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