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mempolicy: update NUMA memory policy documentation

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Updates Documentation/vm/numa_memory_policy.txt and
Documentation/filesystems/tmpfs.txt to describe optional mempolicy mode flags.

Cc: Christoph Lameter <clameter@sgi.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
David Rientjes 2008-04-28 02:12:31 -07:00 committed by Linus Torvalds
parent 4c50bc0116
commit 65d66fc02e
2 changed files with 111 additions and 30 deletions
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12
Documentation/filesystems/tmpfs.txt
View file

@ -92,6 +92,18 @@ NodeList format is a comma-separated list of decimal numbers and ranges,
a range being two hyphen-separated decimal numbers, the smallest and
largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15
NUMA memory allocation policies have optional flags that can be used in
conjunction with their modes. These optional flags can be specified
when tmpfs is mounted by appending them to the mode before the NodeList.
See Documentation/vm/numa_memory_policy.txt for a list of all available
memory allocation policy mode flags.
=static is equivalent to MPOL_F_STATIC_NODES
=relative is equivalent to MPOL_F_RELATIVE_NODES
For example, mpol=bind=static:NodeList, is the equivalent of an
allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
Note that trying to mount a tmpfs with an mpol option will fail if the
running kernel does not support NUMA; and will fail if its nodelist
specifies a node which is not online. If your system relies on that

129
Documentation/vm/numa_memory_policy.txt
View file

@ -135,9 +135,11 @@ most general to most specific:
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.
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
@ -179,7 +181,8 @@ Components of Memory Policies
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.
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
@ -226,6 +229,80 @@ Components of Memory Policies
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.
Without this flag, anytime a mempolicy is rebound because of a
change in the set of allowed nodes, the node (Preferred) or
nodemask (Bind, Interleave) is remapped to the new set of
allowed nodes. This may result in nodes being used that were
previously undesired.
With this flag, if the user-specified nodes overlap with the
nodes allowed by the task's cpuset, then the memory policy is
applied to their intersection. If the two sets of nodes do not
overlap, the Default policy is used.
For example, consider a task that is attached to a cpuset with
mems 1-3 that sets an Interleave policy over the same set. If
the cpuset's mems change to 3-5, the Interleave will now occur
over nodes 3, 4, and 5. With this flag, however, since only node
3 is allowed from the user's nodemask, the "interleave" only
occurs over that node. If no nodes from the user's nodemask are
now allowed, the Default behavior is used.
MPOL_F_STATIC_NODES cannot be used with MPOL_F_RELATIVE_NODES.
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
be remapped relative to the new set of allowed nodes.
Without this flag (and without MPOL_F_STATIC_NODES), anytime a
mempolicy is rebound because of a change in the set of allowed
nodes, the node (Preferred) or nodemask (Bind, Interleave) is
remapped to the new set of allowed nodes. That remap may not
preserve the relative nature of the user's passed nodemask to its
set of allowed nodes upon successive rebinds: a nodemask of
1,3,5 may be remapped to 7-9 and then to 1-3 if the set of
allowed nodes is restored to its original state.
With this flag, the remap is done so that the node numbers from
the user's passed nodemask are relative to the set of allowed
nodes. In other words, if nodes 0, 2, and 4 are set in the user's
nodemask, the policy will be effected over the first (and in the
Bind or Interleave case, the third and fifth) nodes in the set of
allowed nodes. The nodemask passed by the user represents nodes
relative to task or VMA's set of allowed nodes.
If the user's nodemask includes nodes that are outside the range
of the new set of allowed nodes (for example, node 5 is set in
the user's nodemask when the set of allowed nodes is only 0-3),
then the remap wraps around to the beginning of the nodemask and,
if not already set, sets the node in the mempolicy nodemask.
For example, consider a task that is attached to a cpuset with
mems 2-5 that sets an Interleave policy over the same set with
MPOL_F_RELATIVE_NODES. If the cpuset's mems change to 3-7, the
interleave now occurs over nodes 3,5-6. If the cpuset's mems
then change to 0,2-3,5, then the interleave occurs over nodes
0,3,5.
Thanks to the consistent remapping, applications preparing
nodemasks to specify memory policies using this flag should
disregard their current, actual cpuset imposed memory placement
and prepare the nodemask as if they were always located on
memory nodes 0 to N-1, where N is the number of memory nodes the
policy is intended to manage. Let the kernel then remap to the
set of memory nodes allowed by the task's cpuset, as that may
change over time.
MPOL_F_RELATIVE_NODES cannot be used with MPOL_F_STATIC_NODES.
MEMORY POLICY APIs
Linux supports 3 system calls for controlling memory policy. These APIS
@ -246,7 +323,9 @@ Set [Task] Memory Policy:
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.
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
@ -298,29 +377,19 @@ 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.
specified for the policy contains nodes that are not allowed by the cpuset and
MPOL_F_RELATIVE_NODES is not used, the intersection of the set of nodes
specified for the policy and the set of nodes with memory is used. If the
result is the empty set, the policy is considered invalid and cannot be
installed. If MPOL_F_RELATIVE_NODES is used, the policy's nodes are mapped
onto and folded into the task's set of allowed nodes as previously described.
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.
The interaction of memory policies and cpusets can be problematic 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.