freebsd-dev/lib/libc/sys/madvise.2
1996-07-20 04:37:26 +00:00

103 lines
4.3 KiB
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.\" @(#)madvise.2 8.1 (Berkeley) 6/9/93
.\"
.Dd Jul 19, 1996
.Dt MADVISE 2
.Os
.Sh NAME
.Nm madvise
.Nd give advise about use of memory
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/mman.h>
.Fn madvise "caddr_t addr" "size_t len" "int behav"
.Sh DESCRIPTION
The
.Fn madvise
system call
allows a process that has knowledge of its memory behavior
to describe it to the system.
The known behaviors are given in
.Pa <sys/mman.h> :
.Bd -literal
#define MADV_NORMAL 0 /* no further special treatment */
#define MADV_RANDOM 1 /* expect random page references */
#define MADV_SEQUENTIAL 2 /* expect sequential references */
#define MADV_WILLNEED 3 /* will need these pages */
#define MADV_DONTNEED 4 /* don't need these pages */
#define MADV_FREE 5 /* data is now unimportant */
.Ed
.sp
MADV_NORMAL tells the system to revert to the default paging
behavior.
.sp
MADV_RANDOM is a hint that pages will be accessed randomly, and prefetching
is likely not advantageous.
.sp
MADV_SEQUENTIAL causes the VM system to depress the priority of
pages immediately preceeding a given page when it is faulted in.
.sp
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.
.sp
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.
.sp
MADV_FREE gives the VM system the freedom to free pages,
and 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.
.Sh SEE ALSO
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr mprotect 2 ,
.Xr mincore 2 .
.Sh HISTORY
The
.Nm madvise
function first appeared in 4.4BSD.