频繁的文件访问会导致系统的Cache使用量大增, 系统运行缓慢。
1 首先用free 命令查看内存的使用：
$ free -m
total used free shared buffers cached
Mem: 3955 3926 28 0 55 3459
2 运行sync将dirty的内容写回硬盘
$sync
3 通过修改proc系统的drop_caches清理free的cache
$echo 3 > /proc/sys/vm/drop_caches
关于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:
* 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.
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