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

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  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 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 only available for privileged (CAP_SYS_ADMIN) pro-
	      cesses.  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_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 only available 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 only merges
	      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 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.

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, 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 can-
       not 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)

COLOPHON

       This page is part of release 3.27 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/.

Linux								    2010-06-20								MADVISE(2)