清除 Linux memory的 cache

有很多種方法可以查看Linux 記憶體的使用狀況，如vmstat、free、nmon等，這裡以 free 指令為例

[root@db01 ~]# free
                                 total           used                  free     shared    buffers     cached
Mem:                       5144296    3008448    2135848          0       8600        2819464
-/+ buffers/cache:       180384    4963912
Swap:      6160376      18484    6141892

這裡buffers是放即將被寫入硬碟資料的緩衝區；而cache則是資料預先自硬碟讀取暫存在記憶體的暫存區。Linux 作業系統傾向會把資料cache在記憶體中，即使應用程式結束，也不會立即將cache的資料清除。

以下介紹如何把cache的資料清除：

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 in order to make sure all cached objects are freed.