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