SMP(4)

📦 version 15.1, in freebsd 14.5, freebsd 15.1, nixpkgs unstable

okay bestie, welcome to SMP(4) from the Kernel Interfaces Manual — this one is all about the FreeBSD Symmetric Multi-Processor kernel and it's lowkey iconic 💖✨

🎀 NAME 🎀

SMP — the description of the FreeBSD Symmetric Multi-Processor kernel, aka the page that explains how your many-core machine gets its whole vibe on 🦋

🌸 SYNOPSIS 🌸

options SMP

that's it, that's the synopsis — one line, no notes, periodt 💅

✨ DESCRIPTION ✨

The SMP kernel implements symmetric multi-processor support. fr fr, that's the whole point of it.

SMP support can be disabled by setting the loader tunable kern.smp.disabled to 1 — like a little off switch for the whole multithreading situation 🙈

The number of CPUs detected by the system is available in the read-only sysctl variable hw.ncpu. cute, love a read-only receipt.

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. both read-only, both doing their lil job 💅

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 corresponding CPU being disabled. bye-bye specific CPU, you will not be missed 🫶

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. zero means off, bestie, no cap.

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. the kernel is basically doing a vibe check and putting the right tasks with the right CPU besties 🔥

💻 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 — so yes, you can absolutely run the SMP config on non-SMP hardware, it's giving flexible queen behavior 👑

🧸 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. cute little i386-specific lore, we love a footnote moment 🎀

👀 SEE ALSO 👀

go peek at these besties:

📜 HISTORY 📜

The SMP kernel's early history is not (properly) recorded — like, the receipts are missing 😭 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. ancient lore, honestly.

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. shoutout BSDi, real ones 🫶

FreeBSD 5.0 also introduced support for SMP on the sparc64 architecture. sparc64 said "I'm here too" and we respect it 💅

👑 AUTHORS 👑

Steve Passe <fsmp@FreeBSD.org> — icon, legend, the moment, we stan 💖

💀 CAVEATS 💀

⚠️ caveat bestie, listen up: the kern.smp.threads_per_core and kern.smp.cores sysctl variables are provided as a best-effort guess. If an architecture or platform adds SMT and FreeBSD has not yet implemented detection, the reported values may be inaccurate. In this case, kern.smp.threads_per_core will report 1 and kern.smp.cores will report the same value as hw.ncpu. so like, don't take those numbers as gospel if the hardware is doing something new and shiny 😭

this lil page is from Debian, dated January 4, 2019, and it's SMP(4) — that's the whole vibe, bye bestie 💖🎀

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