4f79d873c1
madvise().
This feature prevents the update daemon from gratuitously flushing
dirty pages associated with a mapped file-backed region of memory. The
system pager will still page the memory as necessary and the VM system
will still be fully coherent with the filesystem. Modifications made
by other means to the same area of memory, for example by write(), are
unaffected. The feature works on a page-granularity basis.
MAP_NOSYNC allows one to use mmap() to share memory between processes
without incuring any significant filesystem overhead, putting it in
the same performance category as SysV Shared memory and anonymous memory.
Reviewed by: julian, alc, dg
155 lines
6.1 KiB
Groff
155 lines
6.1 KiB
Groff
.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)madvise.2 8.1 (Berkeley) 6/9/93
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.\" $FreeBSD$
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.\"
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.Dd Jul 19, 1996
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.Dt MADVISE 2
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.Os
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.Sh NAME
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.Nm madvise
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.Nd give advice about use of memory
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.Sh SYNOPSIS
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.Fd #include <sys/types.h>
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.Fd #include <sys/mman.h>
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.Ft int
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.Fn madvise "void *addr" "size_t len" "int behav"
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.Sh DESCRIPTION
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The
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.Fn madvise
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system call
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allows a process that has knowledge of its memory behavior
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to describe it to the system.
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The known behaviors are given in
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.Aq Pa sys/mman.h :
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.Bd -literal
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#define MADV_NORMAL 0 /* no further special treatment */
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#define MADV_RANDOM 1 /* expect random page references */
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#define MADV_SEQUENTIAL 2 /* expect sequential references */
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#define MADV_WILLNEED 3 /* will need these pages */
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#define MADV_DONTNEED 4 /* don't need these pages */
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#define MADV_FREE 5 /* data is now unimportant */
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#define MADV_NOSYNC 6 /* no explicit commit to physical backing store */
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#define MADV_AUTOSYNC 7 /* default commit method to physical backing store */
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.Ed
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.Pp
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.Bl -tag -width MADV_SEQUENTIAL
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.It Dv MADV_NORMAL
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Tells the system to revert to the default paging
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behavior.
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.It Dv MADV_RANDOM
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Is a hint that pages will be accessed randomly, and prefetching
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is likely not advantageous.
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.It Dv MADV_SEQUENTIAL
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Causes the VM system to depress the priority of
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pages immediately preceding a given page when it is faulted in.
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.It Dv MADV_WILLNEED
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Causes pages that are in a given virtual address range
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to temporarily have higher priority, and if they are in
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memory, decrease the likelihood of them being freed. Additionally,
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the pages that are already in memory will be immediately mapped into
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the process, thereby eliminating unnecessary overhead of going through
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the entire process of faulting the pages in. This WILL NOT fault
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pages in from backing store, but quickly map the pages already in memory
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into the calling process.
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.It Dv MADV_DONTNEED
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Allows the VM system to decrease the in-memory priority
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of pages in the specified range. Additionally future references to
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this address range will incur a page fault.
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.It Dv MADV_FREE
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Gives the VM system the freedom to free pages,
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and tells the system that information in the specified page range
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is no longer important. This is an efficient way of allowing
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.Xr malloc 3
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to free pages anywhere in the address space, while keeping the address space
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valid. The next time that the page is referenced, the page might be demand
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zeroed, or might contain the data that was there before the
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.Dv MADV_FREE
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call.
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References made to that address space range will not make the VM system
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page the information back in from backing store until the page is
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modified again.
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.It Dv MADV_NOSYNC
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Request that the system not flush the data associated with this map to
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physical backing store unless it needs to. Typically this prevents the
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filesystem update daemon from gratuitously writing pages dirtied
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by the VM system to physical disk. Note that VM/filesystem coherency is
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always maintained, this feature simply ensures that the mapped data is
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only flush when it needs to be, usually by the system pager.
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.Pp
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This feature is typically used when you want to use a file-backed shared
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memory area to communicate between processes (IPC) and do not particularly
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need the data being stored in that area to be physically written to disk.
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With this feature you get the equivalent performance with mmap that you
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would expect to get with SysV shared memory calls, but in a more controllable
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and less restrictive manner. However, note that this feature is not portable
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across UNIX platforms (though some may do the right thing by default).
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For more information see the MAP_NOSYNC section of
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.Xr mmap 2
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.It Dv MADV_AUTOSYNC
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Undoes the effects of MADV_NOSYNC for any future pages dirtied within the
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address range. The effect on pages already dirtied is indeterminate - they
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may or may not be reverted. You can guarentee reversion by using the
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.Xr msync 2
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or
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.Xr fsync 2
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system calls.
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.El
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.Sh RETURN VALUES
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Upon successful completion,
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.Fn madvise
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returns 0. Otherwise a value of -1 is returned and
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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The
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.Fn madvise
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function will fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The virtual address range specified by the
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.Fa addr
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and
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.Fa len
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arguments is not valid.
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.El
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.Sh SEE ALSO
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.Xr mincore 2 ,
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.Xr mprotect 2 ,
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.Xr msync 2 ,
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.Xr munmap 2 .
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.Sh HISTORY
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The
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.Fn madvise
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function first appeared in
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.Bx 4.4 .
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