numa_memory_policy.txt

linux 内核源代码

		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 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.