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Commit cb5e4376 authored by Mike Rapoport's avatar Mike Rapoport Committed by Jonathan Corbet
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docs/vm: numa_memory_policy.txt: convert to ReST format 

Signed-off-by: default avatarMike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
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Documentation/vm/numa_memory_policy.txt
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.. _numa_memory_policy:
===================
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
......@@ -9,35 +15,36 @@ document attempts to describe the concepts and APIs of the 2.6 memory policy
support.
Memory policies should not be confused with cpusets
(Documentation/cgroup-v1/cpusets.txt)
(``Documentation/cgroup-v1/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.
takes priority. See :ref:`Memory Policies and cpusets <mem_pol_and_cpusets>`
below for more details.
MEMORY POLICY CONCEPTS
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.
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()
......@@ -58,56 +65,66 @@ most general to most specific:
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
.. _vma_policy:
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.
A VMA policy will govern the allocation of pages that back
this region ofthe 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.
* 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
......@@ -118,11 +135,12 @@ most general to most specific:
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
As mentioned above :ref:`VMA policies <vma_policy>`,
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
......@@ -135,24 +153,27 @@ most general to most specific:
one 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.
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.
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:
Linux memory policy supports the following 4 behavioral modes:
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."
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."
For example, a NULL or default task policy will fall back to the
system default policy. A NULL or default vma policy will fall
......@@ -164,57 +185,63 @@ Components of Memory Policies
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 the
set of nodes specified by the policy. Memory will be allocated from
the node in the set with sufficient free memory that is closest to
the node where the allocation takes place.
MPOL_BIND
This mode specifies that memory must come from the set of
nodes specified by the policy. Memory will be allocated from
the node in the set with sufficient free memory that is
closest to the node where the allocation takes place.
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, in order of
increasing distance from the preferred node based on information
provided by the platform firmware.
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, in order
of increasing distance from the preferred node based on
information provided by the platform firmware.
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 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:
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 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.
Linux memory policy supports the following optional mode flags:
MPOL_F_STATIC_NODES: This flag specifies that the nodemask passed by
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 by
the user should not be remapped if the task or VMA's set of allowed
nodes changes after the memory policy has been defined.
......@@ -242,7 +269,8 @@ Components of Memory Policies
MPOL_PREFERRED policies that were created with an empty nodemask
(local allocation).
MPOL_F_RELATIVE_NODES: This flag specifies that the nodemask passed
MPOL_F_RELATIVE_NODES
This flag specifies that the nodemask passed
by the user will be mapped relative to the set of the task or VMA's
set of allowed nodes. The kernel stores the user-passed nodemask,
and if the allowed nodes changes, then that original nodemask will
......@@ -292,7 +320,8 @@ Components of Memory Policies
MPOL_PREFERRED policies that were created with an empty nodemask
(local allocation).
MEMORY POLICY REFERENCE COUNTING
Memory Policy Reference Counting
================================
To resolve use/free races, struct mempolicy contains an atomic reference
count field. Internal interfaces, mpol_get()/mpol_put() increment and
......@@ -360,60 +389,62 @@ follows:
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 APIs
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_'
.. 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:
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. Optional mode flags may be passed by
combining the 'mode' argument with the flag (for example:
MPOL_INTERLEAVE | MPOL_F_STATIC_NODES).
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. Optional mode flags may be passed by combining the
'mode' argument with the flag (for example: MPOL_INTERLEAVE |
MPOL_F_STATIC_NODES).
See the set_mempolicy(2) man page for more details
See the set_mempolicy(2) man page for more details
Get [Task] Memory Policy or Related Information
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.
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
See the get_mempolicy(2) man page for more details
Install VMA/Shared Policy for a Range of Task's Address Space
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.
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.
See the mbind(2) man page for more details.
MEMORY POLICY COMMAND LINE INTERFACE
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:
......@@ -428,8 +459,10 @@ containing the memory policy system call wrappers. Some distributions
package the headers and compile-time libraries in a separate development
package.
.. _mem_pol_and_cpusets:
MEMORY POLICIES AND CPUSETS
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
......
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