比较新一点版本的free添加了available列，可以直接看出还有多少内存可用
比较老一点版本的free为：
3.3.9 版本的free就没有available列，另外，和内核版本也有关系，3.14 以后的内核是支持的； 2.6.27+ 的内核上是模拟出来的
man free
available
Estimation of how much memory is available for starting new applications, without swapping. Unlike
the data provided by the cache or free fields, this field takes into account page cache and also
that not all reclaimable memory slabs will be reclaimed due to items being in use (MemAvailable in
/proc/meminfo, available on kernels 3.14, emulated on kernels 2.6.27+, otherwise the same as free)