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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 $
40 .Nd give advice about use of memory
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.
55 The known behaviors are:
56 .Bl -tag -width MADV_SEQUENTIAL
58 Tells the system to revert to the default paging
61 Is a hint that pages will be accessed randomly, and prefetching
62 is likely not advantageous.
63 .It Dv MADV_SEQUENTIAL
64 Causes the VM system to depress the priority of
65 pages immediately preceding a given page when it is faulted in.
67 Causes pages that are in a given virtual address range
68 to temporarily have higher priority, and if they are in
69 memory, decrease the likelihood of them being freed. Additionally,
70 the pages that are already in memory will be immediately mapped into
71 the process, thereby eliminating unnecessary overhead of going through
72 the entire process of faulting the pages in. This WILL NOT fault
73 pages in from backing store, but quickly map the pages already in memory
74 into the calling process.
76 Allows the VM system to decrease the in-memory priority
77 of pages in the specified range. Additionally future references to
78 this address range will incur a page fault.
80 Gives the VM system the freedom to free pages,
81 and tells the system that information in the specified page range
82 is no longer important. This is an efficient way of allowing
84 to free pages anywhere in the address space, while keeping the address space
85 valid. The next time that the page is referenced, the page might be demand
86 zeroed, or might contain the data that was there before the
89 References made to that address space range will not make the VM system
90 page the information back in from backing store until the page is
93 Request that the system not flush the data associated with this map to
94 physical backing store unless it needs to. Typically this prevents the
95 filesystem update daemon from gratuitously writing pages dirtied
96 by the VM system to physical disk. Note that VM/filesystem coherency is
97 always maintained, this feature simply ensures that the mapped data is
98 only flush when it needs to be, usually by the system pager.
100 This feature is typically used when you want to use a file-backed shared
101 memory area to communicate between processes (IPC) and do not particularly
102 need the data being stored in that area to be physically written to disk.
103 With this feature you get the equivalent performance with mmap that you
104 would expect to get with SysV shared memory calls, but in a more controllable
105 and less restrictive manner. However, note that this feature is not portable
106 across UNIX platforms (though some may do the right thing by default).
107 For more information see the MAP_NOSYNC section of
110 Undoes the effects of MADV_NOSYNC for any future pages dirtied within the
111 address range. The effect on pages already dirtied is indeterminate - they
112 may or may not be reverted. You can guarantee reversion by using the
118 Region is not included in a core file.
120 Include region in a core file.
127 function will fail if:
130 The virtual address range specified by the
134 arguments is not valid.
144 function first appeared in