[linux-2.6-block.git] / Documentation / x86 / topology.txt

x86 Topology
============

This documents and clarifies the main aspects of x86 topology modelling and
representation in the kernel. Update/change when doing changes to the
respective code.

The architecture-agnostic topology definitions are in
Documentation/cputopology.txt. This file holds x86-specific
differences/specialities which must not necessarily apply to the generic
definitions. Thus, the way to read up on Linux topology on x86 is to start
with the generic one and look at this one in parallel for the x86 specifics.

Needless to say, code should use the generic functions - this file is *only*
here to *document* the inner workings of x86 topology.

Started by Thomas Gleixner <tglx@linutronix.de> and Borislav Petkov <bp@alien8.de>.

The main aim of the topology facilities is to present adequate interfaces to
code which needs to know/query/use the structure of the running system wrt
threads, cores, packages, etc.

The kernel does not care about the concept of physical sockets because a
socket has no relevance to software. It's an electromechanical component. In
the past a socket always contained a single package (see below), but with the
advent of Multi Chip Modules (MCM) a socket can hold more than one package. So
there might be still references to sockets in the code, but they are of
historical nature and should be cleaned up.

The topology of a system is described in the units of:

    - packages
    - cores
    - threads

* Package:

  Packages contain a number of cores plus shared resources, e.g. DRAM
  controller, shared caches etc.

  AMD nomenclature for package is 'Node'.

  Package-related topology information in the kernel:

  - cpuinfo_x86.x86_max_cores:

    The number of cores in a package. This information is retrieved via CPUID.

  - cpuinfo_x86.phys_proc_id:

    The physical ID of the package. This information is retrieved via CPUID
    and deduced from the APIC IDs of the cores in the package.

  - cpuinfo_x86.logical_id:

    The logical ID of the package. As we do not trust BIOSes to enumerate the
    packages in a consistent way, we introduced the concept of logical package
    ID so we can sanely calculate the number of maximum possible packages in
    the system and have the packages enumerated linearly.

  - topology_max_packages():

    The maximum possible number of packages in the system. Helpful for per
    package facilities to preallocate per package information.


* Cores:

  A core consists of 1 or more threads. It does not matter whether the threads
  are SMT- or CMT-type threads.

  AMDs nomenclature for a CMT core is "Compute Unit". The kernel always uses
  "core".

  Core-related topology information in the kernel:

  - smp_num_siblings:

    The number of threads in a core. The number of threads in a package can be
    calculated by:

	threads_per_package = cpuinfo_x86.x86_max_cores * smp_num_siblings


* Threads:

  A thread is a single scheduling unit. It's the equivalent to a logical Linux
  CPU.

  AMDs nomenclature for CMT threads is "Compute Unit Core". The kernel always
  uses "thread".

  Thread-related topology information in the kernel:

  - topology_core_cpumask():

    The cpumask contains all online threads in the package to which a thread
    belongs.

    The number of online threads is also printed in /proc/cpuinfo "siblings."

  - topology_sibling_mask():

    The cpumask contains all online threads in the core to which a thread
    belongs.

   - topology_logical_package_id():

    The logical package ID to which a thread belongs.

   - topology_physical_package_id():

    The physical package ID to which a thread belongs.

   - topology_core_id();

    The ID of the core to which a thread belongs. It is also printed in /proc/cpuinfo
    "core_id."


System topology examples

Note:

The alternative Linux CPU enumeration depends on how the BIOS enumerates the
threads. Many BIOSes enumerate all threads 0 first and then all threads 1.
That has the "advantage" that the logical Linux CPU numbers of threads 0 stay
the same whether threads are enabled or not. That's merely an implementation
detail and has no practical impact.

1) Single Package, Single Core

   [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0

2) Single Package, Dual Core

   a) One thread per core

	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
		    -> [core 1] -> [thread 0] -> Linux CPU 1

   b) Two threads per core

	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
				-> [thread 1] -> Linux CPU 1
		    -> [core 1] -> [thread 0] -> Linux CPU 2
				-> [thread 1] -> Linux CPU 3

      Alternative enumeration:

	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
				-> [thread 1] -> Linux CPU 2
		    -> [core 1] -> [thread 0] -> Linux CPU 1
				-> [thread 1] -> Linux CPU 3

      AMD nomenclature for CMT systems:

	[node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0
				     -> [Compute Unit Core 1] -> Linux CPU 1
		 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2
				     -> [Compute Unit Core 1] -> Linux CPU 3

4) Dual Package, Dual Core

   a) One thread per core

	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
		    -> [core 1] -> [thread 0] -> Linux CPU 1

	[package 1] -> [core 0] -> [thread 0] -> Linux CPU 2
		    -> [core 1] -> [thread 0] -> Linux CPU 3

   b) Two threads per core

	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
				-> [thread 1] -> Linux CPU 1
		    -> [core 1] -> [thread 0] -> Linux CPU 2
				-> [thread 1] -> Linux CPU 3

	[package 1] -> [core 0] -> [thread 0] -> Linux CPU 4
				-> [thread 1] -> Linux CPU 5
		    -> [core 1] -> [thread 0] -> Linux CPU 6
				-> [thread 1] -> Linux CPU 7

      Alternative enumeration:

	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
				-> [thread 1] -> Linux CPU 4
		    -> [core 1] -> [thread 0] -> Linux CPU 1
				-> [thread 1] -> Linux CPU 5

	[package 1] -> [core 0] -> [thread 0] -> Linux CPU 2
				-> [thread 1] -> Linux CPU 6
		    -> [core 1] -> [thread 0] -> Linux CPU 3
				-> [thread 1] -> Linux CPU 7

      AMD nomenclature for CMT systems:

	[node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0
				     -> [Compute Unit Core 1] -> Linux CPU 1
		 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2
				     -> [Compute Unit Core 1] -> Linux CPU 3

	[node 1] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 4
				     -> [Compute Unit Core 1] -> Linux CPU 5
		 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 6
				     -> [Compute Unit Core 1] -> Linux CPU 7
Commit	Line	Data
f7be8610 BP	1	x86 Topology
	2	============
	3
	4	This documents and clarifies the main aspects of x86 topology modelling and
	5	representation in the kernel. Update/change when doing changes to the
	6	respective code.
	7
	8	The architecture-agnostic topology definitions are in
	9	Documentation/cputopology.txt. This file holds x86-specific
	10	differences/specialities which must not necessarily apply to the generic
	11	definitions. Thus, the way to read up on Linux topology on x86 is to start
	12	with the generic one and look at this one in parallel for the x86 specifics.
	13
	14	Needless to say, code should use the generic functions - this file is only
	15	here to document the inner workings of x86 topology.
	16
	17	Started by Thomas Gleixner <tglx@linutronix.de> and Borislav Petkov <bp@alien8.de>.
	18
	19	The main aim of the topology facilities is to present adequate interfaces to
	20	code which needs to know/query/use the structure of the running system wrt
	21	threads, cores, packages, etc.
	22
	23	The kernel does not care about the concept of physical sockets because a
	24	socket has no relevance to software. It's an electromechanical component. In
	25	the past a socket always contained a single package (see below), but with the
	26	advent of Multi Chip Modules (MCM) a socket can hold more than one package. So
	27	there might be still references to sockets in the code, but they are of
	28	historical nature and should be cleaned up.
	29
	30	The topology of a system is described in the units of:
	31
	32	- packages
	33	- cores
	34	- threads
	35
	36	* Package:
	37
	38	Packages contain a number of cores plus shared resources, e.g. DRAM
	39	controller, shared caches etc.
	40
	41	AMD nomenclature for package is 'Node'.
	42
	43	Package-related topology information in the kernel:
	44
	45	- cpuinfo_x86.x86_max_cores:
	46
	47	The number of cores in a package. This information is retrieved via CPUID.
	48
	49	- cpuinfo_x86.phys_proc_id:
	50
	51	The physical ID of the package. This information is retrieved via CPUID
	52	and deduced from the APIC IDs of the cores in the package.
	53
	54	- cpuinfo_x86.logical_id:
	55
	56	The logical ID of the package. As we do not trust BIOSes to enumerate the
	57	packages in a consistent way, we introduced the concept of logical package
	58	ID so we can sanely calculate the number of maximum possible packages in
	59	the system and have the packages enumerated linearly.
	60
	61	- topology_max_packages():
	62
	63	The maximum possible number of packages in the system. Helpful for per
	64	package facilities to preallocate per package information.
65
66
67	* Cores:
68
69	A core consists of 1 or more threads. It does not matter whether the threads
70	are SMT- or CMT-type threads.
71
72	AMDs nomenclature for a CMT core is "Compute Unit". The kernel always uses
73	"core".
74
75	Core-related topology information in the kernel:
76
77	- smp_num_siblings:
78
79	The number of threads in a core. The number of threads in a package can be
80	calculated by:
81
82	threads_per_package = cpuinfo_x86.x86_max_cores * smp_num_siblings
83
84
85	* Threads:
86
87	A thread is a single scheduling unit. It's the equivalent to a logical Linux
88	CPU.
89
90	AMDs nomenclature for CMT threads is "Compute Unit Core". The kernel always
91	uses "thread".
92
93	Thread-related topology information in the kernel:
94
95	- topology_core_cpumask():
96
97	The cpumask contains all online threads in the package to which a thread
98	belongs.
99
100	The number of online threads is also printed in /proc/cpuinfo "siblings."
101
102	- topology_sibling_mask():
103
104	The cpumask contains all online threads in the core to which a thread
105	belongs.
106
107	- topology_logical_package_id():
108
109	The logical package ID to which a thread belongs.
110
111	- topology_physical_package_id():
112
113	The physical package ID to which a thread belongs.
114
115	- topology_core_id();
116
117	The ID of the core to which a thread belongs. It is also printed in /proc/cpuinfo
118	"core_id."
119
120
121
122	System topology examples
123
124	Note:
125
126	The alternative Linux CPU enumeration depends on how the BIOS enumerates the
127	threads. Many BIOSes enumerate all threads 0 first and then all threads 1.
128	That has the "advantage" that the logical Linux CPU numbers of threads 0 stay
129	the same whether threads are enabled or not. That's merely an implementation
130	detail and has no practical impact.
131
132	1) Single Package, Single Core
133
134	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
135
136	2) Single Package, Dual Core
137
138	a) One thread per core
139
140	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
141	-> [core 1] -> [thread 0] -> Linux CPU 1
142
143	b) Two threads per core
144
145	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
146	-> [thread 1] -> Linux CPU 1
147	-> [core 1] -> [thread 0] -> Linux CPU 2
148	-> [thread 1] -> Linux CPU 3
149
150	Alternative enumeration:
151
152	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
153	-> [thread 1] -> Linux CPU 2
154	-> [core 1] -> [thread 0] -> Linux CPU 1
155	-> [thread 1] -> Linux CPU 3
156
157	AMD nomenclature for CMT systems:
158
159	[node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0
160	-> [Compute Unit Core 1] -> Linux CPU 1
161	-> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2
162	-> [Compute Unit Core 1] -> Linux CPU 3
163
164	4) Dual Package, Dual Core
165
166	a) One thread per core
167
168	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
169	-> [core 1] -> [thread 0] -> Linux CPU 1
170
171	[package 1] -> [core 0] -> [thread 0] -> Linux CPU 2
172	-> [core 1] -> [thread 0] -> Linux CPU 3
173
174	b) Two threads per core
175
176	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
177	-> [thread 1] -> Linux CPU 1
178	-> [core 1] -> [thread 0] -> Linux CPU 2
179	-> [thread 1] -> Linux CPU 3
180
181	[package 1] -> [core 0] -> [thread 0] -> Linux CPU 4
182	-> [thread 1] -> Linux CPU 5
183	-> [core 1] -> [thread 0] -> Linux CPU 6
184	-> [thread 1] -> Linux CPU 7
185
186	Alternative enumeration:
187
188	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
189	-> [thread 1] -> Linux CPU 4
190	-> [core 1] -> [thread 0] -> Linux CPU 1
191	-> [thread 1] -> Linux CPU 5
192
193	[package 1] -> [core 0] -> [thread 0] -> Linux CPU 2
194	-> [thread 1] -> Linux CPU 6
195	-> [core 1] -> [thread 0] -> Linux CPU 3
196	-> [thread 1] -> Linux CPU 7
197
198	AMD nomenclature for CMT systems:
199
200	[node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0
201	-> [Compute Unit Core 1] -> Linux CPU 1
202	-> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2
203	-> [Compute Unit Core 1] -> Linux CPU 3
204
205	[node 1] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 4
206	-> [Compute Unit Core 1] -> Linux CPU 5
207	-> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 6
208	-> [Compute Unit Core 1] -> Linux CPU 7