1.. _numa_memory_policy: 2 3================== 4NUMA Memory Policy 5================== 6 7What is NUMA Memory Policy? 8============================ 9 10In the Linux kernel, "memory policy" determines from which node the kernel will 11allocate memory in a NUMA system or in an emulated NUMA system. Linux has 12supported platforms with Non-Uniform Memory Access architectures since 2.4.?. 13The current memory policy support was added to Linux 2.6 around May 2004. This 14document attempts to describe the concepts and APIs of the 2.6 memory policy 15support. 16 17Memory policies should not be confused with cpusets 18(``Documentation/cgroup-v1/cpusets.txt``) 19which is an administrative mechanism for restricting the nodes from which 20memory may be allocated by a set of processes. Memory policies are a 21programming interface that a NUMA-aware application can take advantage of. When 22both cpusets and policies are applied to a task, the restrictions of the cpuset 23takes priority. See :ref:`Memory Policies and cpusets <mem_pol_and_cpusets>` 24below for more details. 25 26Memory Policy Concepts 27====================== 28 29Scope of Memory Policies 30------------------------ 31 32The Linux kernel supports _scopes_ of memory policy, described here from 33most general to most specific: 34 35System Default Policy 36 this policy is "hard coded" into the kernel. It is the policy 37 that governs all page allocations that aren't controlled by 38 one of the more specific policy scopes discussed below. When 39 the system is "up and running", the system default policy will 40 use "local allocation" described below. However, during boot 41 up, the system default policy will be set to interleave 42 allocations across all nodes with "sufficient" memory, so as 43 not to overload the initial boot node with boot-time 44 allocations. 45 46Task/Process Policy 47 this is an optional, per-task policy. When defined for a 48 specific task, this policy controls all page allocations made 49 by or on behalf of the task that aren't controlled by a more 50 specific scope. If a task does not define a task policy, then 51 all page allocations that would have been controlled by the 52 task policy "fall back" to the System Default Policy. 53 54 The task policy applies to the entire address space of a task. Thus, 55 it is inheritable, and indeed is inherited, across both fork() 56 [clone() w/o the CLONE_VM flag] and exec*(). This allows a parent task 57 to establish the task policy for a child task exec()'d from an 58 executable image that has no awareness of memory policy. See the 59 :ref:`Memory Policy APIs <memory_policy_apis>` section, 60 below, for an overview of the system call 61 that a task may use to set/change its task/process policy. 62 63 In a multi-threaded task, task policies apply only to the thread 64 [Linux kernel task] that installs the policy and any threads 65 subsequently created by that thread. Any sibling threads existing 66 at the time a new task policy is installed retain their current 67 policy. 68 69 A task policy applies only to pages allocated after the policy is 70 installed. Any pages already faulted in by the task when the task 71 changes its task policy remain where they were allocated based on 72 the policy at the time they were allocated. 73 74.. _vma_policy: 75 76VMA Policy 77 A "VMA" or "Virtual Memory Area" refers to a range of a task's 78 virtual address space. A task may define a specific policy for a range 79 of its virtual address space. See the 80 :ref:`Memory Policy APIs <memory_policy_apis>` section, 81 below, for an overview of the mbind() system call used to set a VMA 82 policy. 83 84 A VMA policy will govern the allocation of pages that back 85 this region of the address space. Any regions of the task's 86 address space that don't have an explicit VMA policy will fall 87 back to the task policy, which may itself fall back to the 88 System Default Policy. 89 90 VMA policies have a few complicating details: 91 92 * VMA policy applies ONLY to anonymous pages. These include 93 pages allocated for anonymous segments, such as the task 94 stack and heap, and any regions of the address space 95 mmap()ed with the MAP_ANONYMOUS flag. If a VMA policy is 96 applied to a file mapping, it will be ignored if the mapping 97 used the MAP_SHARED flag. If the file mapping used the 98 MAP_PRIVATE flag, the VMA policy will only be applied when 99 an anonymous page is allocated on an attempt to write to the 100 mapping-- i.e., at Copy-On-Write. 101 102 * VMA policies are shared between all tasks that share a 103 virtual address space--a.k.a. threads--independent of when 104 the policy is installed; and they are inherited across 105 fork(). However, because VMA policies refer to a specific 106 region of a task's address space, and because the address 107 space is discarded and recreated on exec*(), VMA policies 108 are NOT inheritable across exec(). Thus, only NUMA-aware 109 applications may use VMA policies. 110 111 * A task may install a new VMA policy on a sub-range of a 112 previously mmap()ed region. When this happens, Linux splits 113 the existing virtual memory area into 2 or 3 VMAs, each with 114 it's own policy. 115 116 * By default, VMA policy applies only to pages allocated after 117 the policy is installed. Any pages already faulted into the 118 VMA range remain where they were allocated based on the 119 policy at the time they were allocated. However, since 120 2.6.16, Linux supports page migration via the mbind() system 121 call, so that page contents can be moved to match a newly 122 installed policy. 123 124Shared Policy 125 Conceptually, shared policies apply to "memory objects" mapped 126 shared into one or more tasks' distinct address spaces. An 127 application installs shared policies the same way as VMA 128 policies--using the mbind() system call specifying a range of 129 virtual addresses that map the shared object. However, unlike 130 VMA policies, which can be considered to be an attribute of a 131 range of a task's address space, shared policies apply 132 directly to the shared object. Thus, all tasks that attach to 133 the object share the policy, and all pages allocated for the 134 shared object, by any task, will obey the shared policy. 135 136 As of 2.6.22, only shared memory segments, created by shmget() or 137 mmap(MAP_ANONYMOUS|MAP_SHARED), support shared policy. When shared 138 policy support was added to Linux, the associated data structures were 139 added to hugetlbfs shmem segments. At the time, hugetlbfs did not 140 support allocation at fault time--a.k.a lazy allocation--so hugetlbfs 141 shmem segments were never "hooked up" to the shared policy support. 142 Although hugetlbfs segments now support lazy allocation, their support 143 for shared policy has not been completed. 144 145 As mentioned above in :ref:`VMA policies <vma_policy>` section, 146 allocations of page cache pages for regular files mmap()ed 147 with MAP_SHARED ignore any VMA policy installed on the virtual 148 address range backed by the shared file mapping. Rather, 149 shared page cache pages, including pages backing private 150 mappings that have not yet been written by the task, follow 151 task policy, if any, else System Default Policy. 152 153 The shared policy infrastructure supports different policies on subset 154 ranges of the shared object. However, Linux still splits the VMA of 155 the task that installs the policy for each range of distinct policy. 156 Thus, different tasks that attach to a shared memory segment can have 157 different VMA configurations mapping that one shared object. This 158 can be seen by examining the /proc/<pid>/numa_maps of tasks sharing 159 a shared memory region, when one task has installed shared policy on 160 one or more ranges of the region. 161 162Components of Memory Policies 163----------------------------- 164 165A NUMA memory policy consists of a "mode", optional mode flags, and 166an optional set of nodes. The mode determines the behavior of the 167policy, the optional mode flags determine the behavior of the mode, 168and the optional set of nodes can be viewed as the arguments to the 169policy behavior. 170 171Internally, memory policies are implemented by a reference counted 172structure, struct mempolicy. Details of this structure will be 173discussed in context, below, as required to explain the behavior. 174 175NUMA memory policy supports the following 4 behavioral modes: 176 177Default Mode--MPOL_DEFAULT 178 This mode is only used in the memory policy APIs. Internally, 179 MPOL_DEFAULT is converted to the NULL memory policy in all 180 policy scopes. Any existing non-default policy will simply be 181 removed when MPOL_DEFAULT is specified. As a result, 182 MPOL_DEFAULT means "fall back to the next most specific policy 183 scope." 184 185 For example, a NULL or default task policy will fall back to the 186 system default policy. A NULL or default vma policy will fall 187 back to the task policy. 188 189 When specified in one of the memory policy APIs, the Default mode 190 does not use the optional set of nodes. 191 192 It is an error for the set of nodes specified for this policy to 193 be non-empty. 194 195MPOL_BIND 196 This mode specifies that memory must come from the set of 197 nodes specified by the policy. Memory will be allocated from 198 the node in the set with sufficient free memory that is 199 closest to the node where the allocation takes place. 200 201MPOL_PREFERRED 202 This mode specifies that the allocation should be attempted 203 from the single node specified in the policy. If that 204 allocation fails, the kernel will search other nodes, in order 205 of increasing distance from the preferred node based on 206 information provided by the platform firmware. 207 208 Internally, the Preferred policy uses a single node--the 209 preferred_node member of struct mempolicy. When the internal 210 mode flag MPOL_F_LOCAL is set, the preferred_node is ignored 211 and the policy is interpreted as local allocation. "Local" 212 allocation policy can be viewed as a Preferred policy that 213 starts at the node containing the cpu where the allocation 214 takes place. 215 216 It is possible for the user to specify that local allocation 217 is always preferred by passing an empty nodemask with this 218 mode. If an empty nodemask is passed, the policy cannot use 219 the MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES flags 220 described below. 221 222MPOL_INTERLEAVED 223 This mode specifies that page allocations be interleaved, on a 224 page granularity, across the nodes specified in the policy. 225 This mode also behaves slightly differently, based on the 226 context where it is used: 227 228 For allocation of anonymous pages and shared memory pages, 229 Interleave mode indexes the set of nodes specified by the 230 policy using the page offset of the faulting address into the 231 segment [VMA] containing the address modulo the number of 232 nodes specified by the policy. It then attempts to allocate a 233 page, starting at the selected node, as if the node had been 234 specified by a Preferred policy or had been selected by a 235 local allocation. That is, allocation will follow the per 236 node zonelist. 237 238 For allocation of page cache pages, Interleave mode indexes 239 the set of nodes specified by the policy using a node counter 240 maintained per task. This counter wraps around to the lowest 241 specified node after it reaches the highest specified node. 242 This will tend to spread the pages out over the nodes 243 specified by the policy based on the order in which they are 244 allocated, rather than based on any page offset into an 245 address range or file. During system boot up, the temporary 246 interleaved system default policy works in this mode. 247 248NUMA memory policy supports the following optional mode flags: 249 250MPOL_F_STATIC_NODES 251 This flag specifies that the nodemask passed by 252 the user should not be remapped if the task or VMA's set of allowed 253 nodes changes after the memory policy has been defined. 254 255 Without this flag, any time a mempolicy is rebound because of a 256 change in the set of allowed nodes, the node (Preferred) or 257 nodemask (Bind, Interleave) is remapped to the new set of 258 allowed nodes. This may result in nodes being used that were 259 previously undesired. 260 261 With this flag, if the user-specified nodes overlap with the 262 nodes allowed by the task's cpuset, then the memory policy is 263 applied to their intersection. If the two sets of nodes do not 264 overlap, the Default policy is used. 265 266 For example, consider a task that is attached to a cpuset with 267 mems 1-3 that sets an Interleave policy over the same set. If 268 the cpuset's mems change to 3-5, the Interleave will now occur 269 over nodes 3, 4, and 5. With this flag, however, since only node 270 3 is allowed from the user's nodemask, the "interleave" only 271 occurs over that node. If no nodes from the user's nodemask are 272 now allowed, the Default behavior is used. 273 274 MPOL_F_STATIC_NODES cannot be combined with the 275 MPOL_F_RELATIVE_NODES flag. It also cannot be used for 276 MPOL_PREFERRED policies that were created with an empty nodemask 277 (local allocation). 278 279MPOL_F_RELATIVE_NODES 280 This flag specifies that the nodemask passed 281 by the user will be mapped relative to the set of the task or VMA's 282 set of allowed nodes. The kernel stores the user-passed nodemask, 283 and if the allowed nodes changes, then that original nodemask will 284 be remapped relative to the new set of allowed nodes. 285 286 Without this flag (and without MPOL_F_STATIC_NODES), anytime a 287 mempolicy is rebound because of a change in the set of allowed 288 nodes, the node (Preferred) or nodemask (Bind, Interleave) is 289 remapped to the new set of allowed nodes. That remap may not 290 preserve the relative nature of the user's passed nodemask to its 291 set of allowed nodes upon successive rebinds: a nodemask of 292 1,3,5 may be remapped to 7-9 and then to 1-3 if the set of 293 allowed nodes is restored to its original state. 294 295 With this flag, the remap is done so that the node numbers from 296 the user's passed nodemask are relative to the set of allowed 297 nodes. In other words, if nodes 0, 2, and 4 are set in the user's 298 nodemask, the policy will be effected over the first (and in the 299 Bind or Interleave case, the third and fifth) nodes in the set of 300 allowed nodes. The nodemask passed by the user represents nodes 301 relative to task or VMA's set of allowed nodes. 302 303 If the user's nodemask includes nodes that are outside the range 304 of the new set of allowed nodes (for example, node 5 is set in 305 the user's nodemask when the set of allowed nodes is only 0-3), 306 then the remap wraps around to the beginning of the nodemask and, 307 if not already set, sets the node in the mempolicy nodemask. 308 309 For example, consider a task that is attached to a cpuset with 310 mems 2-5 that sets an Interleave policy over the same set with 311 MPOL_F_RELATIVE_NODES. If the cpuset's mems change to 3-7, the 312 interleave now occurs over nodes 3,5-7. If the cpuset's mems 313 then change to 0,2-3,5, then the interleave occurs over nodes 314 0,2-3,5. 315 316 Thanks to the consistent remapping, applications preparing 317 nodemasks to specify memory policies using this flag should 318 disregard their current, actual cpuset imposed memory placement 319 and prepare the nodemask as if they were always located on 320 memory nodes 0 to N-1, where N is the number of memory nodes the 321 policy is intended to manage. Let the kernel then remap to the 322 set of memory nodes allowed by the task's cpuset, as that may 323 change over time. 324 325 MPOL_F_RELATIVE_NODES cannot be combined with the 326 MPOL_F_STATIC_NODES flag. It also cannot be used for 327 MPOL_PREFERRED policies that were created with an empty nodemask 328 (local allocation). 329 330Memory Policy Reference Counting 331================================ 332 333To resolve use/free races, struct mempolicy contains an atomic reference 334count field. Internal interfaces, mpol_get()/mpol_put() increment and 335decrement this reference count, respectively. mpol_put() will only free 336the structure back to the mempolicy kmem cache when the reference count 337goes to zero. 338 339When a new memory policy is allocated, its reference count is initialized 340to '1', representing the reference held by the task that is installing the 341new policy. When a pointer to a memory policy structure is stored in another 342structure, another reference is added, as the task's reference will be dropped 343on completion of the policy installation. 344 345During run-time "usage" of the policy, we attempt to minimize atomic operations 346on the reference count, as this can lead to cache lines bouncing between cpus 347and NUMA nodes. "Usage" here means one of the following: 348 3491) querying of the policy, either by the task itself [using the get_mempolicy() 350 API discussed below] or by another task using the /proc/<pid>/numa_maps 351 interface. 352 3532) examination of the policy to determine the policy mode and associated node 354 or node lists, if any, for page allocation. This is considered a "hot 355 path". Note that for MPOL_BIND, the "usage" extends across the entire 356 allocation process, which may sleep during page reclaimation, because the 357 BIND policy nodemask is used, by reference, to filter ineligible nodes. 358 359We can avoid taking an extra reference during the usages listed above as 360follows: 361 3621) we never need to get/free the system default policy as this is never 363 changed nor freed, once the system is up and running. 364 3652) for querying the policy, we do not need to take an extra reference on the 366 target task's task policy nor vma policies because we always acquire the 367 task's mm's mmap_sem for read during the query. The set_mempolicy() and 368 mbind() APIs [see below] always acquire the mmap_sem for write when 369 installing or replacing task or vma policies. Thus, there is no possibility 370 of a task or thread freeing a policy while another task or thread is 371 querying it. 372 3733) Page allocation usage of task or vma policy occurs in the fault path where 374 we hold them mmap_sem for read. Again, because replacing the task or vma 375 policy requires that the mmap_sem be held for write, the policy can't be 376 freed out from under us while we're using it for page allocation. 377 3784) Shared policies require special consideration. One task can replace a 379 shared memory policy while another task, with a distinct mmap_sem, is 380 querying or allocating a page based on the policy. To resolve this 381 potential race, the shared policy infrastructure adds an extra reference 382 to the shared policy during lookup while holding a spin lock on the shared 383 policy management structure. This requires that we drop this extra 384 reference when we're finished "using" the policy. We must drop the 385 extra reference on shared policies in the same query/allocation paths 386 used for non-shared policies. For this reason, shared policies are marked 387 as such, and the extra reference is dropped "conditionally"--i.e., only 388 for shared policies. 389 390 Because of this extra reference counting, and because we must lookup 391 shared policies in a tree structure under spinlock, shared policies are 392 more expensive to use in the page allocation path. This is especially 393 true for shared policies on shared memory regions shared by tasks running 394 on different NUMA nodes. This extra overhead can be avoided by always 395 falling back to task or system default policy for shared memory regions, 396 or by prefaulting the entire shared memory region into memory and locking 397 it down. However, this might not be appropriate for all applications. 398 399.. _memory_policy_apis: 400 401Memory Policy APIs 402================== 403 404Linux supports 3 system calls for controlling memory policy. These APIS 405always affect only the calling task, the calling task's address space, or 406some shared object mapped into the calling task's address space. 407 408.. note:: 409 the headers that define these APIs and the parameter data types for 410 user space applications reside in a package that is not part of the 411 Linux kernel. The kernel system call interfaces, with the 'sys\_' 412 prefix, are defined in <linux/syscalls.h>; the mode and flag 413 definitions are defined in <linux/mempolicy.h>. 414 415Set [Task] Memory Policy:: 416 417 long set_mempolicy(int mode, const unsigned long *nmask, 418 unsigned long maxnode); 419 420Set's the calling task's "task/process memory policy" to mode 421specified by the 'mode' argument and the set of nodes defined by 422'nmask'. 'nmask' points to a bit mask of node ids containing at least 423'maxnode' ids. Optional mode flags may be passed by combining the 424'mode' argument with the flag (for example: MPOL_INTERLEAVE | 425MPOL_F_STATIC_NODES). 426 427See the set_mempolicy(2) man page for more details 428 429 430Get [Task] Memory Policy or Related Information:: 431 432 long get_mempolicy(int *mode, 433 const unsigned long *nmask, unsigned long maxnode, 434 void *addr, int flags); 435 436Queries the "task/process memory policy" of the calling task, or the 437policy or location of a specified virtual address, depending on the 438'flags' argument. 439 440See the get_mempolicy(2) man page for more details 441 442 443Install VMA/Shared Policy for a Range of Task's Address Space:: 444 445 long mbind(void *start, unsigned long len, int mode, 446 const unsigned long *nmask, unsigned long maxnode, 447 unsigned flags); 448 449mbind() installs the policy specified by (mode, nmask, maxnodes) as a 450VMA policy for the range of the calling task's address space specified 451by the 'start' and 'len' arguments. Additional actions may be 452requested via the 'flags' argument. 453 454See the mbind(2) man page for more details. 455 456Memory Policy Command Line Interface 457==================================== 458 459Although not strictly part of the Linux implementation of memory policy, 460a command line tool, numactl(8), exists that allows one to: 461 462+ set the task policy for a specified program via set_mempolicy(2), fork(2) and 463 exec(2) 464 465+ set the shared policy for a shared memory segment via mbind(2) 466 467The numactl(8) tool is packaged with the run-time version of the library 468containing the memory policy system call wrappers. Some distributions 469package the headers and compile-time libraries in a separate development 470package. 471 472.. _mem_pol_and_cpusets: 473 474Memory Policies and cpusets 475=========================== 476 477Memory policies work within cpusets as described above. For memory policies 478that require a node or set of nodes, the nodes are restricted to the set of 479nodes whose memories are allowed by the cpuset constraints. If the nodemask 480specified for the policy contains nodes that are not allowed by the cpuset and 481MPOL_F_RELATIVE_NODES is not used, the intersection of the set of nodes 482specified for the policy and the set of nodes with memory is used. If the 483result is the empty set, the policy is considered invalid and cannot be 484installed. If MPOL_F_RELATIVE_NODES is used, the policy's nodes are mapped 485onto and folded into the task's set of allowed nodes as previously described. 486 487The interaction of memory policies and cpusets can be problematic when tasks 488in two cpusets share access to a memory region, such as shared memory segments 489created by shmget() of mmap() with the MAP_ANONYMOUS and MAP_SHARED flags, and 490any of the tasks install shared policy on the region, only nodes whose 491memories are allowed in both cpusets may be used in the policies. Obtaining 492this information requires "stepping outside" the memory policy APIs to use the 493cpuset information and requires that one know in what cpusets other task might 494be attaching to the shared region. Furthermore, if the cpusets' allowed 495memory sets are disjoint, "local" allocation is the only valid policy. 496