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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 $
38 .Nd give advice about use of memory
45 .Fn madvise "void *addr" "size_t len" "int behav"
47 .Fn posix_madvise "void *addr" "size_t len" "int behav"
49 .Fn mcontrol "void *addr" "size_t len" "int behav" "off_t value"
54 allows a process that has knowledge of its memory behavior
55 to describe it to the system.
58 interface is identical and is provided for standards conformance.
61 system call is an extension of
63 that takes an additional
65 argument (see the description of the
69 The known behaviors are:
70 .Bl -tag -width ".Dv MADV_SEQUENTIAL"
72 Tells the system to revert to the default paging
75 Is a hint that pages will be accessed randomly, and prefetching
76 is likely not advantageous.
77 .It Dv MADV_SEQUENTIAL
78 Causes the VM system to depress the priority of
79 pages immediately preceding a given page when it is faulted in.
81 Causes pages that are in a given virtual address range
82 to temporarily have higher priority, and if they are in
83 memory, decrease the likelihood of them being freed. Additionally,
84 the pages that are already in memory will be immediately mapped into
85 the process, thereby eliminating unnecessary overhead of going through
86 the entire process of faulting the pages in. This WILL NOT fault
87 pages in from backing store, but quickly map the pages already in memory
88 into the calling process.
90 Allows the VM system to decrease the in-memory priority
91 of pages in the specified range. Additionally future references to
92 this address range will incur a page fault.
94 Gives the VM system the freedom to free pages,
95 and tells the system that information in the specified page range
96 is no longer important. This is an efficient way of allowing
98 to free pages anywhere in the address space, while keeping the address space
99 valid. The next time that the page is referenced, the page might be demand
100 zeroed, or might contain the data that was there before the
103 References made to that address space range will not make the VM system
104 page the information back in from backing store until the page is
107 Request that the system not flush the data associated with this map to
108 physical backing store unless it needs to. Typically this prevents the
109 filesystem update daemon from gratuitously writing pages dirtied
110 by the VM system to physical disk. Note that VM/filesystem coherency is
111 always maintained, this feature simply ensures that the mapped data is
112 only flush when it needs to be, usually by the system pager.
114 This feature is typically used when you want to use a file-backed shared
115 memory area to communicate between processes (IPC) and do not particularly
116 need the data being stored in that area to be physically written to disk.
117 With this feature you get the equivalent performance with mmap that you
118 would expect to get with SysV shared memory calls, but in a more controllable
119 and less restrictive manner. However, note that this feature is not portable
122 platforms (though some may do the right thing by default).
123 For more information see the
128 Undoes the effects of
130 for any future pages dirtied within the
131 address range. The effect on pages already dirtied is indeterminate - they
132 may or may not be reverted. You can guarantee reversion by using the
138 Region is not included in a core file.
140 Include region in a core file.
142 Invalidate the hardware page table for a region of memory, forcing
143 accesses to re-fault the pages.
144 This command is primarily meant to be used in areas of memory
145 governed by a virtual page table after modifications have been made
148 Set the offset of the page directory page to
150 for the virtual page table governing
151 the specified area of memory. The entire memory area under virtual page table
152 management should be specified. You may encounter unexpected effects
153 if you only set the page directory page for part of the mapping.
156 Portable programs that call the
158 interface should use the aliases
159 .Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL ,
160 .Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED ,
162 .Dv POSIX_MADV_DONTNEED
163 rather than the flags described above.
165 .Rv -std madvise posix_madvise mcontrol
172 functions will fail if:
177 argument is not valid or the virtual address range specified by the
181 arguments is not valid.
192 interface conforms to
197 function first appeared in
201 function was added in