有时候，像mongo这种，对内存只吃不吐的，我们要手动释放一下。
drop_caches的详细文档如下：
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:（释放网页内存）
* echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:（释放dentries和inode缓存）
* echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:（释放页面缓存，dentries和inode）
* echo 3 > /proc/sys/vm/drop_caches
As
this is a non-destructive operation, and dirty objects are notfreeable,
the user should run "sync" first in order to make sure allcached
objects are freed.