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SMP — description of the FreeBSD Symmetric Multi-Processor kernel
The SMP kernel implements symmetric multi-processor support.
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.
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.
The sysctl variable machdep.hlt_cpus is an integer bitmask denoting CPUs
to halt, counting from 0. Setting a bit 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
<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.
FreeBSD supports hyperthreading on Intel CPU's on the i386 and AMD64
platforms. Since using logical CPUs can cause performance penalties
under certain loads, the logical CPUs can be disabled by setting the
machdep.hlt_logical_cpus sysctl to one. Note that this operation is
different from the mechanism used by the cpuset(1).
mptable(1), sysctl(8), condvar(9), msleep(9), mtx_pool(9), mutex(9),
sema(9), sx(9), rwlock(9), sched_4bsd(4), sched_ule(4), cpuset(1)
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
Steve Passe ⟨fsmp@FreeBSD.org⟩