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MADVISE(2) Linux Programmer's Manual MADVISE(2)
madvise - give advice about use of memory
#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
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) 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_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.
On success madvise() returns zero. On error, it returns -1 and errno is set
appropriately.
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.
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.
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).
getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)
This page is part of release 3.32 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)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface