madvise - give advice about use of memory
int madvise(void *start, size_t length, int advice );
The madvise system call advises the kernel about how to handle paging
input/output in the address range beginning at address start 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, but may influence
its performance. The kernel is free to ignore the advice.
The advice is indicated in the advice parameter which can be
MADV_NORMAL [Toc] [Back]
No special treatment. This is the default.
MADV_RANDOM [Toc] [Back]
Expect page references in random order. (Hence, read ahead may
be less useful than normally.)
MADV_SEQUENTIAL [Toc] [Back]
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 [Toc] [Back]
Expect access in the near future. (Hence, it might be a good
idea to read some pages ahead.)
MADV_DONTNEED [Toc] [Back]
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.)
On success madvise returns zero. On error, it returns -1 and errno is
EINVAL the value len is negative, start is not page-aligned, advice is
not a valid value, or the application is attempting to release
locked or shared pages (with MADV_DONTNEED).
ENOMEM addresses in the specified range are not currently mapped, or
are outside the address space of the process.
ENOMEM (for MADV_WILLNEED) Not enough memory - paging in failed.
EIO (for MADV_WILLNEED) Paging in this area would exceed the
process's maximum resident set size.
EBADF the map exists, but the area maps something that isn't a file.
EAGAIN a kernel resource was temporarily unavailable.
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 behaviour.
The Linux implementation requires that the address start be pagealigned,
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).
The madvise function first appeared in 4.4BSD.
POSIX.1b (POSIX.4). The Austin draft describes posix_madvise with constants
POSIX_MADV_NORMAL, etc., with a behaviour close to that
described here. There is a similar posix_fadvise for file access.
getrlimit(2), mmap(2), mincore(2), mprotect(2), msync(2), munmap(2)
Linux 2.4.5 2001-06-10 MADVISE(2)
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