madvise − provide advice to VM system
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.
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 */
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.
Tells 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.
Tells 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. This is used in the cat(1) and cp(1) utilities.
Tells 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.
Tells the kernel that the specified address range is no longer needed, so the system starts to free the resources associated with the address range.
Tells 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 contail 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.
The madvise() function should be used by programs 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 specified.
The addr argument is not a multiple of the page size as returned by sysconf(3C); the length of the specified address range is less than or equal to 0, or the advice 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.
See attributes(5) for descriptions of the following attributes: