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 .. _numa_memory_policy:
 
 ==================
 NUMA Memory Policy
 ==================
 
 What is NUMA Memory Policy?
 ============================
 
 In the Linux kernel, "memory policy" determines from which node the kernel will
 allocate memory in a NUMA system or in an emulated NUMA system.  Linux has
 supported platforms with Non-Uniform Memory Access architectures since 2.4.?.
 The current memory policy support was added to Linux 2.6 around May 2004.  This
 document attempts to describe the concepts and APIs of the 2.6 memory policy
 support.
 
 Memory policies should not be confused with cpusets
 (``Documentation/admin-guide/cgroup-v1/cpusets.rst``)
 which is an administrative mechanism for restricting the nodes from which
 memory may be allocated by a set of processes. Memory policies are a
 programming interface that a NUMA-aware application can take advantage of.  When
 both cpusets and policies are applied to a task, the restrictions of the cpuset
 takes priority.  See :ref:`Memory Policies and cpusets <mem_pol_and_cpusets>`
 below for more details.
 
 Memory Policy Concepts
 ======================
 
 Scope of Memory Policies
 ------------------------
 
 The Linux kernel supports _scopes_ of memory policy, described here from
 most general to most specific:
 
 System Default Policy
         this policy is "hard coded" into the kernel.  It is the policy
         that governs all page allocations that aren't controlled by
         one of the more specific policy scopes discussed below.  When
         the system is "up and running", the system default policy will
         use "local allocation" described below.  However, during boot
         up, the system default policy will be set to interleave
         allocations across all nodes with "sufficient" memory, so as
         not to overload the initial boot node with boot-time
         allocations.
 
 Task/Process Policy
         this is an optional, per-task policy.  When defined for a
         specific task, this policy controls all page allocations made
         by or on behalf of the task that aren't controlled by a more
         specific scope. If a task does not define a task policy, then
         all page allocations that would have been controlled by the
         task policy "fall back" to the System Default Policy.
 
         The task policy applies to the entire address space of a task. Thus,
         it is inheritable, and indeed is inherited, across both fork()
         [clone() w/o the CLONE_VM flag] and exec*().  This allows a parent task
         to establish the task policy for a child task exec()'d from an
         executable image that has no awareness of memory policy.  See the
         :ref:`Memory Policy APIs <memory_policy_apis>` section,
         below, for an overview of the system call
         that a task may use to set/change its task/process policy.
 
         In a multi-threaded task, task policies apply only to the thread
         [Linux kernel task] that installs the policy and any threads
         subsequently created by that thread.  Any sibling threads existing
         at the time a new task policy is installed retain their current
         policy.
 
         A task policy applies only to pages allocated after the policy is
         installed.  Any pages already faulted in by the task when the task
         changes its task policy remain where they were allocated based on
         the policy at the time they were allocated.
 
 .. _vma_policy:
 
 VMA Policy
         A "VMA" or "Virtual Memory Area" refers to a range of a task's
         virtual address space.  A task may define a specific policy for a range
         of its virtual address space.   See the
         :ref:`Memory Policy APIs <memory_policy_apis>` section,
         below, for an overview of the mbind() system call used to set a VMA
         policy.
 
         A VMA policy will govern the allocation of pages that back
         this region of the address space.  Any regions of the task's
         address space that don't have an explicit VMA policy will fall
         back to the task policy, which may itself fall back to the
         System Default Policy.
 
         VMA policies have a few complicating details:
 
         * VMA policy applies ONLY to anonymous pages.  These include
           pages allocated for anonymous segments, such as the task
           stack and heap, and any regions of the address space
           mmap()ed with the MAP_ANONYMOUS flag.  If a VMA policy is
           applied to a file mapping, it will be ignored if the mapping
           used the MAP_SHARED flag.  If the file mapping used the
           MAP_PRIVATE flag, the VMA policy will only be applied when
           an anonymous page is allocated on an attempt to write to the
           mapping-- i.e., at Copy-On-Write.
 
         * VMA policies are shared between all tasks that share a
           virtual address space--a.k.a. threads--independent of when
           the policy is installed; and they are inherited across
           fork().  However, because VMA policies refer to a specific
           region of a task's address space, and because the address
           space is discarded and recreated on exec*(), VMA policies
           are NOT inheritable across exec().  Thus, only NUMA-aware
           applications may use VMA policies.
 
         * A task may install a new VMA policy on a sub-range of a
           previously mmap()ed region.  When this happens, Linux splits
           the existing virtual memory area into 2 or 3 VMAs, each with
           it's own policy.
 
         * By default, VMA policy applies only to pages allocated after
           the policy is installed.  Any pages already faulted into the
           VMA range remain where they were allocated based on the
           policy at the time they were allocated.  However, since
           2.6.16, Linux supports page migration via the mbind() system
           call, so that page contents can be moved to match a newly
           installed policy.
 
 Shared Policy
         Conceptually, shared policies apply to "memory objects" mapped
         shared into one or more tasks' distinct address spaces.  An
         application installs shared policies the same way as VMA
         policies--using the mbind() system call specifying a range of
         virtual addresses that map the shared object.  However, unlike
         VMA policies, which can be considered to be an attribute of a
         range of a task's address space, shared policies apply
         directly to the shared object.  Thus, all tasks that attach to
         the object share the policy, and all pages allocated for the
         shared object, by any task, will obey the shared policy.
 
         As of 2.6.22, only shared memory segments, created by shmget() or
         mmap(MAP_ANONYMOUS|MAP_SHARED), support shared policy.  When shared
         policy support was added to Linux, the associated data structures were
         added to hugetlbfs shmem segments.  At the time, hugetlbfs did not
         support allocation at fault time--a.k.a lazy allocation--so hugetlbfs
         shmem segments were never "hooked up" to the shared policy support.
         Although hugetlbfs segments now support lazy allocation, their support
         for shared policy has not been completed.
 
         As mentioned above in :ref:`VMA policies <vma_policy>` section,
         allocations of page cache pages for regular files mmap()ed
         with MAP_SHARED ignore any VMA policy installed on the virtual
         address range backed by the shared file mapping.  Rather,
         shared page cache pages, including pages backing private
         mappings that have not yet been written by the task, follow
         task policy, if any, else System Default Policy.
 
         The shared policy infrastructure supports different policies on subset
         ranges of the shared object.  However, Linux still splits the VMA of
         the task that installs the policy for each range of distinct policy.
         Thus, different tasks that attach to a shared memory segment can have
         different VMA configurations mapping that one shared object.  This
         can be seen by examining the /proc/<pid>/numa_maps of tasks sharing
         a shared memory region, when one task has installed shared policy on
         one or more ranges of the region.
 
 Components of Memory Policies
 -----------------------------
 
 A NUMA memory policy consists of a "mode", optional mode flags, and
 an optional set of nodes.  The mode determines the behavior of the
 policy, the optional mode flags determine the behavior of the mode,
 and the optional set of nodes can be viewed as the arguments to the
 policy behavior.
 
 Internally, memory policies are implemented by a reference counted
 structure, struct mempolicy.  Details of this structure will be
 discussed in context, below, as required to explain the behavior.
 
 NUMA memory policy supports the following 4 behavioral modes:
 
 Default Mode--MPOL_DEFAULT
         This mode is only used in the memory policy APIs.  Internally,
         MPOL_DEFAULT is converted to the NULL memory policy in all
         policy scopes.  Any existing non-default policy will simply be
         removed when MPOL_DEFAULT is specified.  As a result,
         MPOL_DEFAULT means "fall back to the next most specific policy
         scope."
 
         For example, a NULL or default task policy will fall back to the
         system default policy.  A NULL or default vma policy will fall
         back to the task policy.
 
         When specified in one of the memory policy APIs, the Default mode
         does not use the optional set of nodes.
 
         It is an error for the set of nodes specified for this policy to
         be non-empty.
 
 MPOL_BIND
         This mode specifies that memory must come from the set of
         nodes specified by the policy.  Memory will be allocated from
         the node in the set with sufficient free memory that is
         closest to the node where the allocation takes place.
 
 MPOL_PREFERRED
         This mode specifies that the allocation should be attempted
         from the single node specified in the policy.  If that
         allocation fails, the kernel will search other nodes, in order
         of increasing distance from the preferred node based on
         information provided by the platform firmware.
 
         Internally, the Preferred policy uses a single node--the
         preferred_node member of struct mempolicy.  When the internal
         mode flag MPOL_F_LOCAL is set, the preferred_node is ignored
         and the policy is interpreted as local allocation.  "Local"
         allocation policy can be viewed as a Preferred policy that
         starts at the node containing the cpu where the allocation
         takes place.
 
         It is possible for the user to specify that local allocation
         is always preferred by passing an empty nodemask with this
         mode.  If an empty nodemask is passed, the policy cannot use
         the MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES flags
         described below.
 
 MPOL_INTERLEAVED
         This mode specifies that page allocations be interleaved, on a
         page granularity, across the nodes specified in the policy.
         This mode also behaves slightly differently, based on the
         context where it is used:
 
         For allocation of anonymous pages and shared memory pages,
         Interleave mode indexes the set of nodes specified by the
         policy using the page offset of the faulting address into the
         segment [VMA] containing the address modulo the number of
         nodes specified by the policy.  It then attempts to allocate a
         page, starting at the selected node, as if the node had been
         specified by a Preferred policy or had been selected by a
         local allocation.  That is, allocation will follow the per
         node zonelist.
 
         For allocation of page cache pages, Interleave mode indexes
         the set of nodes specified by the policy using a node counter
         maintained per task.  This counter wraps around to the lowest
         specified node after it reaches the highest specified node.
         This will tend to spread the pages out over the nodes
         specified by the policy based on the order in which they are
         allocated, rather than based on any page offset into an
         address range or file.  During system boot up, the temporary
         interleaved system default policy works in this mode.
 
 MPOL_PREFERRED_MANY
         This mode specifices that the allocation should be preferrably
         satisfied from the nodemask specified in the policy. If there is
         a memory pressure on all nodes in the nodemask, the allocation
         can fall back to all existing numa nodes. This is effectively
         MPOL_PREFERRED allowed for a mask rather than a single node.
 
 NUMA memory policy supports the following optional mode flags:
 
 MPOL_F_STATIC_NODES
         This flag specifies that the nodemask passed by
         the user should not be remapped if the task or VMA's set of allowed
         nodes changes after the memory policy has been defined.
 
         Without this flag, any time a mempolicy is rebound because of a
         change in the set of allowed nodes, the preferred nodemask (Preferred
         Many), preferred node (Preferred) or nodemask (Bind, Interleave) is
         remapped to the new set of allowed nodes.  This may result in nodes
         being used that were previously undesired.
 
         With this flag, if the user-specified nodes overlap with the
         nodes allowed by the task's cpuset, then the memory policy is
         applied to their intersection.  If the two sets of nodes do not
         overlap, the Default policy is used.
 
         For example, consider a task that is attached to a cpuset with
         mems 1-3 that sets an Interleave policy over the same set.  If
         the cpuset's mems change to 3-5, the Interleave will now occur
         over nodes 3, 4, and 5.  With this flag, however, since only node
         3 is allowed from the user's nodemask, the "interleave" only
         occurs over that node.  If no nodes from the user's nodemask are
         now allowed, the Default behavior is used.
 
         MPOL_F_STATIC_NODES cannot be combined with the
         MPOL_F_RELATIVE_NODES flag.  It also cannot be used for
         MPOL_PREFERRED policies that were created with an empty nodemask
         (local allocation).
 
 MPOL_F_RELATIVE_NODES
         This flag specifies that the nodemask passed
         by the user will be mapped relative to the set of the task or VMA's
         set of allowed nodes.  The kernel stores the user-passed nodemask,
         and if the allowed nodes changes, then that original nodemask will
         be remapped relative to the new set of allowed nodes.
 
         Without this flag (and without MPOL_F_STATIC_NODES), anytime a
         mempolicy is rebound because of a change in the set of allowed
         nodes, the node (Preferred) or nodemask (Bind, Interleave) is
         remapped to the new set of allowed nodes.  That remap may not
         preserve the relative nature of the user's passed nodemask to its
         set of allowed nodes upon successive rebinds: a nodemask of
         1,3,5 may be remapped to 7-9 and then to 1-3 if the set of
         allowed nodes is restored to its original state.
 
         With this flag, the remap is done so that the node numbers from
         the user's passed nodemask are relative to the set of allowed
         nodes.  In other words, if nodes 0, 2, and 4 are set in the user's
         nodemask, the policy will be effected over the first (and in the
         Bind or Interleave case, the third and fifth) nodes in the set of
         allowed nodes.  The nodemask passed by the user represents nodes
         relative to task or VMA's set of allowed nodes.
 
         If the user's nodemask includes nodes that are outside the range
         of the new set of allowed nodes (for example, node 5 is set in
         the user's nodemask when the set of allowed nodes is only 0-3),
         then the remap wraps around to the beginning of the nodemask and,
         if not already set, sets the node in the mempolicy nodemask.
 
         For example, consider a task that is attached to a cpuset with
         mems 2-5 that sets an Interleave policy over the same set with
         MPOL_F_RELATIVE_NODES.  If the cpuset's mems change to 3-7, the
         interleave now occurs over nodes 3,5-7.  If the cpuset's mems
         then change to 0,2-3,5, then the interleave occurs over nodes
         0,2-3,5.
 
         Thanks to the consistent remapping, applications preparing
         nodemasks to specify memory policies using this flag should
         disregard their current, actual cpuset imposed memory placement
         and prepare the nodemask as if they were always located on
         memory nodes 0 to N-1, where N is the number of memory nodes the
         policy is intended to manage.  Let the kernel then remap to the
         set of memory nodes allowed by the task's cpuset, as that may
         change over time.
 
         MPOL_F_RELATIVE_NODES cannot be combined with the
         MPOL_F_STATIC_NODES flag.  It also cannot be used for
         MPOL_PREFERRED policies that were created with an empty nodemask
         (local allocation).
 
 Memory Policy Reference Counting
 ================================
 
 To resolve use/free races, struct mempolicy contains an atomic reference
 count field.  Internal interfaces, mpol_get()/mpol_put() increment and
 decrement this reference count, respectively.  mpol_put() will only free
 the structure back to the mempolicy kmem cache when the reference count
 goes to zero.
 
 When a new memory policy is allocated, its reference count is initialized
 to '1', representing the reference held by the task that is installing the
 new policy.  When a pointer to a memory policy structure is stored in another
 structure, another reference is added, as the task's reference will be dropped
 on completion of the policy installation.
 
 During run-time "usage" of the policy, we attempt to minimize atomic operations
 on the reference count, as this can lead to cache lines bouncing between cpus
 and NUMA nodes.  "Usage" here means one of the following:
 
 1) querying of the policy, either by the task itself [using the get_mempolicy()
    API discussed below] or by another task using the /proc/<pid>/numa_maps
    interface.
 
 2) examination of the policy to determine the policy mode and associated node
    or node lists, if any, for page allocation.  This is considered a "hot
    path".  Note that for MPOL_BIND, the "usage" extends across the entire
    allocation process, which may sleep during page reclaimation, because the
    BIND policy nodemask is used, by reference, to filter ineligible nodes.
 
 We can avoid taking an extra reference during the usages listed above as
 follows:
 
 1) we never need to get/free the system default policy as this is never
    changed nor freed, once the system is up and running.
 
 2) for querying the policy, we do not need to take an extra reference on the
    target task's task policy nor vma policies because we always acquire the
    task's mm's mmap_lock for read during the query.  The set_mempolicy() and
    mbind() APIs [see below] always acquire the mmap_lock for write when
    installing or replacing task or vma policies.  Thus, there is no possibility
    of a task or thread freeing a policy while another task or thread is
    querying it.
 
 3) Page allocation usage of task or vma policy occurs in the fault path where
    we hold them mmap_lock for read.  Again, because replacing the task or vma
    policy requires that the mmap_lock be held for write, the policy can't be
    freed out from under us while we're using it for page allocation.
 
 4) Shared policies require special consideration.  One task can replace a
    shared memory policy while another task, with a distinct mmap_lock, is
    querying or allocating a page based on the policy.  To resolve this
    potential race, the shared policy infrastructure adds an extra reference
    to the shared policy during lookup while holding a spin lock on the shared
    policy management structure.  This requires that we drop this extra
    reference when we're finished "using" the policy.  We must drop the
    extra reference on shared policies in the same query/allocation paths
    used for non-shared policies.  For this reason, shared policies are marked
    as such, and the extra reference is dropped "conditionally"--i.e., only
    for shared policies.
 
    Because of this extra reference counting, and because we must lookup
    shared policies in a tree structure under spinlock, shared policies are
    more expensive to use in the page allocation path.  This is especially
    true for shared policies on shared memory regions shared by tasks running
    on different NUMA nodes.  This extra overhead can be avoided by always
    falling back to task or system default policy for shared memory regions,
    or by prefaulting the entire shared memory region into memory and locking
    it down.  However, this might not be appropriate for all applications.
 
 .. _memory_policy_apis:
 
 Memory Policy APIs
 ==================
 
 Linux supports 4 system calls for controlling memory policy.  These APIS
 always affect only the calling task, the calling task's address space, or
 some shared object mapped into the calling task's address space.
 
 .. note::
    the headers that define these APIs and the parameter data types for
    user space applications reside in a package that is not part of the
    Linux kernel.  The kernel system call interfaces, with the 'sys\_'
    prefix, are defined in <linux/syscalls.h>; the mode and flag
    definitions are defined in <linux/mempolicy.h>.
 
 Set [Task] Memory Policy::
 
         long set_mempolicy(int mode, const unsigned long *nmask,
                                         unsigned long maxnode);
 
 Set's the calling task's "task/process memory policy" to mode
 specified by the 'mode' argument and the set of nodes defined by
 'nmask'.  'nmask' points to a bit mask of node ids containing at least
 'maxnode' ids.  Optional mode flags may be passed by combining the
 'mode' argument with the flag (for example: MPOL_INTERLEAVE |
 MPOL_F_STATIC_NODES).
 
 See the set_mempolicy(2) man page for more details
 
 
 Get [Task] Memory Policy or Related Information::
 
         long get_mempolicy(int *mode,
                            const unsigned long *nmask, unsigned long maxnode,
                            void *addr, int flags);
 
 Queries the "task/process memory policy" of the calling task, or the
 policy or location of a specified virtual address, depending on the
 'flags' argument.
 
 See the get_mempolicy(2) man page for more details
 
 
 Install VMA/Shared Policy for a Range of Task's Address Space::
 
         long mbind(void *start, unsigned long len, int mode,
                    const unsigned long *nmask, unsigned long maxnode,
                    unsigned flags);
 
 mbind() installs the policy specified by (mode, nmask, maxnodes) as a
 VMA policy for the range of the calling task's address space specified
 by the 'start' and 'len' arguments.  Additional actions may be
 requested via the 'flags' argument.
 
 See the mbind(2) man page for more details.
 
 Set home node for a Range of Task's Address Spacec::
 
         long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
                                          unsigned long home_node,
                                          unsigned long flags);
 
 sys_set_mempolicy_home_node set the home node for a VMA policy present in the
 task's address range. The system call updates the home node only for the existing
 mempolicy range. Other address ranges are ignored. A home node is the NUMA node
 closest to which page allocation will come from. Specifying the home node override
 the default allocation policy to allocate memory close to the local node for an
 executing CPU.
 
 
 Memory Policy Command Line Interface
 ====================================
 
 Although not strictly part of the Linux implementation of memory policy,
 a command line tool, numactl(8), exists that allows one to:
 
 + set the task policy for a specified program via set_mempolicy(2), fork(2) and
   exec(2)
 
 + set the shared policy for a shared memory segment via mbind(2)
 
 The numactl(8) tool is packaged with the run-time version of the library
 containing the memory policy system call wrappers.  Some distributions
 package the headers and compile-time libraries in a separate development
 package.
 
 .. _mem_pol_and_cpusets:
 
 Memory Policies and cpusets
 ===========================
 
 Memory policies work within cpusets as described above.  For memory policies
 that require a node or set of nodes, the nodes are restricted to the set of
 nodes whose memories are allowed by the cpuset constraints.  If the nodemask
 specified for the policy contains nodes that are not allowed by the cpuset and
 MPOL_F_RELATIVE_NODES is not used, the intersection of the set of nodes
 specified for the policy and the set of nodes with memory is used.  If the
 result is the empty set, the policy is considered invalid and cannot be
 installed.  If MPOL_F_RELATIVE_NODES is used, the policy's nodes are mapped
 onto and folded into the task's set of allowed nodes as previously described.
 
 The interaction of memory policies and cpusets can be problematic when tasks
 in two cpusets share access to a memory region, such as shared memory segments
 created by shmget() of mmap() with the MAP_ANONYMOUS and MAP_SHARED flags, and
 any of the tasks install shared policy on the region, only nodes whose
 memories are allowed in both cpusets may be used in the policies.  Obtaining
 this information requires "stepping outside" the memory policy APIs to use the
 cpuset information and requires that one know in what cpusets other task might
 be attaching to the shared region.  Furthermore, if the cpusets' allowed
 memory sets are disjoint, "local" allocation is the only valid policy.

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