man2/get_mempolicy.2

   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 GET_MEMPOLICY 2 2017-09-15 Linux "Linux Programmer's Manual"
  28 .SH NAME
  29 get_mempolicy \- retrieve NUMA memory policy for a thread
  30 .SH SYNOPSIS
  31 .B "#include <numaif.h>"
  32 .nf
  33 .PP
  34 .BI "long get_mempolicy(int *" mode ", unsigned long *" nodemask ,
  35 .BI "                  unsigned long " maxnode ", void *" addr ,
  36 .BI "                  unsigned long " flags );
  37 .PP
  38 Link with \fI\-lnuma\fP.
  39 .fi
  40 .SH DESCRIPTION
  41 .BR get_mempolicy ()
  42 retrieves the NUMA policy of the calling thread or of a memory address,
  43 depending on the setting of
  44 .IR flags .
  45 .PP
  46 A NUMA machine has different
  47 memory controllers with different distances to specific CPUs.
  48 The memory policy defines from which node memory is allocated for
  49 the thread.
  50 .PP
  51 If
  52 .I flags
  53 is specified as 0,
  54 then information about the calling thread's default policy
  55 (as set by
  56 .BR set_mempolicy (2))
  57 is returned, in the buffers pointed to by
  58 .I mode
  59 and
  60 .IR nodemask .
  61 The value returned in these arguments
  62 may be used to restore the thread's policy to its state at
  63 the time of the call to
  64 .BR get_mempolicy ()
  65 using
  66 .BR set_mempolicy (2).
  67 When
  68 .I flags
  69 is 0,
  70 .I addr
  71 must be specified as NULL.
  72 .PP
  73 If
  74 .I flags
  75 specifies
  76 .BR MPOL_F_MEMS_ALLOWED
  77 (available since Linux 2.6.24), the
  78 .I mode
  79 argument is ignored and the set of nodes (memories) that the
  80 thread is allowed to specify in subsequent calls to
  81 .BR mbind (2)
  82 or
  83 .BR set_mempolicy (2)
  84 (in the absence of any
  85 .IR "mode flags" )
  86 is returned in
  87 .IR nodemask .
  88 It is not permitted to combine
  89 .B MPOL_F_MEMS_ALLOWED
  90 with either
  91 .B MPOL_F_ADDR
  92 or
  93 .BR MPOL_F_NODE .
  94 .PP
  95 If
  96 .I flags
  97 specifies
  98 .BR MPOL_F_ADDR ,
  99 then information is returned about the policy governing the memory
 100 address given in
 101 .IR addr .
 102 This policy may be different from the thread's default policy if
 103 .BR mbind (2)
 104 or one of the helper functions described in
 105 .BR numa (3)
 106 has been used to establish a policy for the memory range containing
 107 .IR addr .
 108 .PP
 109 If the
 110 .I mode
 111 argument is not NULL, then
 112 .BR get_mempolicy ()
 113 will store the policy mode and any optional
 114 .I "mode flags"
 115 of the requested NUMA policy in the location pointed to by this argument.
 116 If
 117 .I nodemask
 118 is not NULL, then the nodemask associated with the policy will be stored
 119 in the location pointed to by this argument.
 120 .I maxnode
 121 specifies the number of node IDs
 122 that can be stored into
 123 .IR nodemask \(emthat
 124 is, the maximum node ID plus one.
 125 The value specified by
 126 .I maxnode
 127 is always rounded to a multiple of
 128 .IR "sizeof(unsigned\ long)*8" .
 129 .PP
 130 If
 131 .I flags
 132 specifies both
 133 .B MPOL_F_NODE
 134 and
 135 .BR MPOL_F_ADDR ,
 136 .BR get_mempolicy ()
 137 will return the node ID of the node on which the address
 138 .I addr
 139 is allocated into the location pointed to by
 140 .IR mode .
 141 If no page has yet been allocated for the specified address,
 142 .BR get_mempolicy ()
 143 will allocate a page as if the thread had performed a read
 144 (load) access to that address, and return the ID of the node
 145 where that page was allocated.
 146 .PP
 147 If
 148 .I flags
 149 specifies
 150 .BR MPOL_F_NODE ,
 151 but not
 152 .BR MPOL_F_ADDR ,
 153 and the thread's current policy is
 154 .BR MPOL_INTERLEAVE ,
 155 then
 156 .BR get_mempolicy ()
 157 will return in the location pointed to by a non-NULL
 158 .I mode
 159 argument,
 160 the node ID of the next node that will be used for
 161 interleaving of internal kernel pages allocated on behalf of the thread.
 162 .\" Note:  code returns next interleave node via 'mode' argument -Lee Schermerhorn
 163 These allocations include pages for memory-mapped files in
 164 process memory ranges mapped using the
 165 .BR mmap (2)
 166 call with the
 167 .B MAP_PRIVATE
 168 flag for read accesses, and in memory ranges mapped with the
 169 .B MAP_SHARED
 170 flag for all accesses.
 171 .PP
 172 Other flag values are reserved.
 173 .PP
 174 For an overview of the possible policies see
 175 .BR set_mempolicy (2).
 176 .SH RETURN VALUE
 177 On success,
 178 .BR get_mempolicy ()
 179 returns 0;
 180 on error, \-1 is returned and
 181 .I errno
 182 is set to indicate the error.
 183 .SH ERRORS
 184 .TP
 185 .B EFAULT
 186 Part of all of the memory range specified by
 187 .I nodemask
 188 and
 189 .I maxnode
 190 points outside your accessible address space.
 191 .TP
 192 .B EINVAL
 193 The value specified by
 194 .I maxnode
 195 is less than the number of node IDs supported by the system.
 196 Or
 197 .I flags
 198 specified values other than
 199 .B MPOL_F_NODE
 200 or
 201 .BR MPOL_F_ADDR ;
 202 or
 203 .I flags
 204 specified
 205 .B MPOL_F_ADDR
 206 and
 207 .I addr
 208 is NULL,
 209 or
 210 .I flags
 211 did not specify
 212 .B MPOL_F_ADDR
 213 and
 214 .I addr
 215 is not NULL.
 216 Or,
 217 .I flags
 218 specified
 219 .B MPOL_F_NODE
 220 but not
 221 .B MPOL_F_ADDR
 222 and the current thread policy is not
 223 .BR MPOL_INTERLEAVE .
 224 Or,
 225 .I flags
 226 specified
 227 .B MPOL_F_MEMS_ALLOWED
 228 with either
 229 .B MPOL_F_ADDR
 230 or
 231 .BR MPOL_F_NODE .
 232 (And there are other
 233 .B EINVAL
 234 cases.)
 235 .SH VERSIONS
 236 The
 237 .BR get_mempolicy ()
 238 system call was added to the Linux kernel in version 2.6.7.
 239 .SH CONFORMING TO
 240 This system call is Linux-specific.
 241 .SH NOTES
 242 For information on library support, see
 243 .BR numa (7).
 244 .SH SEE ALSO
 245 .BR getcpu (2),
 246 .BR mbind (2),
 247 .BR mmap (2),
 248 .BR set_mempolicy (2),
 249 .BR numa (3),
 250 .BR numa (7),
 251 .BR numactl (8)