332 lines
16 KiB
Plaintext
332 lines
16 KiB
Plaintext
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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 (Documentation/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 it's 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 adddress 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 is a tuple consisting of a "mode" and an optional set
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of nodes. The mode determine the behavior of the policy, while the
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optional set of nodes can be viewed as the arguments to the 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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Note: in some functions AND in the struct mempolicy itself, the mode
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is called "policy". However, to avoid confusion with the policy tuple,
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this document will continue to use the term "mode".
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Linux memory policy supports the following 4 behavioral modes:
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Default Mode--MPOL_DEFAULT: The behavior specified by this mode is
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context or scope dependent.
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As mentioned in the Policy Scope section above, during normal
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system operation, the System Default Policy is hard coded to
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contain the Default mode.
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In this context, default mode means "local" allocation--that is
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attempt to allocate the page from the node associated with the cpu
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where the fault occurs. If the "local" node has no memory, or the
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node's memory can be exhausted [no free pages available], local
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allocation will "fallback to"--attempt to allocate pages from--
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"nearby" nodes, in order of increasing "distance".
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Implementation detail -- subject to change: "Fallback" uses
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a per node list of sibling nodes--called zonelists--built at
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boot time, or when nodes or memory are added or removed from
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the system [memory hotplug]. These per node zonelist are
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constructed with nodes in order of increasing distance based
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on information provided by the platform firmware.
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When a task/process policy or a shared policy contains the Default
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mode, this also means "local allocation", as described above.
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In the context of a VMA, Default mode means "fall back to task
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policy"--which may or may not specify Default mode. Thus, Default
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mode can not be counted on to mean local allocation when used
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on a non-shared region of the address space. However, see
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MPOL_PREFERRED below.
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The Default mode does not use the optional set of nodes.
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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.
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The memory policy APIs do not specify an order in which the nodes
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will be searched. However, unlike "local allocation", the Bind
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policy does not consider the distance between the nodes. Rather,
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allocations will fallback to the nodes specified by the policy in
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order of numeric node id. Like everything in Linux, this is subject
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to change.
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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, exactly as
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it would for a local allocation that started at the preferred node
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in increasing distance from the preferred node. "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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Internally, the Preferred policy uses a single node--the
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preferred_node member of struct mempolicy. A "distinguished
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value of this preferred_node, currently '-1', is interpreted
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as "the node containing the cpu where the allocation takes
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place"--local allocation. This is the way to specify
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local allocation for a specific range of addresses--i.e. for
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VMA policies.
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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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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.
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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 packages 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, or
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the intersection of the set of nodes specified for the policy and the set of
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nodes with memory is the empty set, the policy is considered invalid
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and cannot be installed.
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The interaction of memory policies and cpusets can be problematic for a
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couple of reasons:
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1) the memory policy APIs take physical node id's as arguments. As mentioned
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above, it is illegal to specify nodes that are not allowed in the cpuset.
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The application must query the allowed nodes using the get_mempolicy()
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API with the MPOL_F_MEMS_ALLOWED flag to determine the allowed nodes and
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restrict itself to those nodes. However, the resources available to a
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cpuset can be changed by the system administrator, or a workload manager
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application, at any time. So, a task may still get errors attempting to
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specify policy nodes, and must query the allowed memories again.
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2) when tasks in two cpusets share access to a memory region, such as shared
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memory segments created by shmget() of mmap() with the MAP_ANONYMOUS and
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MAP_SHARED flags, and any of the tasks install shared policy on the region,
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only nodes whose memories are allowed in both cpusets may be used in the
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policies. Obtaining this information requires "stepping outside" the
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memory policy APIs to use the cpuset information and requires that one
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know in what cpusets other task might be attaching to the shared region.
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Furthermore, if the cpusets' allowed memory sets are disjoint, "local"
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allocation is the only valid policy.
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