Document Linux Memory Policy

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/<pid>/numa_maps of tasks sharing

	a shared memory region, when one task has installed shared policy on

	one or more ranges of the region.

Components of Memory Policies

    A 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 <linux/syscalls.h>; the mode and flag

	definitions are defined in <linux/mempolicy.h>.

Set [Task] Memory Policy:

	long set_mempolicy(int mode, const unsigned long *nmask,

					unsigned long maxnode);

	Set's the calling task's "task/process memory policy" to mode

	specified by the 'mode' argument and the set of nodes defined

	by 'nmask'.  'nmask' points to a bit mask of node ids containing

	at least 'maxnode' ids.

	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

intersection of the set of nodes specified for the policy and the set of nodes

allowed by the cpuset 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.  However, the

   memory policy APIs do not provide a way to determine what nodes are valid

   in the context where the application is running.  An application MAY consult

   the cpuset file system [directly or via an out of tree, and not generally

   available, libcpuset API] to obtain this information, but then the

   application must be aware that it is running in a cpuset and use what are

   intended primarily as administrative APIs.

   However, as long as the policy specifies at least one node that is valid

   in the controlling cpuset, the policy can be used.

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.  Again, obtaining this information requires "stepping outside"

   the memory policy APIs, as well as knowing in what cpusets other task might

   be attaching to the shared region, to use the cpuset information.

   Furthermore, if the cpusets' allowed memory sets are disjoint, "local"

   allocation is the only valid policy.