NAME¶

alloc_hugepages, free_hugepages - allocate or free huge pages

SYNOPSIS¶

void *syscall(SYS_alloc_hugepages, int key, void addr[.len], size_t len,
              int prot, int flag);
int syscall(SYS_free_hugepages, void *addr);

Note: glibc provides no wrappers for these system calls, necessitating the use of syscall(2).

DESCRIPTION¶

The system calls alloc_hugepages() and free_hugepages() were introduced in Linux 2.5.36 and removed again in Linux 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 fail with the error 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's memory or to free them again. Huge pages are locked into memory, and are not swapped.

The key argument 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 argument 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 argument of alloc_hugepages() is a hint, that the kernel may or may not follow. Addresses must be properly aligned.

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

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

The flag argument 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 to indicate the error.

ERRORS¶

ENOSYS

The system call is not supported on this 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.

STANDARDS¶

Linux on Intel processors.

HISTORY¶

These system calls are gone; they existed only in Linux 2.5.36 through to Linux 2.5.54.

NOTES¶

Now the hugetlbfs filesystem can be used instead. Memory backed by huge pages (if the CPU supports them) is obtained by using mmap(2) to map files in this virtual filesystem.

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