SMP on x86

The primary specification for multiprocessor x86-based setups is the Intel MultiProcessor Specification (last updated, AFAIK, to revision-006 on 1995-05-15).

• ACPI
• MP Table
• Interactions with cpuid
• HyperThreading requires CPU, BIOS and OS support. It's present only on some P4's and Xeons, NOT Core2Duo.

Contents 1 /proc/cpuinfo 1.1 Examples 2 Sysctl 3 See also

/proc/cpuinfo

On Linux kernels with the proc filesystem enabled (and FreeBSD kernels with the linprocfs module loaded, although this does not provide all of the information as native Linux /proc/cpuinfo), the mounted proc/linprocfs filesystem contains a file cpuinfo (this is independent of any CPU-related modules being loaded, particularly cpuid on Linux or either OS's cpu module). Interpreting this file, as it pertains to multiple execution units, can be difficult. The following applies to Linux 2.6 kernels:

• A physical_id corresponds to a socket ("physical package"), of which there are >=1 per machine. Physical IDs do not necessarily monotonically increase across processors!
• A core_id corresponds to a core ("logical processor"), of which there are >=1 per physical_id
• A processor ID corresponds to an architectural state (HyperThreading == 2 per HyperThreaded core)
• HyperThreading is in use only if 'siblings' != 'cpu cores' (from http://kbase.redhat.com/faq/FAQ_46_10715.shtm)
• The 'ht' processor capabilities bit corresponds not to HyperThreading, but to the ability to report sibling count

Examples

I've removed all output from the following examples, save that related to SMP identification.

• Core 2 Duo, no HyperThreading support, 2 cores per socket, 1 socket: 2 total execution units (2.6.26)

[recombinator](0) $ cat /proc/cpuinfo 
processor	: 0
model name	: Intel(R) Core(TM)2 CPU          6600  @ 2.40GHz
physical id	: 0
siblings	: 2
core id		: 0
cpu cores	: 2
apicid		: 0
initial apicid	: 0

processor	: 1
model name	: Intel(R) Core(TM)2 CPU          6600  @ 2.40GHz
physical id	: 0
siblings	: 2
core id		: 1
cpu cores	: 2
apicid		: 1
initial apicid	: 1
[recombinator](0) $

• P4 Xeon, HyperThreading enabled, 1 core per socket, 2 sockets: 4 total execution units (2.6.25-2-686)

scurdev@hrududu:~$ cat /proc/cpuinfo 
processor	: 0
model name	: Intel(R) Xeon(TM) CPU 2.40GHz
physical id	: 0
siblings	: 2
core id		: 0
cpu cores	: 1

processor	: 1
model name	: Intel(R) Xeon(TM) CPU 2.40GHz
physical id	: 0
siblings	: 2
core id		: 0
cpu cores	: 1

processor	: 2
model name	: Intel(R) Xeon(TM) CPU 2.40GHz
physical id	: 3
siblings	: 2
core id		: 0
cpu cores	: 1

processor	: 3
model name	: Intel(R) Xeon(TM) CPU 2.40GHz
physical id	: 3
siblings	: 2
core id		: 0
cpu cores	: 1
scurdev@hrududu:~$

• P4 Xeon, HyperThreading disabled, 1 core per socket, 2 sockets: 2 total execution units (2.6.25-2-686)

[aho](0) $ cat /proc/cpuinfo 
processor	: 0
model name	: Intel(R) Xeon(TM) CPU 2.80GHz

processor	: 1
model name	: Intel(R) Xeon(TM) CPU 2.80GHz
[aho](0) $ 

• P4 Xeon Celeron, HyperThreading disabled, 1 core per socket, 1 socket: 1 total execution unit (2.6.25-2-686)

[knuth](0) $ cat /proc/cpuinfo 
processor	: 0
model name	: Intel(R) Celeron(R) CPU 2.00GHz
[knuth](0) $ 

Sysctl

On FreeBSD, CPU/SMP information is primarily exported through the sysctl(8) interface. Seemingly relevant sysctls are listed below:

• kern.smp.cpus and hw.ncpu
• machdep.hyperthreading.allowed
• kern.threads.virtual_cpu

See also

• This LKML thread provides much information