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1 .\" Copyright 2003,2004 Andi Kleen, SuSE Labs.
2 .\" and Copyright 2007 Lee Schermerhorn, Hewlett Packard
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22 .\"
23 .\" 2006-02-03, mtk, substantial wording changes and other improvements
24 .\" 2007-08-27, Lee Schermerhorn <Lee.Schermerhorn@hp.com>
25 .\" more precise specification of behavior.
26 .\"
27 .TH SET_MEMPOLICY 2 2017-09-15 Linux "Linux Programmer's Manual"
28 .SH NAME
29 set_mempolicy \- set default NUMA memory policy for a thread and its children
30 .SH SYNOPSIS
31 .nf
32 .B "#include <numaif.h>"
33 .PP
34 .BI "long set_mempolicy(int " mode ", const unsigned long *" nodemask ,
35 .BI " unsigned long " maxnode );
36 .PP
37 Link with \fI\-lnuma\fP.
38 .fi
39 .SH DESCRIPTION
40 .BR set_mempolicy ()
41 sets the NUMA memory policy of the calling thread,
42 which consists of a policy mode and zero or more nodes,
43 to the values specified by the
44 .IR mode ,
45 .I nodemask
46 and
47 .I maxnode
48 arguments.
49 .PP
50 A NUMA machine has different
51 memory controllers with different distances to specific CPUs.
52 The memory policy defines from which node memory is allocated for
53 the thread.
54 .PP
55 This system call defines the default policy for the thread.
56 The thread policy governs allocation of pages in the process's
57 address space outside of memory ranges
58 controlled by a more specific policy set by
59 .BR mbind (2).
60 The thread default policy also controls allocation of any pages for
61 memory-mapped files mapped using the
62 .BR mmap (2)
63 call with the
64 .B MAP_PRIVATE
65 flag and that are only read (loaded) from by the thread
66 and of memory-mapped files mapped using the
67 .BR mmap (2)
68 call with the
69 .B MAP_SHARED
70 flag, regardless of the access type.
71 The policy is applied only when a new page is allocated
72 for the thread.
73 For anonymous memory this is when the page is first
74 touched by the thread.
75 .PP
76 The
77 .I mode
78 argument must specify one of
79 .BR MPOL_DEFAULT ,
80 .BR MPOL_BIND ,
81 .BR MPOL_INTERLEAVE ,
82 .BR MPOL_PREFERRED ,
83 or
84 .BR MPOL_LOCAL
85 (which are described in detail below).
86 All modes except
87 .B MPOL_DEFAULT
88 require the caller to specify the node or nodes to which the mode applies,
89 via the
90 .I nodemask
91 argument.
92 .PP
93 The
94 .I mode
95 argument may also include an optional
96 .IR "mode flag" .
97 The supported
98 .I "mode flags"
99 are:
100 .TP
101 .BR MPOL_F_STATIC_NODES " (since Linux 2.6.26)"
102 A nonempty
103 .I nodemask
104 specifies physical node IDs.
105 Linux will not remap the
106 .I nodemask
107 when the process moves to a different cpuset context,
108 nor when the set of nodes allowed by the process's
109 current cpuset context changes.
110 .TP
111 .BR MPOL_F_RELATIVE_NODES " (since Linux 2.6.26)"
112 A nonempty
113 .I nodemask
114 specifies node IDs that are relative to the set of
115 node IDs allowed by the process's current cpuset.
116 .PP
117 .I nodemask
118 points to a bit mask of node IDs that contains up to
119 .I maxnode
120 bits.
121 The bit mask size is rounded to the next multiple of
122 .IR "sizeof(unsigned long)" ,
123 but the kernel will use bits only up to
124 .IR maxnode .
125 A NULL value of
126 .I nodemask
127 or a
128 .I maxnode
129 value of zero specifies the empty set of nodes.
130 If the value of
131 .I maxnode
132 is zero,
133 the
134 .I nodemask
135 argument is ignored.
136 .PP
137 Where a
138 .I nodemask
139 is required, it must contain at least one node that is on-line,
140 allowed by the process's current cpuset context,
141 (unless the
142 .B MPOL_F_STATIC_NODES
143 mode flag is specified),
144 and contains memory.
145 If the
146 .B MPOL_F_STATIC_NODES
147 is set in
148 .I mode
149 and a required
150 .I nodemask
151 contains no nodes that are allowed by the process's current cpuset context,
152 the memory policy reverts to
153 .IR "local allocation" .
154 This effectively overrides the specified policy until the process's
155 cpuset context includes one or more of the nodes specified by
156 .IR nodemask .
157 .PP
158 The
159 .I mode
160 argument must include one of the following values:
161 .TP
162 .B MPOL_DEFAULT
163 This mode specifies that any nondefault thread memory policy be removed,
164 so that the memory policy "falls back" to the system default policy.
165 The system default policy is "local allocation"\(emthat is,
166 allocate memory on the node of the CPU that triggered the allocation.
167 .I nodemask
168 must be specified as NULL.
169 If the "local node" contains no free memory, the system will
170 attempt to allocate memory from a "near by" node.
171 .TP
172 .B MPOL_BIND
173 This mode defines a strict policy that restricts memory allocation to the
174 nodes specified in
175 .IR nodemask .
176 If
177 .I nodemask
178 specifies more than one node, page allocations will come from
179 the node with the lowest numeric node ID first, until that node
180 contains no free memory.
181 Allocations will then come from the node with the next highest
182 node ID specified in
183 .I nodemask
184 and so forth, until none of the specified nodes contain free memory.
185 Pages will not be allocated from any node not specified in the
186 .IR nodemask .
187 .IP
188 .TP
189 .B MPOL_INTERLEAVE
190 This mode interleaves page allocations across the nodes specified in
191 .I nodemask
192 in numeric node ID order.
193 This optimizes for bandwidth instead of latency
194 by spreading out pages and memory accesses to those pages across
195 multiple nodes.
196 However, accesses to a single page will still be limited to
197 the memory bandwidth of a single node.
198 .\" NOTE: the following sentence doesn't make sense in the context
199 .\" of set_mempolicy() -- no memory area specified.
200 .\" To be effective the memory area should be fairly large,
201 .\" at least 1 MB or bigger.
202 .TP
203 .B MPOL_PREFERRED
204 This mode sets the preferred node for allocation.
205 The kernel will try to allocate pages from this node first
206 and fall back to "near by" nodes if the preferred node is low on free
207 memory.
208 If
209 .I nodemask
210 specifies more than one node ID, the first node in the
211 mask will be selected as the preferred node.
212 If the
213 .I nodemask
214 and
215 .I maxnode
216 arguments specify the empty set, then the policy
217 specifies "local allocation"
218 (like the system default policy discussed above).
219 .TP
220 .BR MPOL_LOCAL " (since Linux 3.8)"
221 .\" commit 479e2802d09f1e18a97262c4c6f8f17ae5884bd8
222 .\" commit f2a07f40dbc603c15f8b06e6ec7f768af67b424f
223 This mode specifies "local allocation"; the memory is allocated on
224 the node of the CPU that triggered the allocation (the "local node").
225 The
226 .I nodemask
227 and
228 .I maxnode
229 arguments must specify the empty set.
230 If the "local node" is low on free memory,
231 the kernel will try to allocate memory from other nodes.
232 The kernel will allocate memory from the "local node"
233 whenever memory for this node is available.
234 If the "local node" is not allowed by the process's current cpuset context,
235 the kernel will try to allocate memory from other nodes.
236 The kernel will allocate memory from the "local node" whenever
237 it becomes allowed by the process's current cpuset context.
238 .PP
239 The thread memory policy is preserved across an
240 .BR execve (2),
241 and is inherited by child threads created using
242 .BR fork (2)
243 or
244 .BR clone (2).
245 .SH RETURN VALUE
246 On success,
247 .BR set_mempolicy ()
248 returns 0;
249 on error, \-1 is returned and
250 .I errno
251 is set to indicate the error.
252 .SH ERRORS
253 .TP
254 .B EFAULT
255 Part of all of the memory range specified by
256 .I nodemask
257 and
258 .I maxnode
259 points outside your accessible address space.
260 .TP
261 .B EINVAL
262 .I mode
263 is invalid.
264 Or,
265 .I mode
266 is
267 .B MPOL_DEFAULT
268 and
269 .I nodemask
270 is nonempty,
271 or
272 .I mode
273 is
274 .B MPOL_BIND
275 or
276 .B MPOL_INTERLEAVE
277 and
278 .I nodemask
279 is empty.
280 Or,
281 .I maxnode
282 specifies more than a page worth of bits.
283 Or,
284 .I nodemask
285 specifies one or more node IDs that are
286 greater than the maximum supported node ID.
287 Or, none of the node IDs specified by
288 .I nodemask
289 are on-line and allowed by the process's current cpuset context,
290 or none of the specified nodes contain memory.
291 Or, the
292 .I mode
293 argument specified both
294 .B MPOL_F_STATIC_NODES
295 and
296 .BR MPOL_F_RELATIVE_NODES .
297 .TP
298 .B ENOMEM
299 Insufficient kernel memory was available.
300 .SH VERSIONS
301 The
302 .BR set_mempolicy ()
303 system call was added to the Linux kernel in version 2.6.7.
304 .SH CONFORMING TO
305 This system call is Linux-specific.
306 .SH NOTES
307 Memory policy is not remembered if the page is swapped out.
308 When such a page is paged back in, it will use the policy of
309 the thread or memory range that is in effect at the time the
310 page is allocated.
311 .PP
312 For information on library support, see
313 .BR numa (7).
314 .SH SEE ALSO
315 .BR get_mempolicy (2),
316 .BR getcpu (2),
317 .BR mbind (2),
318 .BR mmap (2),
319 .BR numa (3),
320 .BR cpuset (7),
321 .BR numa (7),
322 .BR numactl (8)