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28 .\" @(#)madvise.2 8.1 (Berkeley) 6/9/93
29 .\" $FreeBSD: src/lib/libc/sys/madvise.2,v 1.17.2.8 2003/01/06 23:33:59 trhodes Exp $
30 .\" $DragonFly: src/lib/libc/sys/madvise.2,v 1.7 2008/10/06 21:01:37 swildner Exp $
39 .Nd give advice about use of memory
46 .Fn madvise "void *addr" "size_t len" "int behav"
48 .Fn posix_madvise "void *addr" "size_t len" "int behav"
50 .Fn mcontrol "void *addr" "size_t len" "int behav" "off_t value"
55 allows a process that has knowledge of its memory behavior
56 to describe it to the system.
59 interface is identical and is provided for standards conformance.
62 system call is an extension of
64 that takes an additional
66 argument (see the description of the
70 The known behaviors are:
71 .Bl -tag -width MADV_SEQUENTIAL
73 Tells the system to revert to the default paging
76 Is a hint that pages will be accessed randomly, and prefetching
77 is likely not advantageous.
78 .It Dv MADV_SEQUENTIAL
79 Causes the VM system to depress the priority of
80 pages immediately preceding a given page when it is faulted in.
82 Causes pages that are in a given virtual address range
83 to temporarily have higher priority, and if they are in
84 memory, decrease the likelihood of them being freed. Additionally,
85 the pages that are already in memory will be immediately mapped into
86 the process, thereby eliminating unnecessary overhead of going through
87 the entire process of faulting the pages in. This WILL NOT fault
88 pages in from backing store, but quickly map the pages already in memory
89 into the calling process.
91 Allows the VM system to decrease the in-memory priority
92 of pages in the specified range. Additionally future references to
93 this address range will incur a page fault.
95 Gives the VM system the freedom to free pages,
96 and tells the system that information in the specified page range
97 is no longer important. This is an efficient way of allowing
99 to free pages anywhere in the address space, while keeping the address space
100 valid. The next time that the page is referenced, the page might be demand
101 zeroed, or might contain the data that was there before the
104 References made to that address space range will not make the VM system
105 page the information back in from backing store until the page is
108 Request that the system not flush the data associated with this map to
109 physical backing store unless it needs to. Typically this prevents the
110 filesystem update daemon from gratuitously writing pages dirtied
111 by the VM system to physical disk. Note that VM/filesystem coherency is
112 always maintained, this feature simply ensures that the mapped data is
113 only flush when it needs to be, usually by the system pager.
115 This feature is typically used when you want to use a file-backed shared
116 memory area to communicate between processes (IPC) and do not particularly
117 need the data being stored in that area to be physically written to disk.
118 With this feature you get the equivalent performance with mmap that you
119 would expect to get with SysV shared memory calls, but in a more controllable
120 and less restrictive manner. However, note that this feature is not portable
123 platforms (though some may do the right thing by default).
124 For more information see the MAP_NOSYNC section of
127 Undoes the effects of MADV_NOSYNC for any future pages dirtied within the
128 address range. The effect on pages already dirtied is indeterminate - they
129 may or may not be reverted. You can guarantee reversion by using the
135 Region is not included in a core file.
137 Include region in a core file.
139 Invalidate the hardware page table for a region of memory, forcing
140 accesses to re-fault the pages.
141 This command is primarily meant to be used in areas of memory
142 governed by a virtual page table after modifications have been made
145 Set the offset of the page directory page to
147 for the virtual page table governing
148 the specified area of memory. The entire memory area under virtual page table
149 management should be specified. You may encounter unexpected effects
150 if you only set the page directory page for part of the mapping.
153 Portable programs that call the
155 interface should use the aliases
156 .Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL ,
157 .Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED ,
159 .Dv POSIX_MADV_DONTNEED
160 rather than the flags described above.
162 .Rv -std madvise posix_madvise mcontrol
169 functions will fail if:
174 argument is not valid or the virtual address range specified by the
178 arguments is not valid.
188 interface conforms to
193 function first appeared in
197 function was added in