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31 .Nd description of the FreeBSD Symmetric Multi-Processor kernel
37 kernel implements symmetric multi-processor support.
39 Support for multi-processor systems is present for all Tier-1
42 Currently, this includes amd64, i386, ia64, and sparc64.
43 Support is enabled using
45 It is permissible to use the SMP kernel configuration on non-SMP equipped
50 kernel supports motherboards that follow the Intel MP specification,
58 command may be used to view the status of multi-processor support.
60 The number of CPUs detected by the system is available in
61 the read-only sysctl variable
65 allows specific CPUs on a multi-processor system to be disabled.
68 is an integer bitmask denoting CPUs to halt, counting from 0.
69 Setting a bit to 1 will result in the corresponding CPU being
74 scheduler implements CPU topology detection and adjusts the scheduling
75 algorithms to make better use of modern multi-core CPUs.
77 .Va kern.sched.topology_spec
78 reflects the detected CPU hardware in a parsable XML format.
79 The top level XML tag is <groups>, which encloses one or more <group> tags
80 containing data about individual CPU groups.
81 A CPU group contains CPUs that are detected to be "close" together, usually
82 by being cores in a single multi-core processor.
83 Attributes available in a <group> tag are "level", corresponding to the
84 nesting level of the CPU group and "cache-level", corresponding to the
85 level of CPU caches shared by the CPUs in the group.
86 The <group> tag contains the <cpu> and <flags> tags.
87 The <cpu> tag describes CPUs in the group.
88 Its attributes are "count", corresponding to the number of CPUs in the
89 group and "mask", corresponding to the integer binary mask in which
90 each bit position set to 1 signifies a CPU belonging to the group.
91 The contents (CDATA) of the <cpu> tag is the comma-delimited list
92 of CPU indexes (derived from the "mask" attribute).
93 The <flags> tag contains special tags (if any) describing the relation
94 of the CPUs in the group.
95 The possible flags are currently "HTT" and "SMT", corresponding to
96 the various implementations of hardware multithreading.
97 An example topology_spec output for a system consisting of
98 two quad-core processors is:
101 <group level="1" cache-level="0">
102 <cpu count="8" mask="0xff">0, 1, 2, 3, 4, 5, 6, 7</cpu>
105 <group level="2" cache-level="0">
106 <cpu count="4" mask="0xf">0, 1, 2, 3</cpu>
109 <group level="2" cache-level="0">
110 <cpu count="4" mask="0xf0">4, 5, 6, 7</cpu>
118 This information is used internally by the kernel to schedule related
119 tasks on CPUs that are closely grouped together.
122 supports hyperthreading on Intel CPU's on the i386 and AMD64 platforms.
123 Since using logical CPUs can cause performance penalties under certain loads,
124 the logical CPUs can be disabled by setting the
125 .Va machdep.hlt_logical_cpus
127 Note that this operation is different from the mechanism used by the
145 kernel's early history is not (properly) recorded.
147 in a separate CVS branch until April 26, 1997, at which point it was
148 merged into 3.0-current.
149 By this date 3.0-current had already been
150 merged with Lite2 kernel code.
153 introduced support for a host of new synchronization primitives, and
154 a move towards fine-grained kernel locking rather than reliance on
156 The SMPng Project relied heavily on the support of BSDi, who provided
157 reference source code from the fine-grained SMP implementation found
162 also introduced support for SMP on the ia64 and sparc64 architectures.
164 .An Steve Passe Aq fsmp@FreeBSD.org