linux释放页面缓存drop_caches

关于drop_caches文件：系统默认为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:仅清除页面缓存（PageCache）
echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:清除目录项和inode
echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:清除页面缓存，目录项和inode
sync && 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.