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[linux/fpc-iii.git] / Documentation / admin-guide / mm / numa_memory_policy.rst

.. _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.

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 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 3 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.

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.