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