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32 .\" @(#)madvise.2 8.1 (Berkeley) 6/9/93
33 .\" $FreeBSD: src/lib/libc/sys/madvise.2,v 1.17.2.8 2003/01/06 23:33:59 trhodes Exp $
34 .\" $DragonFly: src/lib/libcr/sys/Attic/madvise.2,v 1.2 2003/06/17 04:26:47 dillon Exp $
41 .Nd give advice about use of memory
48 .Fn 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.
56 The known behaviors are:
57 .Bl -tag -width MADV_SEQUENTIAL
59 Tells the system to revert to the default paging
62 Is a hint that pages will be accessed randomly, and prefetching
63 is likely not advantageous.
64 .It Dv MADV_SEQUENTIAL
65 Causes the VM system to depress the priority of
66 pages immediately preceding a given page when it is faulted in.
68 Causes pages that are in a given virtual address range
69 to temporarily have higher priority, and if they are in
70 memory, decrease the likelihood of them being freed. Additionally,
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. This WILL NOT fault
74 pages in from backing store, but quickly map the pages already in memory
75 into the calling process.
77 Allows the VM system to decrease the in-memory priority
78 of pages in the specified range. Additionally future references to
79 this address range will incur a page fault.
81 Gives the VM system the freedom to free pages,
82 and tells the system that information in the specified page range
83 is no longer important. This is an efficient way of allowing
85 to free pages anywhere in the address space, while keeping the address space
86 valid. The next time that the page is referenced, the page might be demand
87 zeroed, or might contain the data that was there before the
90 References made to that address space range will not make the VM system
91 page the information back in from backing store until the page is
94 Request that the system not flush the data associated with this map to
95 physical backing store unless it needs to. Typically this prevents the
96 filesystem update daemon from gratuitously writing pages dirtied
97 by the VM system to physical disk. Note that VM/filesystem coherency is
98 always maintained, this feature simply ensures that the mapped data is
99 only flush when it needs to be, usually by the system pager.
101 This feature is typically used when you want to use a file-backed shared
102 memory area to communicate between processes (IPC) and do not particularly
103 need the data being stored in that area to be physically written to disk.
104 With this feature you get the equivalent performance with mmap that you
105 would expect to get with SysV shared memory calls, but in a more controllable
106 and less restrictive manner. However, note that this feature is not portable
107 across UNIX platforms (though some may do the right thing by default).
108 For more information see the MAP_NOSYNC section of
111 Undoes the effects of MADV_NOSYNC for any future pages dirtied within the
112 address range. The effect on pages already dirtied is indeterminate - they
113 may or may not be reverted. You can guarantee reversion by using the
119 Region is not included in a core file.
121 Include region in a core file.
128 function will fail if:
131 The virtual address range specified by the
135 arguments is not valid.
145 function first appeared in