free命令可以查看当前系统内存的使用情况
free -m 以MB为单位
free -k 以KB为单位
free -m 以MB为单位显示系统内存的使用情况，同理，也可以使用-k、-g等其他的单位显示
free命令从两个维度统计了内存的使用情况
第一行Mem：从操作系统角度统计内存的total、used、free、buffers、cached
第二行-/+buffers：从应用程序角度统计内存的total、used、free（实际可使用内存，以第二行这个值为准）
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buffer和cache 两者都是Linux下的缓存机制，其中buffer为写操作的缓存，cache为读操作的缓存
第三行：swap 交换空间，磁盘上的一块空间，当系统内存不足时，会使用交换空间
内存实际可用空间为： free+buffers+cached，优先考虑使用 ，其次考虑缓存