MADVISE
Section: Linux Programmer's Manual (2)
Updated: 2014-04-20
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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 feature 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 consume 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 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 primarily aimed at applications that use large mappings of
data and access large regions of that memory at a time (e.g., virtualization
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 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 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 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),
prctl(2),
core(5)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- Linux notes
-
- SEE ALSO
-
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Time: 02:54:49 GMT, September 18, 2014