View source for SMP on x86

The primary specification for multiprocessor [[x86]]-based setups is the [http://www.intel.com/design/pentium/datashts/242016.HTM 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.

==libvirt==
* <tt>virsh(1)</tt>'s <tt>nodeinfo</tt> command can be pretty useful:
<pre>[wopr](0) $ virsh nodeinfo
CPU model:           x86_64
CPU(s):              16
CPU frequency:       1600 MHz
CPU socket(s):       4
Core(s) per socket:  4
Thread(s) per core:  1
NUMA cell(s):        1
Memory size:         66113480 kB

[wopr](0) $ </pre>

<pre>[recombinator](0) $ sudo virsh nodeinfo
CPU model:           x86_64
CPU(s):              2
CPU frequency:       1596 MHz
CPU socket(s):       1
Core(s) per socket:  2
Thread(s) per core:  1
NUMA cell(s):        1
Memory size:         3908568 kB

[recombinator](0) $ </pre>

==/proc/cpuinfo==
On [[Linux]] kernels with the <tt>proc</tt> filesystem enabled (and [[FreeBSD]] kernels with the <tt>linprocfs</tt> module loaded, although this does not provide all of the information as native Linux /proc/cpuinfo), the mounted <tt>proc/linprocfs</tt> filesystem contains a file <tt>cpuinfo</tt> (this is independent of any CPU-related modules being loaded, particularly <tt>cpuid</tt> on Linux or either OS's <tt>cpu</tt> 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, and thus the maximum physical_id does not by itself determine the number of sockets!
* 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.

*EMT64 Xeon, no HyperThreading support, 4 cores per socket, 4 sockets: 16 total execution units (2.6.26)
<pre>[wopr](0) $ cat /proc/cpuinfo | egrep ^proc\|^model\ \|^phys\|^sib\|^core\|^cpu\ c\|^ap\|^init
processor	: 0
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 0
siblings	: 4
core id		: 0
cpu cores	: 4
apicid		: 0
initial apicid	: 0

processor	: 1
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 2
siblings	: 4
core id		: 0
cpu cores	: 4
apicid		: 8
initial apicid	: 8

processor	: 2
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 4
siblings	: 4
core id		: 0
cpu cores	: 4
apicid		: 16
initial apicid	: 16

processor	: 3
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 6
siblings	: 4
core id		: 0
cpu cores	: 4
apicid		: 24
initial apicid	: 24

processor	: 4
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 0
siblings	: 4
core id		: 2
cpu cores	: 4
apicid		: 2
initial apicid	: 2

processor	: 5
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 2
siblings	: 4
core id		: 2
cpu cores	: 4
apicid		: 10
initial apicid	: 10

processor	: 6
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 4
siblings	: 4
core id		: 2
cpu cores	: 4
apicid		: 18
initial apicid	: 18

processor	: 7
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 6
siblings	: 4
core id		: 2
cpu cores	: 4
apicid		: 26
initial apicid	: 26

processor	: 8
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 0
siblings	: 4
core id		: 1
cpu cores	: 4
apicid		: 1
initial apicid	: 1

processor	: 9
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 2
siblings	: 4
core id		: 1
cpu cores	: 4
apicid		: 9
initial apicid	: 9

processor	: 10
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 4
siblings	: 4
core id		: 1
cpu cores	: 4
apicid		: 17
initial apicid	: 17

processor	: 11
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 6
siblings	: 4
core id		: 1
cpu cores	: 4
apicid		: 25
initial apicid	: 25

processor	: 12
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 0
siblings	: 4
core id		: 3
cpu cores	: 4
apicid		: 3
initial apicid	: 3

processor	: 13
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 2
siblings	: 4
core id		: 3
cpu cores	: 4
apicid		: 11
initial apicid	: 11

processor	: 14
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 4
siblings	: 4
core id		: 3
cpu cores	: 4
apicid		: 19
initial apicid	: 19

processor	: 15
model name	: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz
physical id	: 6
siblings	: 4
core id		: 3
cpu cores	: 4
apicid		: 27
initial apicid	: 27
[wopr](0) $ </pre>
*Core 2 Duo, no HyperThreading support, 2 cores per socket, 1 socket: 2 total execution units (2.6.26)
<pre>[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) $</pre>
*P4 Xeon, HyperThreading enabled, 1 core per socket, 2 sockets: 4 total execution units (2.6.25-2-686)
<pre>scurdev@hrududu:~$ 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:~$</pre>
*P4 Xeon, HyperThreading disabled, 1 core per socket, 2 sockets: 2 total execution units (2.6.25-2-686)
<pre>[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) $ </pre>
*P4 Xeon Celeron, HyperThreading disabled, 1 core per socket, 1 socket: 1 total execution unit (2.6.25-2-686)
<pre>[knuth](0) $ cat /proc/cpuinfo 
processor	: 0
model name	: Intel(R) Celeron(R) CPU 2.00GHz
[knuth](0) $ </pre>

==Sysctl==
On FreeBSD, CPU/SMP information is primarily exported through the <tt>sysctl(8)</tt> interface. Seemingly relevant sysctls are listed below:
* <tt>kern.smp.cpus</tt> and <tt>hw.ncpu</tt>
* <tt>machdep.hyperthreading.allowed</tt>
* <tt>kern.threads.virtual_cpu</tt>

==See also==
* [http://osdir.com/ml/linux.ports.x86-64.general/2005-10/msg00022.html This LKML thread] provides much information