1==================
2NUMA Memory Policy
3==================
4
5What is NUMA Memory Policy?
6============================
7
8In the Linux kernel, "memory policy" determines from which node the kernel will
9allocate memory in a NUMA system or in an emulated NUMA system.  Linux has
10supported platforms with Non-Uniform Memory Access architectures since 2.4.?.
11The current memory policy support was added to Linux 2.6 around May 2004.  This
12document attempts to describe the concepts and APIs of the 2.6 memory policy
13support.
14
15Memory policies should not be confused with cpusets
16(``Documentation/admin-guide/cgroup-v1/cpusets.rst``)
17which is an administrative mechanism for restricting the nodes from which
18memory may be allocated by a set of processes. Memory policies are a
19programming interface that a NUMA-aware application can take advantage of.  When
20both cpusets and policies are applied to a task, the restrictions of the cpuset
21takes priority.  See :ref:`Memory Policies and cpusets <mem_pol_and_cpusets>`
22below for more details.
23
24Memory Policy Concepts
25======================
26
27Scope of Memory Policies
28------------------------
29
30The Linux kernel supports _scopes_ of memory policy, described here from
31most general to most specific:
32
33System Default Policy
34	this policy is "hard coded" into the kernel.  It is the policy
35	that governs all page allocations that aren't controlled by
36	one of the more specific policy scopes discussed below.  When
37	the system is "up and running", the system default policy will
38	use "local allocation" described below.  However, during boot
39	up, the system default policy will be set to interleave
40	allocations across all nodes with "sufficient" memory, so as
41	not to overload the initial boot node with boot-time
42	allocations.
43
44Task/Process Policy
45	this is an optional, per-task policy.  When defined for a
46	specific task, this policy controls all page allocations made
47	by or on behalf of the task that aren't controlled by a more
48	specific scope. If a task does not define a task policy, then
49	all page allocations that would have been controlled by the
50	task policy "fall back" to the System Default Policy.
51
52	The task policy applies to the entire address space of a task. Thus,
53	it is inheritable, and indeed is inherited, across both fork()
54	[clone() w/o the CLONE_VM flag] and exec*().  This allows a parent task
55	to establish the task policy for a child task exec()'d from an
56	executable image that has no awareness of memory policy.  See the
57	:ref:`Memory Policy APIs <memory_policy_apis>` section,
58	below, for an overview of the system call
59	that a task may use to set/change its task/process policy.
60
61	In a multi-threaded task, task policies apply only to the thread
62	[Linux kernel task] that installs the policy and any threads
63	subsequently created by that thread.  Any sibling threads existing
64	at the time a new task policy is installed retain their current
65	policy.
66
67	A task policy applies only to pages allocated after the policy is
68	installed.  Any pages already faulted in by the task when the task
69	changes its task policy remain where they were allocated based on
70	the policy at the time they were allocated.
71
72.. _vma_policy:
73
74VMA Policy
75	A "VMA" or "Virtual Memory Area" refers to a range of a task's
76	virtual address space.  A task may define a specific policy for a range
77	of its virtual address space.   See the
78	:ref:`Memory Policy APIs <memory_policy_apis>` section,
79	below, for an overview of the mbind() system call used to set a VMA
80	policy.
81
82	A VMA policy will govern the allocation of pages that back
83	this region of the address space.  Any regions of the task's
84	address space that don't have an explicit VMA policy will fall
85	back to the task policy, which may itself fall back to the
86	System Default Policy.
87
88	VMA policies have a few complicating details:
89
90	* VMA policy applies ONLY to anonymous pages.  These include
91	  pages allocated for anonymous segments, such as the task
92	  stack and heap, and any regions of the address space
93	  mmap()ed with the MAP_ANONYMOUS flag.  If a VMA policy is
94	  applied to a file mapping, it will be ignored if the mapping
95	  used the MAP_SHARED flag.  If the file mapping used the
96	  MAP_PRIVATE flag, the VMA policy will only be applied when
97	  an anonymous page is allocated on an attempt to write to the
98	  mapping-- i.e., at Copy-On-Write.
99
100	* VMA policies are shared between all tasks that share a
101	  virtual address space--a.k.a. threads--independent of when
102	  the policy is installed; and they are inherited across
103	  fork().  However, because VMA policies refer to a specific
104	  region of a task's address space, and because the address
105	  space is discarded and recreated on exec*(), VMA policies
106	  are NOT inheritable across exec().  Thus, only NUMA-aware
107	  applications may use VMA policies.
108
109	* A task may install a new VMA policy on a sub-range of a
110	  previously mmap()ed region.  When this happens, Linux splits
111	  the existing virtual memory area into 2 or 3 VMAs, each with
112	  it's own policy.
113
114	* By default, VMA policy applies only to pages allocated after
115	  the policy is installed.  Any pages already faulted into the
116	  VMA range remain where they were allocated based on the
117	  policy at the time they were allocated.  However, since
118	  2.6.16, Linux supports page migration via the mbind() system
119	  call, so that page contents can be moved to match a newly
120	  installed policy.
121
122Shared Policy
123	Conceptually, shared policies apply to "memory objects" mapped
124	shared into one or more tasks' distinct address spaces.  An
125	application installs shared policies the same way as VMA
126	policies--using the mbind() system call specifying a range of
127	virtual addresses that map the shared object.  However, unlike
128	VMA policies, which can be considered to be an attribute of a
129	range of a task's address space, shared policies apply
130	directly to the shared object.  Thus, all tasks that attach to
131	the object share the policy, and all pages allocated for the
132	shared object, by any task, will obey the shared policy.
133
134	As of 2.6.22, only shared memory segments, created by shmget() or
135	mmap(MAP_ANONYMOUS|MAP_SHARED), support shared policy.  When shared
136	policy support was added to Linux, the associated data structures were
137	added to hugetlbfs shmem segments.  At the time, hugetlbfs did not
138	support allocation at fault time--a.k.a lazy allocation--so hugetlbfs
139	shmem segments were never "hooked up" to the shared policy support.
140	Although hugetlbfs segments now support lazy allocation, their support
141	for shared policy has not been completed.
142
143	As mentioned above in :ref:`VMA policies <vma_policy>` section,
144	allocations of page cache pages for regular files mmap()ed
145	with MAP_SHARED ignore any VMA policy installed on the virtual
146	address range backed by the shared file mapping.  Rather,
147	shared page cache pages, including pages backing private
148	mappings that have not yet been written by the task, follow
149	task policy, if any, else System Default Policy.
150
151	The shared policy infrastructure supports different policies on subset
152	ranges of the shared object.  However, Linux still splits the VMA of
153	the task that installs the policy for each range of distinct policy.
154	Thus, different tasks that attach to a shared memory segment can have
155	different VMA configurations mapping that one shared object.  This
156	can be seen by examining the /proc/<pid>/numa_maps of tasks sharing
157	a shared memory region, when one task has installed shared policy on
158	one or more ranges of the region.
159
160Components of Memory Policies
161-----------------------------
162
163A NUMA memory policy consists of a "mode", optional mode flags, and
164an optional set of nodes.  The mode determines the behavior of the
165policy, the optional mode flags determine the behavior of the mode,
166and the optional set of nodes can be viewed as the arguments to the
167policy behavior.
168
169Internally, memory policies are implemented by a reference counted
170structure, struct mempolicy.  Details of this structure will be
171discussed in context, below, as required to explain the behavior.
172
173NUMA memory policy supports the following 4 behavioral modes:
174
175Default Mode--MPOL_DEFAULT
176	This mode is only used in the memory policy APIs.  Internally,
177	MPOL_DEFAULT is converted to the NULL memory policy in all
178	policy scopes.  Any existing non-default policy will simply be
179	removed when MPOL_DEFAULT is specified.  As a result,
180	MPOL_DEFAULT means "fall back to the next most specific policy
181	scope."
182
183	For example, a NULL or default task policy will fall back to the
184	system default policy.  A NULL or default vma policy will fall
185	back to the task policy.
186
187	When specified in one of the memory policy APIs, the Default mode
188	does not use the optional set of nodes.
189
190	It is an error for the set of nodes specified for this policy to
191	be non-empty.
192
193MPOL_BIND
194	This mode specifies that memory must come from the set of
195	nodes specified by the policy.  Memory will be allocated from
196	the node in the set with sufficient free memory that is
197	closest to the node where the allocation takes place.
198
199MPOL_PREFERRED
200	This mode specifies that the allocation should be attempted
201	from the single node specified in the policy.  If that
202	allocation fails, the kernel will search other nodes, in order
203	of increasing distance from the preferred node based on
204	information provided by the platform firmware.
205
206	Internally, the Preferred policy uses a single node--the
207	preferred_node member of struct mempolicy.  When the internal
208	mode flag MPOL_F_LOCAL is set, the preferred_node is ignored
209	and the policy is interpreted as local allocation.  "Local"
210	allocation policy can be viewed as a Preferred policy that
211	starts at the node containing the cpu where the allocation
212	takes place.
213
214	It is possible for the user to specify that local allocation
215	is always preferred by passing an empty nodemask with this
216	mode.  If an empty nodemask is passed, the policy cannot use
217	the MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES flags
218	described below.
219
220MPOL_INTERLEAVED
221	This mode specifies that page allocations be interleaved, on a
222	page granularity, across the nodes specified in the policy.
223	This mode also behaves slightly differently, based on the
224	context where it is used:
225
226	For allocation of anonymous pages and shared memory pages,
227	Interleave mode indexes the set of nodes specified by the
228	policy using the page offset of the faulting address into the
229	segment [VMA] containing the address modulo the number of
230	nodes specified by the policy.  It then attempts to allocate a
231	page, starting at the selected node, as if the node had been
232	specified by a Preferred policy or had been selected by a
233	local allocation.  That is, allocation will follow the per
234	node zonelist.
235
236	For allocation of page cache pages, Interleave mode indexes
237	the set of nodes specified by the policy using a node counter
238	maintained per task.  This counter wraps around to the lowest
239	specified node after it reaches the highest specified node.
240	This will tend to spread the pages out over the nodes
241	specified by the policy based on the order in which they are
242	allocated, rather than based on any page offset into an
243	address range or file.  During system boot up, the temporary
244	interleaved system default policy works in this mode.
245
246MPOL_PREFERRED_MANY
247	This mode specifies that the allocation should be preferably
248	satisfied from the nodemask specified in the policy. If there is
249	a memory pressure on all nodes in the nodemask, the allocation
250	can fall back to all existing numa nodes. This is effectively
251	MPOL_PREFERRED allowed for a mask rather than a single node.
252
253NUMA memory policy supports the following optional mode flags:
254
255MPOL_F_STATIC_NODES
256	This flag specifies that the nodemask passed by
257	the user should not be remapped if the task or VMA's set of allowed
258	nodes changes after the memory policy has been defined.
259
260	Without this flag, any time a mempolicy is rebound because of a
261        change in the set of allowed nodes, the preferred nodemask (Preferred
262        Many), preferred node (Preferred) or nodemask (Bind, Interleave) is
263        remapped to the new set of allowed nodes.  This may result in nodes
264        being used that were previously undesired.
265
266	With this flag, if the user-specified nodes overlap with the
267	nodes allowed by the task's cpuset, then the memory policy is
268	applied to their intersection.  If the two sets of nodes do not
269	overlap, the Default policy is used.
270
271	For example, consider a task that is attached to a cpuset with
272	mems 1-3 that sets an Interleave policy over the same set.  If
273	the cpuset's mems change to 3-5, the Interleave will now occur
274	over nodes 3, 4, and 5.  With this flag, however, since only node
275	3 is allowed from the user's nodemask, the "interleave" only
276	occurs over that node.  If no nodes from the user's nodemask are
277	now allowed, the Default behavior is used.
278
279	MPOL_F_STATIC_NODES cannot be combined with the
280	MPOL_F_RELATIVE_NODES flag.  It also cannot be used for
281	MPOL_PREFERRED policies that were created with an empty nodemask
282	(local allocation).
283
284MPOL_F_RELATIVE_NODES
285	This flag specifies that the nodemask passed
286	by the user will be mapped relative to the set of the task or VMA's
287	set of allowed nodes.  The kernel stores the user-passed nodemask,
288	and if the allowed nodes changes, then that original nodemask will
289	be remapped relative to the new set of allowed nodes.
290
291	Without this flag (and without MPOL_F_STATIC_NODES), anytime a
292	mempolicy is rebound because of a change in the set of allowed
293	nodes, the node (Preferred) or nodemask (Bind, Interleave) is
294	remapped to the new set of allowed nodes.  That remap may not
295	preserve the relative nature of the user's passed nodemask to its
296	set of allowed nodes upon successive rebinds: a nodemask of
297	1,3,5 may be remapped to 7-9 and then to 1-3 if the set of
298	allowed nodes is restored to its original state.
299
300	With this flag, the remap is done so that the node numbers from
301	the user's passed nodemask are relative to the set of allowed
302	nodes.  In other words, if nodes 0, 2, and 4 are set in the user's
303	nodemask, the policy will be effected over the first (and in the
304	Bind or Interleave case, the third and fifth) nodes in the set of
305	allowed nodes.  The nodemask passed by the user represents nodes
306	relative to task or VMA's set of allowed nodes.
307
308	If the user's nodemask includes nodes that are outside the range
309	of the new set of allowed nodes (for example, node 5 is set in
310	the user's nodemask when the set of allowed nodes is only 0-3),
311	then the remap wraps around to the beginning of the nodemask and,
312	if not already set, sets the node in the mempolicy nodemask.
313
314	For example, consider a task that is attached to a cpuset with
315	mems 2-5 that sets an Interleave policy over the same set with
316	MPOL_F_RELATIVE_NODES.  If the cpuset's mems change to 3-7, the
317	interleave now occurs over nodes 3,5-7.  If the cpuset's mems
318	then change to 0,2-3,5, then the interleave occurs over nodes
319	0,2-3,5.
320
321	Thanks to the consistent remapping, applications preparing
322	nodemasks to specify memory policies using this flag should
323	disregard their current, actual cpuset imposed memory placement
324	and prepare the nodemask as if they were always located on
325	memory nodes 0 to N-1, where N is the number of memory nodes the
326	policy is intended to manage.  Let the kernel then remap to the
327	set of memory nodes allowed by the task's cpuset, as that may
328	change over time.
329
330	MPOL_F_RELATIVE_NODES cannot be combined with the
331	MPOL_F_STATIC_NODES flag.  It also cannot be used for
332	MPOL_PREFERRED policies that were created with an empty nodemask
333	(local allocation).
334
335Memory Policy Reference Counting
336================================
337
338To resolve use/free races, struct mempolicy contains an atomic reference
339count field.  Internal interfaces, mpol_get()/mpol_put() increment and
340decrement this reference count, respectively.  mpol_put() will only free
341the structure back to the mempolicy kmem cache when the reference count
342goes to zero.
343
344When a new memory policy is allocated, its reference count is initialized
345to '1', representing the reference held by the task that is installing the
346new policy.  When a pointer to a memory policy structure is stored in another
347structure, another reference is added, as the task's reference will be dropped
348on completion of the policy installation.
349
350During run-time "usage" of the policy, we attempt to minimize atomic operations
351on the reference count, as this can lead to cache lines bouncing between cpus
352and NUMA nodes.  "Usage" here means one of the following:
353
3541) querying of the policy, either by the task itself [using the get_mempolicy()
355   API discussed below] or by another task using the /proc/<pid>/numa_maps
356   interface.
357
3582) examination of the policy to determine the policy mode and associated node
359   or node lists, if any, for page allocation.  This is considered a "hot
360   path".  Note that for MPOL_BIND, the "usage" extends across the entire
361   allocation process, which may sleep during page reclamation, because the
362   BIND policy nodemask is used, by reference, to filter ineligible nodes.
363
364We can avoid taking an extra reference during the usages listed above as
365follows:
366
3671) we never need to get/free the system default policy as this is never
368   changed nor freed, once the system is up and running.
369
3702) for querying the policy, we do not need to take an extra reference on the
371   target task's task policy nor vma policies because we always acquire the
372   task's mm's mmap_lock for read during the query.  The set_mempolicy() and
373   mbind() APIs [see below] always acquire the mmap_lock for write when
374   installing or replacing task or vma policies.  Thus, there is no possibility
375   of a task or thread freeing a policy while another task or thread is
376   querying it.
377
3783) Page allocation usage of task or vma policy occurs in the fault path where
379   we hold them mmap_lock for read.  Again, because replacing the task or vma
380   policy requires that the mmap_lock be held for write, the policy can't be
381   freed out from under us while we're using it for page allocation.
382
3834) Shared policies require special consideration.  One task can replace a
384   shared memory policy while another task, with a distinct mmap_lock, is
385   querying or allocating a page based on the policy.  To resolve this
386   potential race, the shared policy infrastructure adds an extra reference
387   to the shared policy during lookup while holding a spin lock on the shared
388   policy management structure.  This requires that we drop this extra
389   reference when we're finished "using" the policy.  We must drop the
390   extra reference on shared policies in the same query/allocation paths
391   used for non-shared policies.  For this reason, shared policies are marked
392   as such, and the extra reference is dropped "conditionally"--i.e., only
393   for shared policies.
394
395   Because of this extra reference counting, and because we must lookup
396   shared policies in a tree structure under spinlock, shared policies are
397   more expensive to use in the page allocation path.  This is especially
398   true for shared policies on shared memory regions shared by tasks running
399   on different NUMA nodes.  This extra overhead can be avoided by always
400   falling back to task or system default policy for shared memory regions,
401   or by prefaulting the entire shared memory region into memory and locking
402   it down.  However, this might not be appropriate for all applications.
403
404.. _memory_policy_apis:
405
406Memory Policy APIs
407==================
408
409Linux supports 4 system calls for controlling memory policy.  These APIS
410always affect only the calling task, the calling task's address space, or
411some shared object mapped into the calling task's address space.
412
413.. note::
414   the headers that define these APIs and the parameter data types for
415   user space applications reside in a package that is not part of the
416   Linux kernel.  The kernel system call interfaces, with the 'sys\_'
417   prefix, are defined in <linux/syscalls.h>; the mode and flag
418   definitions are defined in <linux/mempolicy.h>.
419
420Set [Task] Memory Policy::
421
422	long set_mempolicy(int mode, const unsigned long *nmask,
423					unsigned long maxnode);
424
425Set's the calling task's "task/process memory policy" to mode
426specified by the 'mode' argument and the set of nodes defined by
427'nmask'.  'nmask' points to a bit mask of node ids containing at least
428'maxnode' ids.  Optional mode flags may be passed by combining the
429'mode' argument with the flag (for example: MPOL_INTERLEAVE |
430MPOL_F_STATIC_NODES).
431
432See the set_mempolicy(2) man page for more details
433
434
435Get [Task] Memory Policy or Related Information::
436
437	long get_mempolicy(int *mode,
438			   const unsigned long *nmask, unsigned long maxnode,
439			   void *addr, int flags);
440
441Queries the "task/process memory policy" of the calling task, or the
442policy or location of a specified virtual address, depending on the
443'flags' argument.
444
445See the get_mempolicy(2) man page for more details
446
447
448Install VMA/Shared Policy for a Range of Task's Address Space::
449
450	long mbind(void *start, unsigned long len, int mode,
451		   const unsigned long *nmask, unsigned long maxnode,
452		   unsigned flags);
453
454mbind() installs the policy specified by (mode, nmask, maxnodes) as a
455VMA policy for the range of the calling task's address space specified
456by the 'start' and 'len' arguments.  Additional actions may be
457requested via the 'flags' argument.
458
459See the mbind(2) man page for more details.
460
461Set home node for a Range of Task's Address Spacec::
462
463	long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
464					 unsigned long home_node,
465					 unsigned long flags);
466
467sys_set_mempolicy_home_node set the home node for a VMA policy present in the
468task's address range. The system call updates the home node only for the existing
469mempolicy range. Other address ranges are ignored. A home node is the NUMA node
470closest to which page allocation will come from. Specifying the home node override
471the default allocation policy to allocate memory close to the local node for an
472executing CPU.
473
474
475Memory Policy Command Line Interface
476====================================
477
478Although not strictly part of the Linux implementation of memory policy,
479a command line tool, numactl(8), exists that allows one to:
480
481+ set the task policy for a specified program via set_mempolicy(2), fork(2) and
482  exec(2)
483
484+ set the shared policy for a shared memory segment via mbind(2)
485
486The numactl(8) tool is packaged with the run-time version of the library
487containing the memory policy system call wrappers.  Some distributions
488package the headers and compile-time libraries in a separate development
489package.
490
491.. _mem_pol_and_cpusets:
492
493Memory Policies and cpusets
494===========================
495
496Memory policies work within cpusets as described above.  For memory policies
497that require a node or set of nodes, the nodes are restricted to the set of
498nodes whose memories are allowed by the cpuset constraints.  If the nodemask
499specified for the policy contains nodes that are not allowed by the cpuset and
500MPOL_F_RELATIVE_NODES is not used, the intersection of the set of nodes
501specified for the policy and the set of nodes with memory is used.  If the
502result is the empty set, the policy is considered invalid and cannot be
503installed.  If MPOL_F_RELATIVE_NODES is used, the policy's nodes are mapped
504onto and folded into the task's set of allowed nodes as previously described.
505
506The interaction of memory policies and cpusets can be problematic when tasks
507in two cpusets share access to a memory region, such as shared memory segments
508created by shmget() of mmap() with the MAP_ANONYMOUS and MAP_SHARED flags, and
509any of the tasks install shared policy on the region, only nodes whose
510memories are allowed in both cpusets may be used in the policies.  Obtaining
511this information requires "stepping outside" the memory policy APIs to use the
512cpuset information and requires that one know in what cpusets other task might
513be attaching to the shared region.  Furthermore, if the cpusets' allowed
514memory sets are disjoint, "local" allocation is the only valid policy.
515