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