清理操作系統緩存

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:
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

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

[root@server test]# cat /proc/sys/vm/drop_caches
0

/proc/sys/vm/drop_caches的值,默認爲0
簡單說，清除系統對內存的cache，使用root做下面幾步：

sync; echo 3 > /proc/sys/vm/drop_caches
sync; echo 0 > /proc/sys/vm/drop_caches