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