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SET_MEMPOLICY(2)           Linux Programmer's Manual          SET_MEMPOLICY(2)



NAME
       set_mempolicy - set default NUMA memory policy for a process and its children

SYNOPSIS
       #include <numaif.h>

       int set_mempolicy(int mode, unsigned long *nodemask,
                         unsigned long maxnode);

       Link with -lnuma.

DESCRIPTION
       set_mempolicy() sets the NUMA memory policy of the calling process, which consists of a policy mode and zero or
       more nodes, to the values specified by the mode, nodemask and maxnode arguments.

       A NUMA machine has different memory controllers with different distances to specific CPUs.  The  memory  policy
       defines from which node memory is allocated for the process.

       This system call defines the default policy for the process.  The process policy governs allocation of pages in
       the process's address space outside of memory ranges controlled by a more specific policy set by mbind(2).  The
       process  default  policy also controls allocation of any pages for memory mapped files mapped using the mmap(2)
       call with the MAP_PRIVATE flag and that are only read [loaded] from by the process and of memory  mapped  files
       mapped  using  the  mmap(2)  call  with the MAP_SHARED flag, regardless of the access type.  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.

       The  mode  argument  must specify one of MPOL_DEFAULT, MPOL_BIND, MPOL_INTERLEAVE or MPOL_PREFERRED.  All modes
       except MPOL_DEFAULT require the caller to specify via the nodemask argument one or more nodes.

       The mode argument may also include an optional mode flag.  The supported mode flags are:

       MPOL_F_STATIC_NODES (since Linux 2.6.26)
              A non-empty nodemask specifies physical node ids.  Linux does will not remap the nodemask when the  pro-
              cess  moves  to  a  different cpuset context, nor when the set of nodes allowed by the process's current
              cpuset context changes.

       MPOL_F_RELATIVE_NODES (since Linux 2.6.26)
              A non-empty nodemask specifies node ids that are relative to the set of node ids allowed   by  the  pro-
              cess's current cpuset.

       nodemask  points  to  a bit mask of node IDs that contains up to maxnode bits.  The bit mask size is rounded to
       the next multiple of sizeof(unsigned long), but the kernel will only use bits up to maxnode.  A NULL  value  of
       nodemask  or  a  maxnode  value of zero specifies the empty set of nodes.  If the value of maxnode is zero, the
       nodemask argument is ignored.

       Where a nodemask is required, it must contain at least one node that is on-line, allowed by the process's  cur-
       rent  cpuset  context,  [unless  the  MPOL_F_STATIC_NODES mode flag is specified], and contains memory.  If the
       MPOL_F_STATIC_NODES is set in mode and a required nodemask contains no nodes that are allowed by the  process's
       current  cpuset  context, the memory policy reverts to local allocation.  This effectively overrides the speci-
       fied policy until the process's cpuset context includes one or more of the nodes specified by nodemask.

       The MPOL_DEFAULT mode specifies that any non-default process memory policy be removed, so that the memory  pol-
       icy  "falls  back" to the system default policy.  The system default policy is "local allocation"-- i.e., allo-
       cate memory on the node of the CPU that triggered the allocation.  nodemask must be specified as NULL.  If  the
       "local node" contains no free memory, the system will attempt to allocate memory from a "near by" node.

       The MPOL_BIND mode defines a strict policy that restricts memory allocation to the nodes specified in nodemask.
       If nodemask specifies more than one node, page allocations will come from the node with the lowest numeric node
       ID  first,  until  that  node  contains no free memory.  Allocations will then come from the node with the next
       highest node ID specified in nodemask and so forth, until none of the  specified  nodes  contain  free  memory.
       Pages will not be allocated from any node not specified in the nodemask.

       MPOL_INTERLEAVE  interleaves  page allocations across the nodes specified in nodemask in numeric node ID order.
       This optimizes for bandwidth instead of latency by spreading out pages  and  memory  accesses  to  those  pages
       across  multiple  nodes.  However, accesses to a single page will still be limited to the memory bandwidth of a
       single node.

       MPOL_PREFERRED sets the preferred node for allocation.  The kernel will try to allocate pages  from  this  node
       first and fall back to "near by" nodes if the preferred node is low on free memory.  If nodemask specifies more
       than one node ID, the first node in the mask will be selected as the preferred node.  If the nodemask and maxn-
       ode arguments specify the empty set, then the policy specifies "local allocation" (like the system default pol-
       icy discussed above).

       The process memory policy is preserved across an execve(2), and is inherited by child processes  created  using
       fork(2) or clone(2).

RETURN VALUE
       On success, set_mempolicy() returns 0; on error, -1 is returned and errno is set to indicate the error.

ERRORS
       EFAULT Part of all of the memory range specified by nodemask and maxnode points outside your accessible address
              space.

       EINVAL mode is invalid.  Or, mode  is  MPOL_DEFAULT  and  nodemask  is  non-empty,  or  mode  is  MPOL_BIND  or
              MPOL_INTERLEAVE and nodemask is empty.  Or, maxnode specifies more than a page worth of bits.  Or, node-
              mask specifies one or more node IDs that are greater than the maximum supported node ID.   Or,  none  of
              the  node  IDs specified by nodemask are on-line and allowed by the process's current cpuset context, or
              none of the specified nodes contain memory.  Or, the mode argument  specified  both  MPOL_F_STATIC_NODES
              and MPOL_F_RELATIVE_NODES.

       ENOMEM Insufficient kernel memory was available.

VERSIONS
       The set_mempolicy(), system call was added to the Linux kernel in version 2.6.7.

CONFORMING TO
       This system call is Linux-specific.

NOTES
       Process  policy  is  not remembered if the page is swapped out.  When such a page is paged back in, it will use
       the policy of the process or memory range that is in effect at the time the page is allocated.

       For information on library support, see numa(7).

SEE ALSO
       get_mempolicy(2), getcpu(2), mbind(2), mmap(2), numa(3), cpuset(7), numa(7), numactl(8)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-15                  SET_MEMPOLICY(2)