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madvise(2) - phpMan
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 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 fea‐
ture 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 con‐
sume 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 candi‐
dates 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., virtual‐
ization systems such as QEMU). It can very easily waste memory (e.g., a 2MB map‐
ping 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 appropri‐
ately.
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_DONT‐
NEED).
* 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_UNMERGE‐
ABLE 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 behav‐
ior.
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 3.74 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)
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