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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 filesystems re-
	      turn 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	behav-
	      ior, 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 re-
	      ceiving a	SIGBUS and the page being unmapped.  This  feature  is
	      intended for testing of memory error-handling code; it is	avail-
	      able only	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 er-
	      ror-handling  code;  it is available only	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
	      merges only 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
	      Linux kernel source file Documentation/vm/ksm.txt	for  more  de-
	      tails.   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  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 Linux
	      kernel source file Documentation/vm/transhuge.txt	for  more  de-
	      tails.   The  MADV_HUGEPAGE  and	MADV_NOHUGEPAGE	operations are
	      available	only if	the kernel was configured  with	 CONFIG_TRANS-
	      PARENT_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  ER-
       RORS 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),
       prctl(2), core(5)

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-04-20			    MADVISE(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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