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32 .\" @(#)madvise.2 8.1 (Berkeley) 6/9/93
40 .Nd give advice about use of memory
44 .Fd #include <sys/types.h>
45 .Fd #include <sys/mman.h>
47 .Fn madvise "void *addr" "size_t len" "int behav"
52 allows a process that has knowledge of its memory behavior
53 to describe it to the system.
54 The known behaviors are given in
57 #define MADV_NORMAL 0 /* no further special treatment */
58 #define MADV_RANDOM 1 /* expect random page references */
59 #define MADV_SEQUENTIAL 2 /* expect sequential references */
60 #define MADV_WILLNEED 3 /* will need these pages */
61 #define MADV_DONTNEED 4 /* don't need these pages */
62 #define MADV_FREE 5 /* data is now unimportant */
63 #define MADV_NOSYNC 6 /* no explicit commit to physical backing store */
64 #define MADV_AUTOSYNC 7 /* default commit method to physical backing store */
65 #define MADV_NOCORE 8 /* do not include these pages in a core file */
66 #define MADV_CORE 9 /* revert to including pages in a core file */
69 .Bl -tag -width MADV_SEQUENTIAL
71 Tells the system to revert to the default paging
74 Is a hint that pages will be accessed randomly, and prefetching
75 is likely not advantageous.
76 .It Dv MADV_SEQUENTIAL
77 Causes the VM system to depress the priority of
78 pages immediately preceding a given page when it is faulted in.
80 Causes pages that are in a given virtual address range
81 to temporarily have higher priority, and if they are in
82 memory, decrease the likelihood of them being freed. Additionally,
83 the pages that are already in memory will be immediately mapped into
84 the process, thereby eliminating unnecessary overhead of going through
85 the entire process of faulting the pages in. This WILL NOT fault
86 pages in from backing store, but quickly map the pages already in memory
87 into the calling process.
89 Allows the VM system to decrease the in-memory priority
90 of pages in the specified range. Additionally future references to
91 this address range will incur a page fault.
93 Gives the VM system the freedom to free pages,
94 and tells the system that information in the specified page range
95 is no longer important. This is an efficient way of allowing
97 to free pages anywhere in the address space, while keeping the address space
98 valid. The next time that the page is referenced, the page might be demand
99 zeroed, or might contain the data that was there before the
102 References made to that address space range will not make the VM system
103 page the information back in from backing store until the page is
106 Request that the system not flush the data associated with this map to
107 physical backing store unless it needs to. Typically this prevents the
108 filesystem update daemon from gratuitously writing pages dirtied
109 by the VM system to physical disk. Note that VM/filesystem coherency is
110 always maintained, this feature simply ensures that the mapped data is
111 only flush when it needs to be, usually by the system pager.
113 This feature is typically used when you want to use a file-backed shared
114 memory area to communicate between processes (IPC) and do not particularly
115 need the data being stored in that area to be physically written to disk.
116 With this feature you get the equivalent performance with mmap that you
117 would expect to get with SysV shared memory calls, but in a more controllable
118 and less restrictive manner. However, note that this feature is not portable
119 across UNIX platforms (though some may do the right thing by default).
120 For more information see the MAP_NOSYNC section of
123 Undoes the effects of MADV_NOSYNC for any future pages dirtied within the
124 address range. The effect on pages already dirtied is indeterminate - they
125 may or may not be reverted. You can guarentee reversion by using the
131 Region is not included in a core file.
133 Include region in a core file.
140 function will fail if:
143 The virtual address range specified by the
147 arguments is not valid.
157 function first appeared in