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.\" 2006-02-03, mtk, substantial wording changes and other improvements
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.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
.nf
.B "#include <numaif.h>" 
.sp
.BI "int set_mempolicy(int " policy ", unsigned long *" nodemask , 
.BI "                  unsigned long " maxnode );
.fi
.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 preserved across an
.BR execve (2),
and 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)