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.\" Copyright 2003,2004 Andi Kleen, SuSE Labs.
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" 2006-02-03, mtk, substantial wording changes and other improvements
.\"
.TH SET_MEMPOLICY 2 "2006-02-07" "SuSE Labs" "Linux Programmer's Manual"
.SH NAME
set_mempolicy \- Set default NUMA memory policy for a process and its children.
.SH SYNOPSIS
.B "#include <numaif.h>"
.sp
.BI "int set_mempolicy(int " policy ", unsigned long *" nodemask ,
.BI "unsigned long " maxnode );
.sp
.SH DESCRIPTION
.BR set_mempolicy ()
sets the NUMA memory policy of the calling process to
.IR policy .
A NUMA machine has different
memory controllers with different distances to specific CPUs.
The memory policy defines in which node memory is allocated for
the process.
This system call defines the default policy for the process;
in addition a policy can be set for specific memory ranges using
.BR mbind (2).
The policy is only applied when a new page is allocated
for the process. For anonymous memory this is when the page is first
touched by the application.
Available policies are
.BR MPOL_DEFAULT ,
.BR MPOL_BIND ,
.BR MPOL_INTERLEAVE ,
.BR MPOL_PREFERRED .
All policies except
.B MPOL_DEFAULT
require the caller to specify the nodes to which the policy applies in the
.I nodemask
parameter.
.I nodemask
is pointer to a bit field of nodes that contains up to
.I maxnode
bits. The bit field size is rounded to the next multiple of
.IR "sizeof(unsigned long)" ,
but the kernel will only use bits up to
.IR maxnode .
The
.B MPOL_DEFAULT
policy is the default and means to allocate memory locally,
i.e., on the node of the CPU that triggered the allocation.
.I nodemask
should be specified as NULL.
The
.B MPOL_BIND
policy is a strict policy that restricts memory allocation to the
nodes specified in
.IR nodemask .
There won't be allocations on other nodes.
.B MPOL_INTERLEAVE
interleaves allocations to the nodes specified in
.IR nodemask .
This optimizes for bandwidth instead of latency.
To be effective the memory area should be fairly large,
at least 1MB or bigger.
.B MPOL_PREFERRED
sets the preferred node for allocation.
The kernel will try to allocate in this
node first and fall back to other nodes if the preferred node is low on free
memory. Only the first node in the
.I nodemask
is used.
If no node is set in the mask, then the memory is allocated on
the node of the CPU that triggered the allocation allocation (like
.BR MPOL_DEFAULT ).
The memory policy is inherited by child processes created using
.BR fork (2)
or
.BR clone (2).
.SH NOTES
Process policy is not remembered if the page is swapped out.
.SH RETURN VALUE
On success,
.BR set_mempolicy ()
returns 0;
on error, \-1 is returned and
.I errno
is set to indicate the error.
.\" .SH ERRORS
.\" FIXME writeme -- no errors are listed on this page
.\" .
.\" .TP
.\" .B EINVAL
.\" .I mode is invalid.
.SH "VERSIONS AND LIBRARY SUPPORT"
See
.BR mbind (2).
.SH CONFORMING TO
This system call is Linux specific.
.SH SEE ALSO
.BR mbind (2),
.BR get_mempolicy (2),
.BR numactl (8),
.BR numa (3)
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