# sync && echo 3 >/proc/sys/vm/drop_caches && sleep 2 && echo 0 >/proc/sys/vm/drop_cachesfree -m
各参数含义：
1: total：总物理内存
2: used：已使用内存
3: free：完全未被使用的内存
4: shared：应用程序共享内存
5: buffers：缓存，主要用于目录方面,inode值等
6: cached：缓存，用于已打开的文件
7: -buffers/cache：应用程序使用的内存大小，used减去缓存值
8: +buffers/cache：所有可供应用程序使用的内存大小，free加上缓存值
9: 其中：
10: total = used + free
11: -buffers/cache=used-buffers-cached，应用程序真实使用的内存大小
12: +buffers/cache=free+buffers+cached，服务器真实还可利用的内存大小
Writing to this file causes the kernel to drop clean caches,dentries and inodes from memory, causing that memory to becomefree. To free pagecache, use echo 1 > /proc/sys/vm/drop_caches; to free dentries and inodes, use echo 2 > /proc/sys/vm/drop_caches; to free pagecache, dentries and inodes, use echo 3 >/proc/sys/vm/drop_caches. Because this is a non-destructive operation and dirty objects are not freeable, the user should run sync first.
