How to evict file from system cache on Linux?

Question

When running performance tests file system cache hit or miss can significantly influence test results. Therefore generally before running such tests used files are evicted from system cache. How to do that on Linux?

Clarification: If possible, the solution should not require root privileges.

Answer 1

As a superuser you can do the following:

To free pagecache:

• echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:

• echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:

• echo 3 > /proc/sys/vm/drop_caches

This operation will not "lose" any data (caches are written out to disk before their data is dropped), however, to really make sure all cache is cleaned, you should sync first. E.g. all caches should be cleared if you run

sync; echo 3 > /proc/sys/vm/drop_caches

As I said, only a superuser (root) may do so.

Answer 2

Ha, I have the answer:

#include <unistd.h>
#include <fcntl.h>
int main(int argc, char *argv[]) {
  int fd;
  fd = open(argv[1], O_RDONLY);
  fdatasync(fd);
  posix_fadvise(fd, 0,0,POSIX_FADV_DONTNEED);
  close(fd);
  return 0;
}

This is from http://insights.oetiker.ch/linux/fadvise.html

Answer 3

There is a command line utility by Eric Wong that makes it easy to invoke posix_fadvise:

http://git.bogomips.org/cgit/pcu.git/tree/README

It's then as simple as

$ pcu-fadvise -a dontneed filename-to-evict