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SMP(4) Kernel Interfaces Manual SMP(4) NAME SMP -- description of the FreeBSD Symmetric Multi-Processor kernel SYNOPSIS options SMP DESCRIPTION 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 hw.ncpu. 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 correspond- ing 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" to- gether, usually by being cores in a single multi-core processor. Attrib- utes 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 attrib- utes 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) describ- ing the relation of the CPUs in the group. The possible flags are cur- rently "HTT" and "SMT", corresponding to the various implementations of hardware multithreading. An example topology_spec output for a system con- sisting 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. COMPATIBILITY Support for multi-processor systems is present for all Tier-1 and Tier-2 architectures on FreeBSD. Currently, this includes x86, powerpc, mips, arm and arm64. Support is enabled using options SMP. It is permissible to use the SMP kernel configuration on non-SMP hardware. I386 NOTES For i386 systems, the SMP kernel supports motherboards that follow the In- tel 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 sta- tus of multi-processor support. SEE ALSO 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) HISTORY The SMP kernel's early history is not (properly) recorded. It was devel- oped 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 primi- tives, 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 imple- mentation found in BSD/OS. FreeBSD 5.0 also introduced support for SMP on the sparc64 architecture. AUTHORS Steve Passe <fsmp@FreeBSD.org> CAVEATS The kern.smp.threads_per_core and kern.smp.cores sysctl variables are pro- vided as a best-effort guess. If an architecture or platform adds SMT and FreeBSD has not yet implemented detection, the reported values may be inac- curate. In this case, kern.smp.threads_per_core will report 1 and kern.smp.cores will report the same value as hw.ncpu. FreeBSD ports 15.quarterly January 4, 2019 SMP(4)
NAME | SYNOPSIS | DESCRIPTION | COMPATIBILITY | I386 NOTES | SEE ALSO | HISTORY | AUTHORS | CAVEATS
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