fbbd9655e5
Renumber cluase 4 to 3, per what everybody else did when BSD granted them permission to remove clause 3. My insistance on keeping the same numbering for legal reasons is too pedantic, so give up on that point. Submitted by: Jan Schaumann <jschauma@stevens.edu> Pull Request: https://github.com/freebsd/freebsd/pull/96
187 lines
6.3 KiB
Groff
187 lines
6.3 KiB
Groff
.\" Copyright (c) 1991, 1993
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)madvise.2 8.1 (Berkeley) 6/9/93
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd July 12, 2015
|
|
.Dt MADVISE 2
|
|
.Os
|
|
.Sh NAME
|
|
.Nm madvise , posix_madvise
|
|
.Nd give advice about use of memory
|
|
.Sh LIBRARY
|
|
.Lb libc
|
|
.Sh SYNOPSIS
|
|
.In sys/mman.h
|
|
.Ft int
|
|
.Fn madvise "void *addr" "size_t len" "int behav"
|
|
.Ft int
|
|
.Fn posix_madvise "void *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
|
|
.Fn posix_madvise
|
|
interface is identical, except it returns an error number on error and does
|
|
not modify
|
|
.Va errno ,
|
|
and is provided for standards conformance.
|
|
.Pp
|
|
The known behaviors are:
|
|
.Bl -tag -width MADV_SEQUENTIAL
|
|
.It Dv MADV_NORMAL
|
|
Tells the system to revert to the default paging
|
|
behavior.
|
|
.It Dv MADV_RANDOM
|
|
Is a hint that pages will be accessed randomly, and prefetching
|
|
is likely not advantageous.
|
|
.It Dv MADV_SEQUENTIAL
|
|
Causes the VM system to depress the priority of
|
|
pages immediately preceding a given page when it is faulted in.
|
|
.It Dv 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.
|
|
.It Dv MADV_DONTNEED
|
|
Allows the VM system to decrease the in-memory priority
|
|
of pages in the specified address range.
|
|
Consequently, future references to this address range are more likely
|
|
to incur a page fault.
|
|
.It Dv 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
|
|
.Xr 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
|
|
.Dv 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.
|
|
.It Dv 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
|
|
file system update daemon from gratuitously writing pages dirtied
|
|
by the VM system to physical disk.
|
|
Note that VM/file system 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.
|
|
.Pp
|
|
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
|
|
.Xr mmap 2
|
|
.It Dv 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
|
|
.Xr msync 2
|
|
or
|
|
.Xr fsync 2
|
|
system calls.
|
|
.It Dv MADV_NOCORE
|
|
Region is not included in a core file.
|
|
.It Dv MADV_CORE
|
|
Include region in a core file.
|
|
.It Dv MADV_PROTECT
|
|
Informs the VM system this process should not be killed when the
|
|
swap space is exhausted.
|
|
The process must have superuser privileges.
|
|
This should be used judiciously in processes that must remain running
|
|
for the system to properly function.
|
|
.El
|
|
.Pp
|
|
Portable programs that call the
|
|
.Fn posix_madvise
|
|
interface should use the aliases
|
|
.Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL ,
|
|
.Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED ,
|
|
and
|
|
.Dv POSIX_MADV_DONTNEED
|
|
rather than the flags described above.
|
|
.Sh RETURN VALUES
|
|
.Rv -std madvise
|
|
.Sh ERRORS
|
|
The
|
|
.Fn madvise
|
|
system call will fail if:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa behav
|
|
argument is not valid.
|
|
.It Bq Er ENOMEM
|
|
The virtual address range specified by the
|
|
.Fa addr
|
|
and
|
|
.Fa len
|
|
arguments is not valid.
|
|
.It Bq Er EPERM
|
|
.Dv MADV_PROTECT
|
|
was specified and the process does not have superuser privileges.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr mincore 2 ,
|
|
.Xr mprotect 2 ,
|
|
.Xr msync 2 ,
|
|
.Xr munmap 2 ,
|
|
.Xr posix_fadvise 2
|
|
.Sh STANDARDS
|
|
The
|
|
.Fn posix_madvise
|
|
interface conforms to
|
|
.St -p1003.1-2001 .
|
|
.Sh HISTORY
|
|
The
|
|
.Fn madvise
|
|
system call first appeared in
|
|
.Bx 4.4 .
|