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


madvise - provide advice to VM system


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

int madvise(caddr_t addr, size_t len, int advice);


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 */

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.


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


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

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

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.

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.

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

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.

Stale NFS file handle.


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Stable |
|MT-Level | MT-Safe |


meminfo(2), mmap(2), sysconf(3C), attributes(7)

March 28, 2016 MADVISE(3C)