     kern.smp.threads_per_core.  The number of physical CPU cores detected by the system is
     available in the read-only sysctl variable kern.smp.cores.

     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.

     FreeBSD supports simultaneous multithreading on x86 and powerpc platforms.  On x86, the
     logical CPUs can be disabled by setting the machdep.hyperthreading_allowed tunable to zero.

     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:

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

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


     Support for multi-processor systems is present for all Tier-1 and Tier-2 architectures on
     FreeBSD.  Currently, this includes x86, powerpc, arm, and sparc64.  Support is enabled using
     options SMP.  It is permissible to use the SMP kernel configuration on non-SMP hardware.


     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.


     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)


     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.


     Steve Passe <>


     The kern.smp.threads_per_core and kern.smp.cores sysctl variables are provided as a best-
     effort guess.  If an architecture or platform adds SMT and FreeBSD has not yet implemented
     detection, the reported values may be inaccurate.  In this case, kern.smp.threads_per_core
     will report 1 and kern.smp.cores will report the same value as hw.ncpu.