What is the difference between Memory, Real Mem, and Compressed Mem? I've already seen this question:


What&#39;s the difference between Real, Virtual, Shared, and Private Memory?


but I think it might be outdated. Specifically, there is a Memory column, as well as Real Mem and Compressed Mem column. What is the difference, and why would the Real Mem ever be smaller than Compressed Mem?

(I'm using macOS Sierra 10.12, but I think I've seen this in slightly older versions as well.)
The relation between them is this:

Memory = The amount of memory the process has reserved for its use.

Compressed mem = The amount of memory (a subset of "Memory") that the operating system has used its compression algorithm on

Real mem = The amount of real memory that is actually used by the process (i.e. a sum of the memory that hasn't been compressed, and the size of the result of the compression algorithm)

So in this case the process thinks it has reserved 976,2 MB for its use. The system took 649,7 MB of that memory and compressed it. That leaves 326,5 MB that wasn't compressed plus the size of the compressed memory after it was compressed - which in this case is approx. 77,2 MB. The total of actual memory used by the process is then 403,7 MB.