free

free是查看内存的命令

1             2                   3                4           5                6
1                                              total       used              free        shared    buffers     cached
2 Mem:                              24677460   23276064    1401396      0         870540    12084008
3 -/+ buffers/cache:                               10321516   14355944
4 Swap:                                25151484     224188   24927296

尝试着理解下面三个等式：

FO[2][1] = FO[2][2] + FO[2][3]

FO[3][2] = FO[2][2] - FO[2][5] - FO[2][6]
FO[3][3] = FO[2][3] + FO[2][5] + FO[2][6]

这里主要注意两个概念：

buffers : A buffer i ssomething that has yet to be "writen" to disk

cache   : A cache is something that has been "read" from the disk and stored for later use

buffer是用于存放要输出到disk（块设备）的数据

cache是存放从disk上读出的数据