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, ia64,
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.
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 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. 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).
SEE ALSO¶
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)
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 ia64
and sparc64 architectures.
AUTHORS¶
Steve Passe ⟨fsmp@FreeBSD.org⟩