madvise(2) - Manual pages

NAME

madvise, posix_madvise, mcontrol — give advice about use of memory

SYNOPSIS

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

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

int
posix_madvise(void *addr, size_t len, int behav);

int
mcontrol(void *addr, size_t len, int behav, off_t value);

The madvise() system call allows a process that has knowledge of its memory behavior to describe it to the system. The posix_madvise() interface is identical and is provided for standards conformance. The mcontrol() system call is an extension of madvise() that takes an additional value argument (see the description of the MADV_SETMAP behavior below).

The known behaviors are:

MADV_NORMAL: Tells the system to revert to the default paging behavior.
MADV_RANDOM: Is a hint that pages will be accessed randomly, and prefetching is likely not advantageous.
MADV_SEQUENTIAL: Causes the VM system to depress the priority of pages immediately preceding a given page when it is faulted in.
MADV_WILLNEED: Causes pages that are in a given virtual address range to temporarily have higher priority, and if they are in memory, decrease the likelihood of them being freed. Additionally, the pages that are already in memory will be immediately mapped into the process, thereby eliminating unnecessary overhead of going through the entire process of faulting the pages in. This WILL NOT fault pages in from backing store, but quickly map the pages already in memory into the calling process.
MADV_DONTNEED: Allows the VM system to decrease the in-memory priority of pages in the specified range. Additionally future references to this address range will incur a page fault.
MADV_FREE: Gives the VM system the freedom to free pages, and tells the system that information in the specified page range is no longer important. This is an efficient way of allowing malloc(3) to free pages anywhere in the address space, while keeping the address space valid. The next time that the page is referenced, the page might be demand zeroed, or might contain the data that was there before the MADV_FREE call. References made to that address space range will not make the VM system page the information back in from backing store until the page is modified again.
MADV_NOSYNC: Request that the system not flush the data associated with this map to physical backing store unless it needs to. Typically this prevents the filesystem update daemon from gratuitously writing pages dirtied by the VM system to physical disk. Note that VM/filesystem coherency is always maintained, this feature simply ensures that the mapped data is only flush when it needs to be, usually by the system pager.
This feature is typically used when you want to use a file-backed shared memory area to communicate between processes (IPC) and do not particularly need the data being stored in that area to be physically written to disk. With this feature you get the equivalent performance with mmap that you would expect to get with SysV shared memory calls, but in a more controllable and less restrictive manner. However, note that this feature is not portable across UNIX platforms (though some may do the right thing by default). For more information see the MAP_NOSYNC section of mmap(2)
MADV_AUTOSYNC: Undoes the effects of MADV_NOSYNC for any future pages dirtied within the address range. The effect on pages already dirtied is indeterminate - they may or may not be reverted. You can guarantee reversion by using the msync(2) or fsync(2) system calls.
MADV_NOCORE: Region is not included in a core file.
MADV_CORE: Include region in a core file.
MADV_INVAL: Invalidate the hardware page table for a region of memory, forcing accesses to re-fault the pages. This command is primarily meant to be used in areas of memory governed by a virtual page table after modifications have been made to it.
MADV_SETMAP: Set the offset of the page directory page to value for the virtual page table governing the specified area of memory. The entire memory area under virtual page table management should be specified. You may encounter unexpected effects if you only set the page directory page for part of the mapping.

Portable programs that call the posix_madvise() interface should use the aliases POSIX_MADV_NORMAL, POSIX_MADV_SEQUENTIAL, POSIX_MADV_RANDOM, POSIX_MADV_WILLNEED, and POSIX_MADV_DONTNEED rather than the flags described above.

RETURN VALUES

The madvise(), posix_madvise(), and mcontrol() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.