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madvise - give advice about use of memory
int madvise(void *addr, size_t length, int advice);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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
No special treatment. This is the default.
Expect page references in random order. (Hence, read ahead may
be less useful than normally.)
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.)
Expect access in the near future. (Hence, it might be a good
idea to read some pages ahead.)
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
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.
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
only work 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 kernel source file
Documentation/vm/transhuge.txt for more details. The
MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are only available
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.
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)
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