mirror of https://github.com/mkerrisk/man-pages
get_mempolicy.2, set_mempolicy: Policy is per thread, not per process
set/get_mempolicy manpages say that the memory allocation policy is per process while reading the code and testing shows that it's actually per thread. Here's a quick fix, which may need to be improved to better explain that we're allocating in the context of a thread within a process address space. Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Brice Goglin
Michael Kerrisk
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commit
856778162b
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@ -26,7 +26,7 @@
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.\"
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.TH GET_MEMPOLICY 2 2008-08-15 Linux "Linux Programmer's Manual"
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.SH NAME
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get_mempolicy \- retrieve NUMA memory policy for a process
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get_mempolicy \- retrieve NUMA memory policy for a thread
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.SH SYNOPSIS
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.B "#include <numaif.h>"
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.nf
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@ -39,19 +39,19 @@ Link with \fI\-lnuma\fP.
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.fi
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.SH DESCRIPTION
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.BR get_mempolicy ()
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retrieves the NUMA policy of the calling process or of a memory address,
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retrieves the NUMA policy of the calling thread or of a memory address,
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depending on the setting of
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.IR flags .
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A NUMA machine has different
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memory controllers with different distances to specific CPUs.
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The memory policy defines from which node memory is allocated for
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the process.
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the thread.
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If
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.I flags
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is specified as 0,
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then information about the calling process's default policy
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then information about the calling thread's default policy
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(as set by
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.BR set_mempolicy (2))
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is returned.
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@ -59,7 +59,7 @@ The policy returned
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.RI [ mode
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and
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.IR nodemask ]
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may be used to restore the process's policy to its state at
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may be used to restore the thread's policy to its state at
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the time of the call to
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.BR get_mempolicy ()
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using
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@ -72,7 +72,7 @@ specifies
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(available since Linux 2.6.24), the
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.I mode
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argument is ignored and the set of nodes [memories] that the
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process is allowed to specify in subsequent calls to
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thread is allowed to specify in subsequent calls to
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.BR mbind (2)
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or
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.BR set_mempolicy (2)
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@ -94,7 +94,7 @@ specifies
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then information is returned about the policy governing the memory
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address given in
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.IR addr .
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This policy may be different from the process's default policy if
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This policy may be different from the thread's default policy if
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.BR mbind (2)
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or one of the helper functions described in
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.BR numa (3)
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@ -135,7 +135,7 @@ is allocated into the location pointed to by
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.IR mode .
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If no page has yet been allocated for the specified address,
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.BR get_mempolicy ()
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will allocate a page as if the process had performed a read
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will allocate a page as if the thread had performed a read
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[load] access to that address, and return the ID of the node
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where that page was allocated.
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@ -145,7 +145,7 @@ specifies
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.BR MPOL_F_NODE ,
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but not
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.BR MPOL_F_ADDR ,
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and the process's current policy is
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and the thread's current policy is
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.BR MPOL_INTERLEAVE ,
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then
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.BR get_mempolicy ()
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@ -153,7 +153,7 @@ will return in the location pointed to by a non-NULL
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.I mode
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argument,
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the node ID of the next node that will be used for
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interleaving of internal kernel pages allocated on behalf of the process.
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interleaving of internal kernel pages allocated on behalf of the thread.
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.\" Note: code returns next interleave node via 'mode' argument -Lee Schermerhorn
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These allocations include pages for memory-mapped files in
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process memory ranges mapped using the
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@ -214,7 +214,7 @@ specified
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.B MPOL_F_NODE
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but not
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.B MPOL_F_ADDR
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and the current process policy is not
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and the current thread policy is not
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.BR MPOL_INTERLEAVE .
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Or,
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.I flags
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@ -26,7 +26,7 @@
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.\"
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.TH SET_MEMPOLICY 2 2014-05-28 Linux "Linux Programmer's Manual"
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.SH NAME
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set_mempolicy \- set default NUMA memory policy for a process and its children
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set_mempolicy \- set default NUMA memory policy for a thread and its children
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.SH SYNOPSIS
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.nf
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.B "#include <numaif.h>"
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@ -38,7 +38,7 @@ Link with \fI\-lnuma\fP.
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.fi
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.SH DESCRIPTION
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.BR set_mempolicy ()
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sets the NUMA memory policy of the calling process,
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sets the NUMA memory policy of the calling thread,
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which consists of a policy mode and zero or more nodes,
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to the values specified by the
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.IR mode ,
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@ -50,28 +50,28 @@ arguments.
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A NUMA machine has different
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memory controllers with different distances to specific CPUs.
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The memory policy defines from which node memory is allocated for
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the process.
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the thread.
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This system call defines the default policy for the process.
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The process policy governs allocation of pages in the process's
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This system call defines the default policy for the thread.
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The thread policy governs allocation of pages in the process's
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address space outside of memory ranges
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controlled by a more specific policy set by
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.BR mbind (2).
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The process default policy also controls allocation of any pages for
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The thread default policy also controls allocation of any pages for
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memory-mapped files mapped using the
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.BR mmap (2)
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call with the
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.B MAP_PRIVATE
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flag and that are only read [loaded] from by the process
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flag and that are only read [loaded] from by the thread
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and of memory-mapped files mapped using the
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.BR mmap (2)
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call with the
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.B MAP_SHARED
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flag, regardless of the access type.
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The policy is applied only when a new page is allocated
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for the process.
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for the thread.
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For anonymous memory this is when the page is first
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touched by the application.
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touched by the thread.
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The
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.I mode
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@ -154,7 +154,7 @@ cpuset context includes one or more of the nodes specified by
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The
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.B MPOL_DEFAULT
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mode specifies that any nondefault process memory policy be removed,
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mode specifies that any nondefault thread memory policy be removed,
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so that the memory policy "falls back" to the system default policy.
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The system default policy is "local allocation"\(emthat is,
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allocate memory on the node of the CPU that triggered the allocation.
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@ -211,9 +211,9 @@ arguments specify the empty set, then the policy
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specifies "local allocation"
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(like the system default policy discussed above).
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The process memory policy is preserved across an
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The thread memory policy is preserved across an
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.BR execve (2),
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and is inherited by child processes created using
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and is inherited by child threads created using
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.BR fork (2)
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or
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.BR clone (2).
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@ -279,9 +279,9 @@ system call was added to the Linux kernel in version 2.6.7.
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.SH CONFORMING TO
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This system call is Linux-specific.
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.SH NOTES
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Process policy is not remembered if the page is swapped out.
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Memory policy is not remembered if the page is swapped out.
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When such a page is paged back in, it will use the policy of
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the process or memory range that is in effect at the time the
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the thread or memory range that is in effect at the time the
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page is allocated.
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For information on library support, see
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