This is about the drop_caches tunability. It's available in kernel 2.6.16 and above, and exists in /proc/sys/vm.
If you echo various values to it, various kernel cache data structures are dropped. This is a non-destructive operation, so if you still see stuff hanging out after it, it's likely that it was dirty cache. Anyhow, on to the values:
1 - drop the pagecache
2 - drop the dentry and inode caches
3 - drop both the dentry and inode caches, as well as the pagecache.
echo 3 > /proc/sys/vm/drop_caches as a root or admin user
If you echo various values to it, various kernel cache data structures are dropped. This is a non-destructive operation, so if you still see stuff hanging out after it, it's likely that it was dirty cache. Anyhow, on to the values:
1 - drop the pagecache
2 - drop the dentry and inode caches
3 - drop both the dentry and inode caches, as well as the pagecache.
echo 3 > /proc/sys/vm/drop_caches as a root or admin user