[Go] MemStats in runtime package

Reference:
https://vsdmars.blogspot.com/2019/01/golang-golangs-memory-management-eben.html


https://golang.org/pkg/runtime/#MemStats

// HeapInuse is bytes in in-use spans.

// In-use spans have at least one object in them. These spans 

// can only be used for other objects of roughly the same 

// size. 

// HeapInuse minus HeapAlloc estimates the amount of memory 

// that has been dedicated to particular size classes, but is 

// not currently being used. This is an upper bound on 

// fragmentation, but in general this memory can be reused  

// efficiently.