131041fa2a
MADV_DONTNEED, while pages faults on the affected address range are more likely to occur, they are not guaranteed to occur. MFC after: 3 days
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.
|
|
.\" 4. 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 .
|