Linux Memory Policy

转自：

https://www.kernel.org/doc/Documentation/vm/numa_memory_policy.txt

 

What is Linux Memory Policy?In the Linux kernel, "memory policy" determines from which node the kernel willallocate memory in a NUMA system or in an emulated NUMA system.  Linux hassupported platforms with Non-Uniform Memory Access architectures since 2.4.?.The current memory policy support was added to Linux 2.6 around May 2004.  Thisdocument attempts to describe the concepts and APIs of the 2.6 memory policysupport.Memory policies should not be confused with cpusets(Documentation/cgroups/cpusets.txt)which is an administrative mechanism for restricting the nodes from whichmemory may be allocated by a set of processes. Memory policies are aprogramming interface that a NUMA-aware application can take advantage of.  Whenboth cpusets and policies are applied to a task, the restrictions of the cpusettakes priority.  See "MEMORY POLICIES AND CPUSETS" below for more details.MEMORY POLICY CONCEPTSScope of Memory PoliciesThe Linux kernel supports _scopes_ of memory policy, described here frommost 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 taskto establish the task policy for a child task exec()'d from anexecutable image that has no awareness of memory policy.  See theMEMORY POLICY APIS section, below, for an overview of the system callthat 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 threadssubsequently created by that thread.  Any sibling threads existingat the time a new task policy is installed retain their currentpolicy.A task policy applies only to pages allocated after the policy isinstalled.  Any pages already faulted in by the task when the taskchanges its task policy remain where they were allocated based onthe policy at the time they were allocated.    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 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 pagesallocated for anonymous segments, such as the task stack and heap, andany 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 ifthe mapping used the MAP_SHARED flag.  If the file mapping used theMAP_PRIVATE flag, the VMA policy will only be applied when ananonymous 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 addressspace--a.k.a. threads--independent of when the policy is installed; andthey are inherited across fork().  However, because VMA policies referto a specific region of a task's address space, and because the addressspace is discarded and recreated on exec*(), VMA policies are NOTinheritable across exec().  Thus, only NUMA-aware applications mayuse VMA policies.A task may install a new VMA policy on a sub-range of a previouslymmap()ed region.  When this happens, Linux splits the existing virtualmemory area into 2 or 3 VMAs, each with it's own policy.By default, VMA policy applies only to pages allocated after the policyis installed.  Any pages already faulted into the VMA range remainwhere they were allocated based on the policy at the time they wereallocated.  However, since 2.6.16, Linux supports page migration viathe mbind() system call, so that page contents can be moved to matcha 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() ormmap(MAP_ANONYMOUS|MAP_SHARED), support shared policy.  When sharedpolicy support was added to Linux, the associated data structures wereadded to hugetlbfs shmem segments.  At the time, hugetlbfs did notsupport allocation at fault time--a.k.a lazy allocation--so hugetlbfsshmem segments were never "hooked up" to the shared policy support.Although hugetlbfs segments now support lazy allocation, their supportfor shared policy has not been completed.As mentioned above [re: VMA policies], allocations of page cachepages for regular files mmap()ed with MAP_SHARED ignore any VMApolicy installed on the virtual address range backed by the sharedfile mapping.  Rather, shared page cache pages, including pages backingprivate mappings that have not yet been written by the task, followtask policy, if any, else System Default Policy.The shared policy infrastructure supports different policies on subsetranges of the shared object.  However, Linux still splits the VMA ofthe task that installs the policy for each range of distinct policy.Thus, different tasks that attach to a shared memory segment can havedifferent VMA configurations mapping that one shared object.  Thiscan be seen by examining the /proc/<pid>/numa_maps of tasks sharinga shared memory region, when one task has installed shared policy onone or more ranges of the region.Components of Memory Policies    A Linux 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.   Linux memory policy supports the following 4 behavioral modes:Default Mode--MPOL_DEFAULT:  This mode is only used in the memorypolicy APIs.  Internally, MPOL_DEFAULT is converted to the NULLmemory policy in all policy scopes.  Any existing non-default policywill 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 theset of nodes specified by the policy.  Memory will be allocated fromthe node in the set with sufficient free memory that is closest tothe node where the allocation takes place.MPOL_PREFERRED:  This mode specifies that the allocation should beattempted from the single node specified in the policy.  If thatallocation fails, the kernel will search other nodes, in order ofincreasing distance from the preferred node based on informationprovided by the platform firmware.containing the cpu where the allocation takes place.    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 beinterleaved, on a page granularity, across the nodes specified inthe policy.  This mode also behaves slightly differently, based onthe 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.   Linux memory policy supports the following optional mode flags:MPOL_F_STATIC_NODES:  This flag specifies that the nodemask passed bythe user should not be remapped if the task or VMA's set of allowednodes changes after the memory policy has been defined.    Without this flag, 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.  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 passedby the user will be mapped relative to the set of the task or VMA'sset of allowed nodes.  The kernel stores the user-passed nodemask,and if the allowed nodes changes, then that original nodemask willbe 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 3,5-6.  If the cpuset's mems    then change to 0,2-3,5, then the interleave occurs over nodes    0,3,5.    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 COUNTINGTo resolve use/free races, struct mempolicy contains an atomic referencecount field.  Internal interfaces, mpol_get()/mpol_put() increment anddecrement this reference count, respectively.  mpol_put() will only freethe structure back to the mempolicy kmem cache when the reference countgoes to zero.When a new memory policy is allocated, its reference count is initializedto '1', representing the reference held by the task that is installing thenew policy.  When a pointer to a memory policy structure is stored in anotherstructure, another reference is added, as the task's reference will be droppedon completion of the policy installation.During run-time "usage" of the policy, we attempt to minimize atomic operationson the reference count, as this can lead to cache lines bouncing between cpusand 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 asfollows: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_sem for read during the query.  The set_mempolicy() and   mbind() APIs [see below] always acquire the mmap_sem 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_sem for read.  Again, because replacing the task or vma   policy requires that the mmap_sem 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_sem, 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 APIsLinux supports 3 system calls for controlling memory policy.  These APISalways affect only the calling task, the calling task's address space, orsome shared object mapped into the calling task's address space.Note:  the headers that define these APIs and the parameter data typesfor user space applications reside in a package that is not part ofthe Linux kernel.  The kernel system call interfaces, with the 'sys_'prefix, are defined in <linux/syscalls.h>; the mode and flagdefinitions 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 modespecified by the 'mode' argument and the set of nodes definedby 'nmask'.  'nmask' points to a bit mask of node ids containingat least 'maxnode' ids.  Optional mode flags may be passed bycombining the 'mode' argument with the flag (for example:MPOL_INTERLEAVE | MPOL_F_STATIC_NODES).See the set_mempolicy(2) man page for more detailsGet [Task] Memory Policy or Related Informationlong 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, orthe policy or location of a specified virtual address, dependingon the 'flags' argument.See the get_mempolicy(2) man page for more detailsInstall VMA/Shared Policy for a Range of Task's Address Spacelong 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) asa VMA policy for the range of the calling task's address spacespecified by the 'start' and 'len' arguments.  Additional actionsmay be requested via the 'flags' argument.See the mbind(2) man page for more details.MEMORY POLICY COMMAND LINE INTERFACEAlthough 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 librarycontaining the memory policy system call wrappers.  Some distributionspackage the headers and compile-time libraries in a separate developmentpackage.MEMORY POLICIES AND CPUSETSMemory policies work within cpusets as described above.  For memory policiesthat require a node or set of nodes, the nodes are restricted to the set ofnodes whose memories are allowed by the cpuset constraints.  If the nodemaskspecified for the policy contains nodes that are not allowed by the cpuset andMPOL_F_RELATIVE_NODES is not used, the intersection of the set of nodesspecified for the policy and the set of nodes with memory is used.  If theresult is the empty set, the policy is considered invalid and cannot beinstalled.  If MPOL_F_RELATIVE_NODES is used, the policy's nodes are mappedonto and folded into the task's set of allowed nodes as previously described.The interaction of memory policies and cpusets can be problematic when tasksin two cpusets share access to a memory region, such as shared memory segmentscreated by shmget() of mmap() with the MAP_ANONYMOUS and MAP_SHARED flags, andany of the tasks install shared policy on the region, only nodes whosememories are allowed in both cpusets may be used in the policies.  Obtainingthis information requires "stepping outside" the memory policy APIs to use thecpuset information and requires that one know in what cpusets other task mightbe attaching to the shared region.  Furthermore, if the cpusets' allowedmemory sets are disjoint, "local" allocation is the only valid policy.