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31 .Nd description of the FreeBSD Symmetric Multi-Processor kernel
37 kernel implements symmetric multi-processor support.
40 support can be disabled by setting the loader tunable
44 The number of CPUs detected by the system is available in
45 the read-only sysctl variable
48 The number of online threads per CPU core is available in the read-only sysctl
50 .Va kern.smp.threads_per_core .
51 The number of physical CPU cores detected by the system is available in the
52 read-only sysctl variable
56 allows specific CPUs on a multi-processor system to be disabled.
57 This can be done using the
58 .Va hint.lapic.X.disabled
59 tunable, where X is the APIC ID of a CPU.
60 Setting this tunable to 1 will result in the corresponding CPU being
64 supports simultaneous multithreading on x86 and powerpc platforms.
65 On x86, the logical CPUs can be disabled by setting the
66 .Va machdep.hyperthreading_allowed
71 scheduler implements CPU topology detection and adjusts the scheduling
72 algorithms to make better use of modern multi-core CPUs.
74 .Va kern.sched.topology_spec
75 reflects the detected CPU hardware in a parsable XML format.
76 The top level XML tag is <groups>, which encloses one or more <group> tags
77 containing data about individual CPU groups.
78 A CPU group contains CPUs that are detected to be "close" together, usually
79 by being cores in a single multi-core processor.
80 Attributes available in a <group> tag are "level", corresponding to the
81 nesting level of the CPU group and "cache-level", corresponding to the
82 level of CPU caches shared by the CPUs in the group.
83 The <group> tag contains the <cpu> and <flags> tags.
84 The <cpu> tag describes CPUs in the group.
85 Its attributes are "count", corresponding to the number of CPUs in the
86 group and "mask", corresponding to the integer binary mask in which
87 each bit position set to 1 signifies a CPU belonging to the group.
88 The contents (CDATA) of the <cpu> tag is the comma-delimited list
89 of CPU indexes (derived from the "mask" attribute).
90 The <flags> tag contains special tags (if any) describing the relation
91 of the CPUs in the group.
92 The possible flags are currently "HTT" and "SMT", corresponding to
93 the various implementations of hardware multithreading.
94 An example topology_spec output for a system consisting of
95 two quad-core processors is:
98 <group level="1" cache-level="0">
99 <cpu count="8" mask="0xff">0, 1, 2, 3, 4, 5, 6, 7</cpu>
102 <group level="2" cache-level="0">
103 <cpu count="4" mask="0xf">0, 1, 2, 3</cpu>
106 <group level="2" cache-level="0">
107 <cpu count="4" mask="0xf0">4, 5, 6, 7</cpu>
115 This information is used internally by the kernel to schedule related
116 tasks on CPUs that are closely grouped together.
118 Support for multi-processor systems is present for all Tier-1 and Tier-2
121 Currently, this includes x86, powerpc, mips, arm and arm64.
122 Support is enabled using
124 It is permissible to use the SMP kernel configuration on non-SMP hardware.
126 For i386 systems, the
128 kernel supports motherboards that follow the Intel MP specification,
136 command may be used to view the status of multi-processor support.
154 kernel's early history is not (properly) recorded.
156 in a separate CVS branch until April 26, 1997, at which point it was
157 merged into 3.0-current.
158 By this date 3.0-current had already been
159 merged with Lite2 kernel code.
162 introduced support for a host of new synchronization primitives, and
163 a move towards fine-grained kernel locking rather than reliance on
165 The SMPng Project relied heavily on the support of BSDi, who provided
166 reference source code from the fine-grained SMP implementation found
171 also introduced support for SMP on the sparc64 architecture.
173 .An Steve Passe Aq Mt fsmp@FreeBSD.org
176 .Va kern.smp.threads_per_core
179 sysctl variables are provided as a best-effort guess.
180 If an architecture or platform adds SMT and
182 has not yet implemented detection, the reported values may be inaccurate.
184 .Va kern.smp.threads_per_core
189 will report the same value as