1130b656e5
This will make a number of things easier in the future, as well as (finally!) avoiding the Id-smashing problem which has plagued developers for so long. Boy, I'm glad we're not using sup anymore. This update would have been insane otherwise.
128 lines
4.7 KiB
Groff
128 lines
4.7 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 advise 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 "caddr_t 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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.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 preceeding 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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.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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.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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