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__builtin_prefetch(3) [netbsd man page]

__BUILTIN_PREFETCH(3)					   BSD Library Functions Manual 				     __BUILTIN_PREFETCH(3)

__builtin_prefetch -- GNU extension to prefetch memory SYNOPSIS
void __builtin_prefetch(const void *addr, ...); DESCRIPTION
The __builtin_prefetch() function prefetches memory from addr. The rationale is to minimize cache-miss latency by trying to move data into a cache before accessing the data. Possible use cases include frequently called sections of code in which it is known that the data in a given address is likely to be accessed soon. In addition to addr, there are two optional stdarg(3) arguments, rw and locality. The value of the latter should be a compile-time constant integer between 0 and 3. The higher the value, the higher the temporal locality in the data. When locality is 0, it is assumed that there is little or no temporal locality in the data; after access, it is not necessary to leave the data in the cache. The default value is 3. The value of rw is either 0 or 1, corresponding with read and write prefetch, respectively. The default value of rw is 0. Also rw must be a compile-time constant integer. The __builtin_prefetch() function translates into prefetch instructions only if the architecture has support for these. If there is no sup- port, addr is evaluated only if it includes side effects, although no warnings are issued by gcc(1). EXAMPLES
The following optimization appears in the heavily used cpu_in_cksum() function that calculates checksums for the inet(4) headers: while (mlen >= 32) { __builtin_prefetch(data + 32); partial += *(uint16_t *)data; partial += *(uint16_t *)(data + 2); partial += *(uint16_t *)(data + 4); ... partial += *(uint16_t *)(data + 28); partial += *(uint16_t *)(data + 30); data += 32; mlen -= 32; ... SEE ALSO
gcc(1), attribute(3) Ulrich Drepper, What Every Programmer Should Know About Memory,, November 21, 2007. CAVEATS
This is a non-standard, compiler-specific extension. BSD
December 22, 2010 BSD

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madvise(3C)															       madvise(3C)

madvise - provide advice to VM system SYNOPSIS
#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 procedure 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 */ MADV_NORMAL 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. MADV_RANDOM 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. MADV_SEQUENTIAL 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. MADV_WILLNEED Tell the system that a certain address range is definitely needed so the kernel will start reading 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. MADV_DONTNEED Tell the kernel that the specified address range is no longer needed, so the system starts to free the resources associated with the address range. MADV_FREE Tell the kernel that contents in the specified address range are no longer important and the range will be over- written. When there is demand for memory, the system will free pages associated 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 system read from backing store (swap space) until the page is modified again. This value cannot be used on mappings that have underlying file objects. MADV_ACCESS_LWP 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. MADV_ACCESS_MANY 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. MADV_ACCESS_DEFAULT Reset the kernel's expectation for how the specified 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. EAGAIN Some or all mappings in the address range [addr, addr + len) are locked for I/O. EBUSY Some or all of the addresses in the range [addr, addr + len) are locked and MS_SYNC with the MS_INVALIDATE option is speci- fied. EFAULT 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. EINVAL 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. EIO An I/O error occurred while reading from or writing to the file system. ENOMEM 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. ESTALE Stale NFS file handle. See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Stable | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ meminfo(2), mmap(2), sysconf(3C), attributes(5) 23 Feb 2005 madvise(3C)
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