DragonFly On-Line Manual Pages
MADVISE(2) DragonFly System Calls Manual MADVISE(2)
madvise, posix_madvise, mcontrol -- give advice about use of memory
Standard C Library (libc, -lc)
madvise(void *addr, size_t len, int behav);
posix_madvise(void *addr, size_t len, int behav);
mcontrol(void *addr, size_t len, int behav, off_t value);
The madvise() system call allows a process that has knowledge of its mem-
ory 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 behav-
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
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 elimi-
nating 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
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
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 dae-
mon 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 perfor-
mance 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 mem-
ory, forcing accesses to re-fault the pages. This com-
mand 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 unex-
pected 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
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.
The madvise(), posix_madvise(), and mcontrol() functions will fail if:
[EINVAL] The behav argument is not valid or the virtual address
range specified by the addr and len arguments is not
mincore(2), mprotect(2), msync(2), munmap(2)
The posix_madvise() interface conforms to IEEE Std 1003.1-2001
The madvise() function first appeared in 4.4BSD. The mcontrol() function
was added in DragonFly 1.7.
DragonFly 4.9 October 6, 2008 DragonFly 4.9