arm_fadvise64_64 - Linux

Overview

arm_fadvise64_64 is a Linux kernel system call that provides advisory information about file accesses to the kernel. It is a variant of the more common fadvise64_64 system call that operates on 64-bit file offsets and lengths.

It can be used to inform the kernel about expected access patterns of a given file, allowing the kernel to optimize its caching and I/O scheduling algorithms.

Syntax

int arm_fadvise64_64(int fd, uint64_t offset, uint64_t len, int advice);

The arm_fadvise64_64 system call takes the following arguments:

  • fd: The file descriptor of the file to advise about.

  • offset: The file offset from which the advice applies.

  • len: The length of the file region to which the advice applies.

  • advice: The advice to give to the kernel. This can be one of the following values:

    • POSIX_FADV_NORMAL: No special advice.
    • POSIX_FADV_SEQUENTIAL: The file will be accessed sequentially from the given offset.
    • POSIX_FADV_RANDOM: The file will be accessed randomly.
    • POSIX_FADV_NOREUSE: The file data should not be cached.
    • POSIX_FADV_WILLNEED: The file data will be needed soon.
    • POSIX_FADV_DONTNEED: The file data is not needed anymore.
    • POSIX_FADV_SYNC: Synchronize changes to the file with stable storage.
    • POSIX_FADV_NOCACHE: Do not cache the file data.

Options/Flags

There are no additional options or flags available for arm_fadvise64_64.

Examples

The following example shows how to use arm_fadvise64_64 to advise the kernel that a file will be accessed sequentially from a given offset:

#include <fcntl.h>
#include <sys/types.h>
#include <sys/syscall.h>

int main(void) {
  int fd = open("myfile.txt", O_RDONLY);
  if (fd == -1) {
    /* Handle error */
  }

  off64_t offset = 1024;
  size_t len = 4096;
  int advice = POSIX_FADV_SEQUENTIAL;

  if (syscall(SYS_arm_fadvise64_64, fd, offset, len, advice) == -1) {
    /* Handle error */
  }

  /* Continue using the file */

  return 0;
}

Common Issues

One common issue when using arm_fadvise64_64 is that the advice may not always be followed by the kernel. The kernel may decide to ignore the advice if it believes that doing so will improve performance.

Another potential issue is that arm_fadvise64_64 can only provide advisory information to the kernel. It does not guarantee that the kernel will actually change its behavior based on the advice.

Integration

arm_fadvise64_64 can be used in conjunction with other Linux commands and tools to improve performance. For example, it can be used with the dd command to prefetch data into the kernel’s buffer cache:

dd if=myfile.txt of=/dev/null bs=4096 count=1024 conv=fdatasync,fadvise=sequential

This command will tell the kernel to prefetch the first 4MB of the file myfile.txt into the buffer cache, and to advise the kernel that the data will be accessed sequentially.

Related Commands

  • fadvise64_64
  • posix_fadvise
  • sync