madvise


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 file systems
              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
              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 only
              available if the kernel was configured with  CONFIG_MEMORY_FAIL-
              URE.

       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  pre-
              served (i.e., when next accessed, the same content will be visi-
              ble, 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 call-
              ing process.  This feature is intended  for  testing  of  memory
              error-handling code; it is only available if the kernel was con-
              figured 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 sin-
              gle write-protected page (which is  automatically  copied  if  a
              process  later  wants  to  update the content of the page).  KSM
              only merges private anonymous pages (see mmap(2)).  The KSM fea-
              ture  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
              kernel source file Documentation/vm/ksm.txt  for  more  details.
              The  MADV_MERGEABLE  and  MADV_UNMERGEABLE  operations  are only
              available 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
              only work with private anonymous pages (see mmap(2)).  The  ker-
              nel 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 primar-
              ily  aimed  at  applications that use large mappings of data and
              access large regions of that memory at a time (e.g.  virtualiza-
              tion  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 kernel
              source file  Documentation/vm/transhuge.txt  for  more  details.
              The MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are only avail-
              able  if  the  kernel  was  configured   with   CONFIG_TRANSPAR-

       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, etc., 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 mad-
       vise() 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)

COLOPHON
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