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I've already seen this question:

but I think it might be outdated.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.)

Activity monitor screenshot

Another screenshot

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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.

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    Thanks for the answer. Sometimes I see a Real Mem value greater than Memory, which doesn't seem to fit with the explanation (or I'm misunderstanding). Do you know how that's possible? (I added another screenshot as example)
    – dkv
    Jun 13, 2019 at 1:09
  • @dkv Shared memory maybe?
    – Feuermurmel
    May 17, 2021 at 16:10
  • @dkv The difference is typically made up of swapped out memory. I.e. in your screenshot you have 3.35 GB of real memory used by the process, and 940 MB of RAM reserved for the process - the rest is swapped out to disk.
    – jksoegaard
    May 17, 2021 at 20:23

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