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)
+Memory policies should not be confused with cpusets
+(Documentation/cgroups/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
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
+ 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.
Linux memory policy supports the following 4 behavioral modes:
- Default Mode--MPOL_DEFAULT: The behavior specified by this mode is
- context or scope dependent.
+ Default Mode--MPOL_DEFAULT: This mode is only used in the memory
+ policy APIs. Internally, MPOL_DEFAULT is converted to the NULL
+ memory policy in all policy scopes. Any existing non-default policy
+ will simply be removed when MPOL_DEFAULT is specified. As a result,
+ MPOL_DEFAULT means "fall back to the next most specific policy scope."
- As mentioned in the Policy Scope section above, during normal
- system operation, the System Default Policy is hard coded to
- contain the Default mode.
+ 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.
- 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.
+ 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_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
+ allocation fails, the kernel will search other nodes, in order of
+ increasing distance from the preferred node based on information
+ provided 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. 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.
+ 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.
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 expecially
+ 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,
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