Re: [PATCH] Document Linux Memory Policy - V3

[mel@skynet.ie (Mel Gorman)]

On (31/07/07 16:48), Lee Schermerhorn didst pronounce:
> [PATCH] Document Linux Memory Policy - V3
> 
> V3 -> V2:
> + edits and rework suggested by Randy Dunlap, Mel Gorman and Christoph
>   Lameter.  N.B., I couldn't make all of the changes exactly as suggested
>   and retain what I consider important semantics.  Therefor, I tried to
>   capture the spirit of the suggestions as best I could.
> 
> V1 -> V2:
> +  Uh, I forget the details.  Rework based on suggestions by Andi Kleen
>    and Christoph Lameter.  E.g., dropped syscall details and updated
>    the man pages, instead.
> 
> I couldn't find any memory policy documentation in the Documentation
> directory, so here is my attempt to document it.
> 
> There's lots more that could be written about the internal design--including
> data structures, functions, etc.  However, if you agree that this is better
> that the nothing that exists now, perhaps it could be merged.  This will
> provide a baseline for updates to document the many policy patches that are
> currently being worked.
> 
> Signed-off-by:  Lee Schermerhorn <lee.schermerhorn@hp.com>
> 

I'm happy with that. It gets lots of useful information out in as clear
as manner as you get.

Acked-by: Mel Gorman <mel@csn.ul.ie>

>  Documentation/vm/memory_policy.txt |  332 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 332 insertions(+)
> 
> Index: Linux/Documentation/vm/memory_policy.txt
> ===================================================================
> --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> +++ Linux/Documentation/vm/memory_policy.txt	2007-07-31 15:54:50.000000000 -0400
> @@ -0,0 +1,332 @@
> +
> +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.
> 

-- 
-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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