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# alloc_hugepages() - Unix, Linux System Call

# NAME

alloc_hugepages, free_hugepages - allocate or free huge pages.

# SYNOPSIS

    void *alloc_hugepages(int key, void *addr, size_t len, int prot, int flag);
    int free_hugepages(void *addr);

# DESCRIPTION

The system calls alloc_hugepages() and free_hugepages() were introduced in Linux 2.5.36 and removed again in 2.5.54. They existed only on i386 and ia64 (when built with CONFIG_HUGETLB_PAGE). In Linux 2.4.20 the syscall numbers exist, but the calls return ENOSYS.

On i386 the memory management hardware knows about ordinary pages (4 KiB) and huge pages (2 or 4 MiB). Similarly ia64 knows about huge pages of several sizes. These system calls serve to map huge pages into the process’ memory or to free them again. Huge pages are locked into memory, and are not swapped.

The key parameter is an identifier. When zero the pages are private, and not inherited by children. When positive the pages are shared with other applications using the same key, and inherited by child processes.

The addr parameter of free_hugepages() tells which page is being freed: it was the return value of a call to alloc_hugepages(). (The memory is first actually freed when all users have released it.) The addr parameter of alloc_hugepages() is a hint, that the kernel may or may not follow. Addresses must be properly aligned.

The len parameter is the length of the required segment. It must be a multiple of the huge page size.

The prot parameter specifies the memory protection of the segment. It is one of PROT_READ, PROT_WRITE, PROT_EXEC.

The flag parameter is ignored, unless key is positive. In that case, if flag is IPC_CREAT, then a new huge page segment is created when none with the given key existed. If this flag is not set, then ENOENT is returned when no segment with the given key exists.

# RETURN VALUE

On success, alloc_hugepages() returns the allocated virtual address, and free_hugepages() returns zero. On error, -1 is returned, and errno is set appropriately.

# ERRORS

Tag | Description
--- | ---
ENOSYS | The system call is not supported on this kernel.

# CONFORMING TO

These calls existed only in Linux 2.5.36 through to 2.5.54. These calls are specific to Linux on Intel processors, and should not be used in programs intended to be portable. Indeed, the system call numbers are marked for reuse, so programs using these may do something random on a future kernel.

# FILES

/proc/sys/vm/nr_hugepages Number of configured hugetlb pages. This can be read and written.

/proc/meminfo Gives info on the number of configured hugetlb pages and on their size in the three variables HugePages_Total, HugePages_Free, Hugepagesize.

# NOTES

The system calls are gone. Now the hugetlbfs filesystem can be used instead. Memory backed by huge pages (if the CPU supports them) is obtained by using mmap() to map files in this virtual filesystem.

The maximal number of huge pages can be specified using the hugepages= boot parameter.

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