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