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
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
The known behaviors are:
MADV_NORMAL Tells the system to revert to the default paging
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
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
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), posix_fadvise(3)
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 5.5-DEVELOPMENT May 4, 2019 DragonFly 5.5-DEVELOPMENT