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

madvise — provide advice to VM system

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

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

The () 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 () 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 ; otherwise, it returns 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 specified.
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 .
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 system.
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.

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

February 17, 2023 OmniOS