mirror of https://github.com/mkerrisk/man-pages
344 lines
9.3 KiB
Groff
344 lines
9.3 KiB
Groff
.\" Copyright 2003,2004 Andi Kleen, SuSE Labs.
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.\" and Copyright 2007 Lee Schermerhorn, Hewlett Packard
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.\"
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.\" %%%LICENSE_START(VERBATIM_PROF)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" 2006-02-03, mtk, substantial wording changes and other improvements
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.\" 2007-08-27, Lee Schermerhorn <Lee.Schermerhorn@hp.com>
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.\" more precise specification of behavior.
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.\"
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.TH SET_MEMPOLICY 2 2021-06-20 Linux "Linux Programmer's Manual"
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.SH NAME
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set_mempolicy \- set default NUMA memory policy for a thread and its children
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.SH SYNOPSIS
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.nf
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.B "#include <numaif.h>"
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.PP
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.BI "long set_mempolicy(int " mode ", const unsigned long *" nodemask ,
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.BI " unsigned long " maxnode );
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.PP
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Link with \fI\-lnuma\fP.
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.fi
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.SH DESCRIPTION
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.BR set_mempolicy ()
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sets the NUMA memory policy of the calling thread,
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which consists of a policy mode and zero or more nodes,
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to the values specified by the
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.IR mode ,
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.IR nodemask ,
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and
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.I maxnode
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arguments.
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.PP
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A NUMA machine has different
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memory controllers with different distances to specific CPUs.
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The memory policy defines from which node memory is allocated for
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the thread.
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.PP
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This system call defines the default policy for the thread.
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The thread policy governs allocation of pages in the process's
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address space outside of memory ranges
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controlled by a more specific policy set by
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.BR mbind (2).
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The thread default policy also controls allocation of any pages for
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memory-mapped files mapped using the
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.BR mmap (2)
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call with the
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.B MAP_PRIVATE
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flag and that are only read (loaded) from by the thread
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and of memory-mapped files mapped using the
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.BR mmap (2)
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call with the
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.B MAP_SHARED
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flag, regardless of the access type.
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The policy is applied only when a new page is allocated
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for the thread.
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For anonymous memory this is when the page is first
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touched by the thread.
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.PP
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The
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.I mode
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argument must specify one of
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.BR MPOL_DEFAULT ,
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.BR MPOL_BIND ,
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.BR MPOL_INTERLEAVE ,
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.BR MPOL_PREFERRED ,
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or
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.BR MPOL_LOCAL
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(which are described in detail below).
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All modes except
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.B MPOL_DEFAULT
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require the caller to specify the node or nodes to which the mode applies,
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via the
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.I nodemask
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argument.
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.PP
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The
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.I mode
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argument may also include an optional
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.IR "mode flag" .
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The supported
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.I "mode flags"
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are:
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.TP
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.BR MPOL_F_NUMA_BALANCING " (since Linux 5.12)"
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.\" commit bda420b985054a3badafef23807c4b4fa38a3dff
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When
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.I mode
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is
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.BR MPOL_BIND ,
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enable the kernel NUMA balancing for the task if it is supported by the kernel.
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If the flag isn't supported by the kernel, or is used with
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.I mode
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other than
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.BR MPOL_BIND ,
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\-1 is returned and
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.I errno
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is set to
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.BR EINVAL .
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.TP
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.BR MPOL_F_RELATIVE_NODES " (since Linux 2.6.26)"
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A nonempty
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.I nodemask
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specifies node IDs that are relative to the
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set of node IDs allowed by the process's current cpuset.
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.TP
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.BR MPOL_F_STATIC_NODES " (since Linux 2.6.26)"
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A nonempty
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.I nodemask
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specifies physical node IDs.
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Linux will not remap the
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.I nodemask
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when the process moves to a different cpuset context,
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nor when the set of nodes allowed by the process's
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current cpuset context changes.
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.PP
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.I nodemask
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points to a bit mask of node IDs that contains up to
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.I maxnode
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bits.
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The bit mask size is rounded to the next multiple of
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.IR "sizeof(unsigned long)" ,
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but the kernel will use bits only up to
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.IR maxnode .
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A NULL value of
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.I nodemask
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or a
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.I maxnode
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value of zero specifies the empty set of nodes.
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If the value of
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.I maxnode
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is zero,
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the
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.I nodemask
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argument is ignored.
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.PP
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Where a
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.I nodemask
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is required, it must contain at least one node that is on-line,
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allowed by the process's current cpuset context,
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(unless the
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.B MPOL_F_STATIC_NODES
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mode flag is specified),
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and contains memory.
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If the
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.B MPOL_F_STATIC_NODES
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is set in
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.I mode
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and a required
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.I nodemask
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contains no nodes that are allowed by the process's current cpuset context,
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the memory policy reverts to
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.IR "local allocation" .
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This effectively overrides the specified policy until the process's
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cpuset context includes one or more of the nodes specified by
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.IR nodemask .
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.PP
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The
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.I mode
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argument must include one of the following values:
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.TP
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.B MPOL_DEFAULT
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This mode specifies that any nondefault thread memory policy be removed,
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so that the memory policy "falls back" to the system default policy.
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The system default policy is "local allocation"\(emthat is,
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allocate memory on the node of the CPU that triggered the allocation.
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.I nodemask
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must be specified as NULL.
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If the "local node" contains no free memory, the system will
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attempt to allocate memory from a "near by" node.
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.TP
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.B MPOL_BIND
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This mode defines a strict policy that restricts memory allocation to the
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nodes specified in
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.IR nodemask .
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If
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.I nodemask
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specifies more than one node, page allocations will come from
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the node with the lowest numeric node ID first, until that node
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contains no free memory.
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Allocations will then come from the node with the next highest
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node ID specified in
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.I nodemask
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and so forth, until none of the specified nodes contain free memory.
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Pages will not be allocated from any node not specified in the
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.IR nodemask .
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.TP
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.B MPOL_INTERLEAVE
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This mode interleaves page allocations across the nodes specified in
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.I nodemask
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in numeric node ID order.
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This optimizes for bandwidth instead of latency
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by spreading out pages and memory accesses to those pages across
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multiple nodes.
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However, accesses to a single page will still be limited to
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the memory bandwidth of a single node.
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.\" NOTE: the following sentence doesn't make sense in the context
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.\" of set_mempolicy() -- no memory area specified.
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.\" To be effective the memory area should be fairly large,
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.\" at least 1 MB or bigger.
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.TP
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.B MPOL_PREFERRED
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This mode sets the preferred node for allocation.
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The kernel will try to allocate pages from this node first
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and fall back to "near by" nodes if the preferred node is low on free
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memory.
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If
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.I nodemask
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specifies more than one node ID, the first node in the
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mask will be selected as the preferred node.
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If the
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.I nodemask
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and
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.I maxnode
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arguments specify the empty set, then the policy
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specifies "local allocation"
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(like the system default policy discussed above).
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.TP
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.BR MPOL_LOCAL " (since Linux 3.8)"
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.\" commit 479e2802d09f1e18a97262c4c6f8f17ae5884bd8
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.\" commit f2a07f40dbc603c15f8b06e6ec7f768af67b424f
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This mode specifies "local allocation"; the memory is allocated on
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the node of the CPU that triggered the allocation (the "local node").
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The
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.I nodemask
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and
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.I maxnode
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arguments must specify the empty set.
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If the "local node" is low on free memory,
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the kernel will try to allocate memory from other nodes.
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The kernel will allocate memory from the "local node"
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whenever memory for this node is available.
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If the "local node" is not allowed by the process's current cpuset context,
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the kernel will try to allocate memory from other nodes.
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The kernel will allocate memory from the "local node" whenever
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it becomes allowed by the process's current cpuset context.
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.PP
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The thread memory policy is preserved across an
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.BR execve (2),
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and is inherited by child threads created using
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.BR fork (2)
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or
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.BR clone (2).
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.SH RETURN VALUE
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On success,
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.BR set_mempolicy ()
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returns 0;
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on error, \-1 is returned and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EFAULT
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Part of all of the memory range specified by
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.I nodemask
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and
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.I maxnode
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points outside your accessible address space.
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.TP
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.B EINVAL
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.I mode
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is invalid.
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Or,
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.I mode
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is
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.B MPOL_DEFAULT
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and
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.I nodemask
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is nonempty,
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or
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.I mode
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is
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.B MPOL_BIND
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or
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.B MPOL_INTERLEAVE
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and
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.I nodemask
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is empty.
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Or,
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.I maxnode
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specifies more than a page worth of bits.
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Or,
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.I nodemask
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specifies one or more node IDs that are
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greater than the maximum supported node ID.
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Or, none of the node IDs specified by
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.I nodemask
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are on-line and allowed by the process's current cpuset context,
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or none of the specified nodes contain memory.
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Or, the
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.I mode
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argument specified both
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.B MPOL_F_STATIC_NODES
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and
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.BR MPOL_F_RELATIVE_NODES .
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Or, the
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.B MPOL_F_NUMA_BALANCING
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isn't supported by the kernel, or is used with
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.I mode
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other than
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.BR MPOL_BIND .
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.TP
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.B ENOMEM
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Insufficient kernel memory was available.
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.SH VERSIONS
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The
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.BR set_mempolicy ()
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system call was added to the Linux kernel in version 2.6.7.
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.SH CONFORMING TO
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This system call is Linux-specific.
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.SH NOTES
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Memory policy is not remembered if the page is swapped out.
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When such a page is paged back in, it will use the policy of
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the thread or memory range that is in effect at the time the
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page is allocated.
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.PP
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For information on library support, see
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.BR numa (7).
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.SH SEE ALSO
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.BR get_mempolicy (2),
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.BR getcpu (2),
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.BR mbind (2),
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.BR mmap (2),
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.BR numa (3),
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.BR cpuset (7),
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.BR numa (7),
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.BR numactl (8)
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