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28 .\" @(#)madvise.2 8.1 (Berkeley) 6/9/93
35 .Nm madvise , posix_madvise
36 .Nd give advice about use of memory
42 .Fn madvise "void *addr" "size_t len" "int behav"
44 .Fn posix_madvise "void *addr" "size_t len" "int behav"
49 allows a process that has knowledge of its memory behavior
50 to describe it to the system.
53 interface is identical and is provided for standards conformance.
55 The known behaviors are:
56 .Bl -tag -width MADV_SEQUENTIAL
58 Tells the system to revert to the default paging
61 Is a hint that pages will be accessed randomly, and prefetching
62 is likely not advantageous.
63 .It Dv MADV_SEQUENTIAL
64 Causes the VM system to depress the priority of
65 pages immediately preceding a given page when it is faulted in.
67 Causes pages that are in a given virtual address range
68 to temporarily have higher priority, and if they are in
69 memory, decrease the likelihood of them being freed.
71 the pages that are already in memory will be immediately mapped into
72 the process, thereby eliminating unnecessary overhead of going through
73 the entire process of faulting the pages in.
75 pages in from backing store, but quickly map the pages already in memory
76 into the calling process.
78 Allows the VM system to decrease the in-memory priority
79 of pages in the specified range.
80 Additionally future references to
81 this address range will incur a page fault.
83 Gives the VM system the freedom to free pages,
84 and tells the system that information in the specified page range
85 is no longer important.
86 This is an efficient way of allowing
88 to free pages anywhere in the address space, while keeping the address space
90 The next time that the page is referenced, the page might be demand
91 zeroed, or might contain the data that was there before the
94 References made to that address space range will not make the VM system
95 page the information back in from backing store until the page is
98 Request that the system not flush the data associated with this map to
99 physical backing store unless it needs to.
100 Typically this prevents the
101 file system update daemon from gratuitously writing pages dirtied
102 by the VM system to physical disk.
103 Note that VM/file system coherency is
104 always maintained, this feature simply ensures that the mapped data is
105 only flush when it needs to be, usually by the system pager.
107 This feature is typically used when you want to use a file-backed shared
108 memory area to communicate between processes (IPC) and do not particularly
109 need the data being stored in that area to be physically written to disk.
110 With this feature you get the equivalent performance with mmap that you
111 would expect to get with SysV shared memory calls, but in a more controllable
112 and less restrictive manner.
113 However, note that this feature is not portable
114 across UNIX platforms (though some may do the right thing by default).
115 For more information see the MAP_NOSYNC section of
118 Undoes the effects of MADV_NOSYNC for any future pages dirtied within the
120 The effect on pages already dirtied is indeterminate - they
121 may or may not be reverted.
122 You can guarantee reversion by using the
128 Region is not included in a core file.
130 Include region in a core file.
132 Informs the VM system this process should not be killed when the
133 swap space is exhausted.
134 The process must have superuser privileges.
135 This should be used judiciously in processes that must remain running
136 for the system to properly function.
139 Portable programs that call the
141 interface should use the aliases
142 .Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL ,
143 .Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED ,
145 .Dv POSIX_MADV_DONTNEED
146 rather than the flags described above.
152 system call will fail if:
157 argument is not valid.
159 The virtual address range specified by the
163 arguments is not valid.
166 was specified and the process does not have superuser privileges.
176 interface conforms to
181 system call first appeared in