illumos: manual page: madvise.3c

MADVISE(3C) Standard C Library Functions MADVISE(3C)

NAME

madvise - provide advice to VM system

SYNOPSIS

#include <sys/types.h>
#include <sys/mman.h>

int
madvise(void *addr, size_t len, int advice);

DESCRIPTION

The madvise() function advises the kernel that a region of user mapped
memory in the range [addr, addr + len) will be accessed following a type of
pattern. The kernel uses this information to optimize the procedure for
manipulating and maintaining the resources associated with the specified
mapping range. In general (and true to the name of the function), the
advice is merely advisory, and the only user-visible ramifications are in
terms of performance, not semantics. Note that MADV_PURGE is an exception
to this; see below for details.

Values for advice are defined in <sys/mman.h> as:

#define MADV_NORMAL 0x0 /* No further special treatment */
#define MADV_RANDOM 0x1 /* Expect random page references */
#define MADV_SEQUENTIAL 0x2 /* Expect sequential page references */
#define MADV_WILLNEED 0x3 /* Will need these pages */
#define MADV_DONTNEED 0x4 /* Don't need these pages */
#define MADV_FREE 0x5 /* Contents can be freed */
#define MADV_ACCESS_DEFAULT 0x6 /* default access */
#define MADV_ACCESS_LWP 0x7 /* next LWP to access heavily */
#define MADV_ACCESS_MANY 0x8 /* many processes to access heavily */
#define MADV_PURGE 0x9 /* contents will be purged */

MADV_NORMAL
This is the default system characteristic where accessing memory
within the address range causes the system to read data from the
mapped file. The kernel reads all data from files into pages which
are retained for a period of time as a "cache". System pages can
be a scarce resource, so the kernel steals pages from other
mappings when needed. This is a likely occurrence, but adversely
affects system performance only if a large amount of memory is
accessed.

MADV_RANDOM
Tell the kernel to read in a minimum amount of data from a mapped
file on any single particular access. If MADV_NORMAL is in effect
when an address of a mapped file is accessed, the system tries to
read in as much data from the file as reasonable, in anticipation
of other accesses within a certain locality.

MADV_SEQUENTIAL
Tell the system that addresses in this range are likely to be
accessed only once, so the system will free the resources mapping
the address range as quickly as possible.

MADV_WILLNEED
Tell the system that a certain address range is definitely needed
so the kernel will start reading the specified range into memory.
This can benefit programs wanting to minimize the time needed to
access memory the first time, as the kernel would need to read in
from the file.

MADV_DONTNEED
Tell the kernel that the specified address range is no longer
needed, so the system starts to free the resources associated with
the address range. While the semantics of MADV_DONTNEED are
similar to other systems, they differ significantly from the
semantics on Linux, where MADV_DONTNEED will actually synchronously
purge the address range, and subsequent faults will load from
either backing store or be zero-filled on demand. If the peculiar
Linux semantics are desired, MADV_PURGE should be used in lieu of
MADV_DONTNEED.

MADV_FREE
Tell the kernel that contents in the specified address range are no
longer important and the range will be overwritten. When there is
demand for memory, the system will free pages associated with the
specified address range. In this instance, the next time a page in
the address range is referenced, it will contain all zeroes.
Otherwise, it will contain the data that was there prior to the
MADV_FREE call. References made to the address range will not make
the system read from backing store (swap space) until the page is
modified again.

This value cannot be used on mappings that have underlying file
objects.

MADV_PURGE
Tell the kernel to purge the specified address range. The mapping
will be retained, but the pages themselves will be destroyed;
subsequent faults on the range will result in the page being read
from backing store (if file-backed) or being zero-filled on demand
(if anonymous). Note that these semantics are generally inferior
to MADV_FREE, which gives the system more flexibility and results
in better performance when pages are, in fact, reused by the
caller. Indeed, MADV_PURGE only exists to provide an equivalent to
the unfortunate MADV_DONTNEED semantics found in Linux, upon which
some programs have (regrettably) come to depend. In de novo
applications, MADV_PURGE should be avoided; MADV_FREE should always
be preferred.

MADV_ACCESS_LWP
Tell the kernel that the next LWP to touch the specified address
range will access it most heavily, so the kernel should try to
allocate the memory and other resources for this range and the LWP
accordingly.

MADV_ACCESS_MANY
Tell the kernel that many processes and/or LWPs will access the
specified address range randomly across the machine, so the kernel
should try to allocate the memory and other resources for this
range accordingly.

MADV_ACCESS_DEFAULT
Reset the kernel's expectation for how the specified range will be
accessed to the default.

The madvise() function should be used by applications with specific
knowledge of their access patterns over a memory object, such as a mapped
file, to increase system performance.

RETURN VALUES

Upon successful completion, madvise() returns 0; otherwise, it returns -1
and sets errno to indicate the error.

ERRORS

EAGAIN Some or all mappings in the address range [addr, addr +
len) are locked for I/O.

EBUSY Some or all of the addresses in the range [addr, addr +
len) are locked and MS_SYNC with the MS_INVALIDATE
option is specified.

EFAULT Some or all of the addresses in the specified range
could not be read into memory from the underlying object
when performing MADV_WILLNEED. The madvise() function
could return prior to this condition being detected, in
which case errno will not be set to EFAULT.

EINVAL The addr argument is not a multiple of the page size as
returned by sysconf(3C), the length of the specified
address range is equal to 0, or the advice argument was
invalid.

EIO An I/O error occurred while reading from or writing to
the file system.

ENOMEM Addresses in the range [addr, addr + len) are outside
the valid range for the address space of a process, or
specify one or more pages that are not mapped.

ESTALE Stale NFS file handle.

INTERFACE STABILITY