關於drop_caches檔案：系統預設為0

在Documentation/sysctl/vm.txt中有如下描述:

drop_caches

Writing to this will cause the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.

To free pagecache:僅清除頁面快取（PageCache）
 echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:清除目錄項和inode
 echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:清除頁面快取，目錄項和inode
 echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation and dirty objects are not freeable, the user should run `sync' first.