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32 .\" @(#)madvise.2 8.1 (Berkeley) 6/9/93
39 .Nm madvise , posix_madvise
40 .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"
53 allows a process that has knowledge of its memory behavior
54 to describe it to the system.
57 interface is identical and is provided for standards conformance.
59 The known behaviors are:
60 .Bl -tag -width MADV_SEQUENTIAL
62 Tells the system to revert to the default paging
65 Is a hint that pages will be accessed randomly, and prefetching
66 is likely not advantageous.
67 .It Dv MADV_SEQUENTIAL
68 Causes the VM system to depress the priority of
69 pages immediately preceding a given page when it is faulted in.
71 Causes pages that are in a given virtual address range
72 to temporarily have higher priority, and if they are in
73 memory, decrease the likelihood of them being freed.
75 the pages that are already in memory will be immediately mapped into
76 the process, thereby eliminating unnecessary overhead of going through
77 the entire process of faulting the pages in.
79 pages in from backing store, but quickly map the pages already in memory
80 into the calling process.
82 Allows the VM system to decrease the in-memory priority
83 of pages in the specified range.
84 Additionally future references to
85 this address range will incur a page fault.
87 Gives the VM system the freedom to free pages,
88 and tells the system that information in the specified page range
89 is no longer important.
90 This is an efficient way of allowing
92 to free pages anywhere in the address space, while keeping the address space
94 The next time that the page is referenced, the page might be demand
95 zeroed, or might contain the data that was there before the
98 References made to that address space range will not make the VM system
99 page the information back in from backing store until the page is
102 Request that the system not flush the data associated with this map to
103 physical backing store unless it needs to.
104 Typically this prevents the
105 file system update daemon from gratuitously writing pages dirtied
106 by the VM system to physical disk.
107 Note that VM/file system coherency is
108 always maintained, this feature simply ensures that the mapped data is
109 only flush when it needs to be, usually by the system pager.
111 This feature is typically used when you want to use a file-backed shared
112 memory area to communicate between processes (IPC) and do not particularly
113 need the data being stored in that area to be physically written to disk.
114 With this feature you get the equivalent performance with mmap that you
115 would expect to get with SysV shared memory calls, but in a more controllable
116 and less restrictive manner.
117 However, note that this feature is not portable
118 across UNIX platforms (though some may do the right thing by default).
119 For more information see the MAP_NOSYNC section of
122 Undoes the effects of MADV_NOSYNC for any future pages dirtied within the
124 The effect on pages already dirtied is indeterminate - they
125 may or may not be reverted.
126 You can guarantee reversion by using the
132 Region is not included in a core file.
134 Include region in a core file.
136 Informs the VM system this process should not be killed when the
137 swap space is exhausted.
138 The process must have superuser privileges.
139 This should be used judiciously in processes that must remain running
140 for the system to properly function.
143 Portable programs that call the
145 interface should use the aliases
146 .Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL ,
147 .Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED ,
149 .Dv POSIX_MADV_DONTNEED
150 rather than the flags described above.
156 system call will fail if:
161 argument is not valid.
163 The virtual address range specified by the
167 arguments is not valid.
170 was specified and the process does not have superuser privileges.
180 interface conforms to
185 system call first appeared in