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MADVISE(3C)							   MADVISE(3C)

       madvise - provide advice to VM system

       #include <sys/types.h>
       #include <sys/mman.h>

       int madvise(caddr_t addr, size_t len, int advice);

       The  madvise() function advises the kernel that a region of user mapped
       memory in the range [addr, addr + len) will  be	accessed  following  a
       type  of pattern. The kernel uses this information to optimize the pro‐
       cedure for manipulating and maintaining the resources  associated  with
       the specified mapping range.

       Values for advice are defined in <sys/mman.h> as:

	 #define MADV_NORMAL	       0x0  /* No further special treatment */
	 #define MADV_RANDOM	       0x1  /* Expect random page references */
	 #define MADV_SEQUENTIAL       0x2  /* Expect sequential page references */
	 #define MADV_WILLNEED	       0x3  /* Will need these pages */
	 #define MADV_DONTNEED	       0x4  /* Don't need these pages */
	 #define MADV_FREE	       0x5  /* Contents can be freed */
	 #define MADV_ACCESS_DEFAULT   0x6  /* default access */
	 #define MADV_ACCESS_LWP       0x7  /* next LWP to access heavily */
	 #define MADV_ACCESS_MANY      0x8  /* many processes to access heavily */

			      This  is the default system characteristic where
			      accessing memory within the address range causes
			      the  system  to  read data from the mapped file.
			      The kernel reads all data from files into	 pages
			      which  are  retained  for	 a period of time as a
			      "cache." System pages can be a scarce  resource,
			      so  the  kernel steals pages from other mappings
			      when needed. This is a  likely  occurrence,  but
			      adversely	 affects  system performance only if a
			      large amount of memory is accessed.

			      Tell the kernel to read in a minimum  amount  of
			      data from a mapped file on any single particular
			      access. If  MADV_NORMAL is  in  effect  when  an
			      address of a mapped file is accessed, the system
			      tries to read in as much data from the  file  as
			      reasonable,  in  anticipation  of other accesses
			      within a certain locality.

			      Tell the system that addresses in this range are
			      likely  to  be accessed only once, so the system
			      will free	 the  resources	 mapping  the  address
			      range as quickly as possible.

			      Tell  the system that a certain address range is
			      definitely needed so the kernel will start read‐
			      ing  the	specified  range into memory. This can
			      benefit programs wanting to  minimize  the  time
			      needed  to  access memory the first time, as the
			      kernel would need to read in from the file.

			      Tell the kernel that the specified address range
			      is  no  longer  needed,  so the system starts to
			      free the resources associated with  the  address

			      Tell  the	 kernel that contents in the specified
			      address range are no longer  important  and  the
			      range  will be overwritten. When there is demand
			      for memory, the system will free	pages  associ‐
			      ated  with  the specified address range. In this
			      instance, the next time a page  in  the  address
			      range is referenced, it will contain all zeroes.
			      Otherwise, it will contain  the  data  that  was
			      there  prior  to	the MADV_FREE call. References
			      made to the address range will not make the sys‐
			      tem  read	 from backing store (swap space) until
			      the page is modified again.

			      This value cannot be used on mappings that  have
			      underlying file objects.

			      Tell  the	 kernel that the next LWP to touch the
			      specified address	 range	will  access  it  most
			      heavily,	so  the	 kernel should try to allocate
			      the memory and other resources  for  this	 range
			      and the LWP accordingly.

			      Tell  the kernel that many processes and/or LWPs
			      will access the specified address range randomly
			      across  the machine, so the kernel should try to
			      allocate the memory and other resources for this
			      range accordingly.

			      Reset the kernel's expectation for how the spec‐
			      ified range will be accessed to the default.

       The madvise() function should be used  by  applications	with  specific
       knowledge  of  their  access  patterns  over a memory object, such as a
       mapped file, to increase system performance.

       Upon successful completion, madvise() returns 0; otherwise, it  returns
       −1 and sets errno to indicate the error.

		 Some  or all mappings	in the	address	 range [addr,  addr  +
		 len) are locked for I/O.

		 Some or all of the addresses in the range [addr, addr +  len)
		 are locked and MS_SYNC with the MS_INVALIDATE option is spec‐

		 Some or all of the addresses in the specified range could not
		 be  read into memory from the underlying object when perform‐
		 ing MADV_WILLNEED. The madvise() function could return	 prior
		 to  this  condition  being detected, in which case errno will
		 not be set to EFAULT.

		 The addr argument is not a  multiple  of  the	page  size  as
		 returned  by sysconf(3C), the length of the specified address
		 range is equal to 0, or the advice argument was invalid.

		 An I/O error occurred while reading from or  writing  to  the
		 file system.

		 Addresses  in	the  range  [addr, addr + len) are outside the
		 valid range for the address space of a	 process,  or  specify
		 one or more pages that are not mapped.

		 Stale NFS file handle.

       See attributes(5) for descriptions of the following attributes:

       │Interface Stability │ Stable	      │
       │MT-Level	    │ MT-Safe	      │

       meminfo(2), mmap(2), sysconf(3C), attributes(5)

				 Feb 23, 2005			   MADVISE(3C)

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