183 lines
6.2 KiB
Groff
183 lines
6.2 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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.\" 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 July 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 , posix_madvise
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.Nd give advice about use of memory
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In 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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.Ft int
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.Fn posix_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
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.Fn posix_madvise
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interface is identical and is provided for standards conformance.
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.Pp
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The known behaviors are:
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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.
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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.
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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.
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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.
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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.
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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.
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Typically this prevents the
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file system update daemon from gratuitously writing pages dirtied
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by the VM system to physical disk.
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Note that VM/file system 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.
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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.
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The effect on pages already dirtied is indeterminate - they
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may or may not be reverted.
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You can guarantee 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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.It Dv MADV_NOCORE
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Region is not included in a core file.
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.It Dv MADV_CORE
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Include region in a core file.
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.It Dv MADV_PROTECT
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Informs the VM system this process should not be killed when the
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swap space is exhausted.
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The process must have superuser privileges.
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This should be used judiciously in processes that must remain running
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for the system to properly function.
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.El
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.Pp
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Portable programs that call the
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.Fn posix_madvise
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interface should use the aliases
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.Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL ,
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.Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED ,
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and
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.Dv POSIX_MADV_DONTNEED
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rather than the flags described above.
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.Sh RETURN VALUES
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.Rv -std madvise
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.Sh ERRORS
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The
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.Fn madvise
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system call will fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The
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.Fa behav
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argument is not valid.
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.It Bq Er ENOMEM
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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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.It Bq Er EPERM
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.Dv MADV_PROTECT
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was specified and the process does not have superuser privileges.
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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 STANDARDS
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The
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.Fn posix_madvise
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interface conforms to
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.St -p1003.1-2001 .
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.Sh HISTORY
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The
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.Fn madvise
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system call first appeared in
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.Bx 4.4 .
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