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NAME

       madvise - give advice about use of memory

SYNOPSIS

       #include <sys/mman.h>

       int madvise(void *addr, size_t length, int advice);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       madvise(): _BSD_SOURCE

DESCRIPTION

       The madvise() system call advises the kernel about how to handle paging
       input/output in the address range beginning at address  addr  and  with
       size  length bytes.  It allows an application to tell the kernel how it
       expects to use some mapped or shared memory areas, so that  the  kernel
       can  choose  appropriate  read-ahead and caching techniques.  This call
       does not influence the semantics of the application (except in the case
       of  MADV_DONTNEED),  but  may influence its performance.  The kernel is
       free to ignore the advice.

       The advice is indicated in the advice argument which can be

       MADV_NORMAL
              No special treatment.  This is the default.

       MADV_RANDOM
              Expect page references in random order.  (Hence, read ahead  may
              be less useful than normally.)

       MADV_SEQUENTIAL
              Expect  page  references  in sequential order.  (Hence, pages in
              the given range can be aggressively read ahead, and may be freed
              soon after they are accessed.)

       MADV_WILLNEED
              Expect  access  in  the near future.  (Hence, it might be a good
              idea to read some pages ahead.)

       MADV_DONTNEED
              Do not expect access in the near future.  (For the  time  being,
              the  application is finished with the given range, so the kernel
              can free resources associated with it.)  Subsequent accesses  of
              pages  in  this  range  will  succeed, but will result either in
              reloading of the memory contents from the underlying mapped file
              (see  mmap(2)) or zero-fill-on-demand pages for mappings without
              an underlying file.

       MADV_REMOVE (since Linux 2.6.16)
              Free up a given range of pages and its associated backing store.
              Currently,  only  shmfs/tmpfs  supports  this; other filesystems
              return with the error ENOSYS.

       MADV_DONTFORK (since Linux 2.6.16)
              Do not make the pages in this range available to the child after
              a  fork(2).   This  is useful to prevent copy-on-write semantics
              from changing the physical location of a page(s) if  the  parent
              writes  to  it  after  a  fork(2).  (Such page relocations cause
              problems for hardware that DMAs into the page(s).)

       MADV_DOFORK (since Linux 2.6.16)
              Undo  the  effect  of  MADV_DONTFORK,  restoring   the   default
              behavior, whereby a mapping is inherited across fork(2).

       MADV_HWPOISON (since Linux 2.6.32)
              Poison  a  page and handle it like a hardware memory corruption.
              This operation is available only for privileged  (CAP_SYS_ADMIN)
              processes.   This  operation  may  result in the calling process
              receiving a SIGBUS and the page being unmapped.  This feature is
              intended  for  testing  of  memory  error-handling  code;  it is
              available   only   if   the   kernel   was    configured    with
              CONFIG_MEMORY_FAILURE.

       MADV_SOFT_OFFLINE (since Linux 2.6.33)
              Soft  offline  the  pages  in  the  range  specified by addr and
              length.  The memory of each  page  in  the  specified  range  is
              preserved  (i.e.,  when  next accessed, the same content will be
              visible, but in a new physical page  frame),  and  the  original
              page  is offlined (i.e., no longer used, and taken out of normal
              memory  management).   The  effect  of   the   MADV_SOFT_OFFLINE
              operation  is  invisible to (i.e., does not change the semantics
              of) the calling process.  This feature is intended  for  testing
              of  memory  error-handling  code;  it  is  available only if the
              kernel was configured with CONFIG_MEMORY_FAILURE.

       MADV_MERGEABLE (since Linux 2.6.32)
              Enable Kernel Samepage Merging (KSM) for the pages in the  range
              specified  by addr and length.  The kernel regularly scans those
              areas of user memory that have been marked as mergeable, looking
              for  pages  with  identical  content.   These  are replaced by a
              single write-protected page (which is automatically copied if  a
              process  later  wants  to  update the content of the page).  KSM
              merges only private anonymous  pages  (see  mmap(2)).   The  KSM
              feature   is   intended  for  applications  that  generate  many
              instances of the same data (e.g., virtualization systems such as
              KVM).   It can consume a lot of processing power; use with care.
              See the Linux kernel source  file  Documentation/vm/ksm.txt  for
              more   details.    The   MADV_MERGEABLE   and   MADV_UNMERGEABLE
              operations are available only if the kernel was configured  with
              CONFIG_KSM.

       MADV_UNMERGEABLE (since Linux 2.6.32)
              Undo  the  effect  of an earlier MADV_MERGEABLE operation on the
              specified address range; KSM  unmerges  whatever  pages  it  had
              merged in the address range specified by addr and length.

       MADV_HUGEPAGE (since Linux 2.6.38)
              Enables  Transparent  Huge  Pages  (THP)  for pages in the range
              specified by addr and length.  Currently, Transparent Huge Pages
              work  only  with  private  anonymous  pages  (see mmap(2)).  The
              kernel will  regularly  scan  the  areas  marked  as  huge  page
              candidates  to  replace  them  with huge pages.  The kernel will
              also allocate huge pages directly when the region  is  naturally
              aligned  to  the  huge  page size (see posix_memalign(2)).  This
              feature is  primarily  aimed  at  applications  that  use  large
              mappings  of  data  and access large regions of that memory at a
              time (e.g., virtualization systems such as QEMU).  It  can  very
              easily waste memory (e.g., a 2MB mapping that only ever accesses
              1 byte will result in 2MB of wired memory  instead  of  one  4KB
              page).      See     the     Linux     kernel     source     file
              Documentation/vm/transhuge.txt   for    more    details.     The
              MADV_HUGEPAGE  and MADV_NOHUGEPAGE operations are available only
              if the kernel was configured with CONFIG_TRANSPARENT_HUGEPAGE.

       MADV_NOHUGEPAGE (since Linux 2.6.38)
              Ensures that memory in the address range specified by  addr  and
              length will not be collapsed into huge pages.

       MADV_DONTDUMP (since Linux 3.4)
              Exclude  from  a core dump those pages in the range specified by
              addr and length.  This is useful in applications that have large
              areas  of memory that are known not to be useful in a core dump.
              The effect of MADV_DONTDUMP takes precedence over the  bit  mask
              that   is   set  via  the  /proc/PID/coredump_filter  file  (see
              core(5)).

       MADV_DODUMP (since Linux 3.4)
              Undo the effect of an earlier MADV_DONTDUMP.

RETURN VALUE

       On success madvise() returns zero.  On error, it returns -1  and  errno
       is set appropriately.

ERRORS

       EAGAIN A kernel resource was temporarily unavailable.

       EBADF  The map exists, but the area maps something that isn't a file.

       EINVAL This error can occur for the following reasons:

              *  The value len is negative.

              *  addr is not page-aligned.

              *  advice is not a valid value

              *  The  application  is  attempting  to release locked or shared
                 pages (with MADV_DONTNEED).

              *  MADV_MERGEABLE or MADV_UNMERGEABLE was specified  in  advice,
                 but the kernel was not configured with CONFIG_KSM.

       EIO    (for  MADV_WILLNEED)  Paging  in  this  area  would  exceed  the
              process's maximum resident set size.

       ENOMEM (for MADV_WILLNEED) Not enough memory: paging in failed.

       ENOMEM Addresses in the specified range are not  currently  mapped,  or
              are outside the address space of the process.

CONFORMING TO

       POSIX.1b.    POSIX.1-2001  describes  posix_madvise(3)  with  constants
       POSIX_MADV_NORMAL, POSIX_MADV_RANDOM, and so on, with a behavior  close
       to  that  described here.  There is a similar posix_fadvise(2) for file
       access.

       MADV_REMOVE, MADV_DONTFORK, MADV_DOFORK, MADV_HWPOISON, MADV_MERGEABLE,
       and MADV_UNMERGEABLE are Linux-specific.

NOTES

   Linux notes
       The current Linux implementation (2.4.0) views this system call more as
       a command than as advice and hence may return an error when  it  cannot
       do  what  it  usually  would  do  in response to this advice.  (See the
       ERRORS description above.)  This is nonstandard behavior.

       The Linux implementation  requires  that  the  address  addr  be  page-
       aligned,  and allows length to be zero.  If there are some parts of the
       specified address range that are  not  mapped,  the  Linux  version  of
       madvise()  ignores  them  and applies the call to the rest (but returns
       ENOMEM from the system call, as it should).

SEE ALSO

       getrlimit(2), mincore(2), mmap(2),  mprotect(2),  msync(2),  munmap(2),
       prctl(2), core(5)

COLOPHON

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



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