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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, except it returns an error number on error and does
56 and is provided for standards conformance.
58 The known behaviors are:
59 .Bl -tag -width MADV_SEQUENTIAL
61 Tells the system to revert to the default paging
64 Is a hint that pages will be accessed randomly, and prefetching
65 is likely not advantageous.
66 .It Dv MADV_SEQUENTIAL
67 Causes the VM system to depress the priority of
68 pages immediately preceding a given page when it is faulted in.
70 Causes pages that are in a given virtual address range
71 to temporarily have higher priority, and if they are in
72 memory, decrease the likelihood of them being freed.
74 the pages that are already in memory will be immediately mapped into
75 the process, thereby eliminating unnecessary overhead of going through
76 the entire process of faulting the pages in.
78 pages in from backing store, but quickly map the pages already in memory
79 into the calling process.
81 Allows the VM system to decrease the in-memory priority
82 of pages in the specified range.
83 Additionally future references to
84 this address range will incur a page fault.
86 Gives the VM system the freedom to free pages,
87 and tells the system that information in the specified page range
88 is no longer important.
89 This is an efficient way of allowing
91 to free pages anywhere in the address space, while keeping the address space
93 The next time that the page is referenced, the page might be demand
94 zeroed, or might contain the data that was there before the
97 References made to that address space range will not make the VM system
98 page the information back in from backing store until the page is
101 Request that the system not flush the data associated with this map to
102 physical backing store unless it needs to.
103 Typically this prevents the
104 file system update daemon from gratuitously writing pages dirtied
105 by the VM system to physical disk.
106 Note that VM/file system coherency is
107 always maintained, this feature simply ensures that the mapped data is
108 only flush when it needs to be, usually by the system pager.
110 This feature is typically used when you want to use a file-backed shared
111 memory area to communicate between processes (IPC) and do not particularly
112 need the data being stored in that area to be physically written to disk.
113 With this feature you get the equivalent performance with mmap that you
114 would expect to get with SysV shared memory calls, but in a more controllable
115 and less restrictive manner.
116 However, note that this feature is not portable
117 across UNIX platforms (though some may do the right thing by default).
118 For more information see the MAP_NOSYNC section of
121 Undoes the effects of MADV_NOSYNC for any future pages dirtied within the
123 The effect on pages already dirtied is indeterminate - they
124 may or may not be reverted.
125 You can guarantee reversion by using the
131 Region is not included in a core file.
133 Include region in a core file.
135 Informs the VM system this process should not be killed when the
136 swap space is exhausted.
137 The process must have superuser privileges.
138 This should be used judiciously in processes that must remain running
139 for the system to properly function.
142 Portable programs that call the
144 interface should use the aliases
145 .Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL ,
146 .Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED ,
148 .Dv POSIX_MADV_DONTNEED
149 rather than the flags described above.
155 system call will fail if:
160 argument is not valid.
162 The virtual address range specified by the
166 arguments is not valid.
169 was specified and the process does not have superuser privileges.
180 interface conforms to
185 system call first appeared in