In Mavericks, the Activity Monitor has changed its memory terminology. The columns available when viewing processes in the memory tab now include "Memory", "Real Memory", "Real Private Memory", "Real Shared Memory" and "Purgeable Memory".

I would have thought that since "Virtual Memory" has disappeared and "Memory" has appeared that they are equivalent, however that doesn't seem to add up because shouldn't then "Memory" be always greater than or equal to "Real Memory"? (If I'm interpreting the terminology correctly, Real Memory is the number of pages currently resident in RAM, and Virtual memory is the number of pages total, including those swapped out to disk, compressed, etc....) However in Activity monitor, "Memory" is always less than "Real Memory". Can anyone explain what this mysterious "Memory" metric is?

  • I think you'll find your answer at apple.stackexchange.com/a/107/46950 .
    – Dave
    Commented Nov 14, 2013 at 17:10
  • 2
    @Dave That answer may be outdated, since Mavericks appears to have changed the terminology. For instance, that answer does not explain what the "Memory" column indicates.
    – Max Nanasy
    Commented Nov 19, 2013 at 23:27
  • has things changed since question was asked. Using latest Mavericks. And I do not follow your terminology?
    – Joop
    Commented Nov 21, 2013 at 20:30
  • Is there something that needs to be improved in the current answer?
    – bmike
    Commented Jun 13, 2019 at 1:35

1 Answer 1


For each process there is

  • Real Memory (always at least as big as Memory)
    • Total Memory currently consumed by an application (including Virtual pages)
  • Memory
    • Memory used in RAM
  • Purgeable Memory
    • Memory which can be cleaned by MMU, if another process needs more real memory.

Then, for the system in total

  • Physical Memory
    • The amount of RAM installed.
  • Memory Used
    • The amount of RAM being used and not immediately available.
  • Virtual Memory
    • The amount of disk or flash drive space being used as virtual memory.
  • Swap Used
    • The space on your drive being used to swap unused files to and from RAM.
  • App Memory
    • The amount of space being used by apps.
  • Wired Memory
    • Memory that can’t be cached to disk, so it must stay in RAM. This memory can’t be borrowed by other apps.
  • Compressed
    • The amount of memory in RAM that is compressed.
  • File Cache
    • The space being used to temporarily store files that are not currently being used.
  • 2
    Do you have a source for this information that I can use to read abotu this more? The part I find most confusing is that in your answer "Real Memory" seems to correspond to the "Virtual Memory" in basically any other OS. Would it be accurate to state that your "Memory" is the "Real Memory" in most other OS's? Commented Dec 2, 2013 at 22:45
  • 1
    It definitely looks like apple have chosen to "blurr the lines" by using uncommon memory terms such as real memory and memory. For all the "System" ones, i.e. the last 8 definitions, they are lifted straight from the Activity Monitor Help pages. The Top three are educated guesses and observation, based on what memory is in use, and some documentation on apple's dev center developer.apple.com/library/mac/documentation/Performance/…
    – wrossmck
    Commented Dec 4, 2013 at 10:32
  • 2
    What exactly does the number in the compressed memory column mean? If the activity monitor says a given process uses 621.4 MB of memory and 615.4 MB of compressed memory, does that mean that the process is really only using up 6 MB of memory? Or that 615.4 out of 621.4 MB was compressed down to some unspecified size? Or something else? Commented May 2, 2015 at 16:49
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    On Sierra, at least, "Real Memory" can be significantly less than "Memory". i.imgur.com/RvAG9oWl.png
    – Phil Cohen
    Commented Oct 28, 2016 at 2:17
  • 1
    @JamesKPolk From what I can tell, the "Compressed Memory" process column is pre-compression. "Memory" is then "Real Memory" + the post-compression size. E.g., right now my Finder has Memory 613M, Real 556M, Compressed 429M. 613-556=57M, which isn't reported in another column. The system-wide "Compressed:" entry is, I think, the actual space used by all processes post-compression though. On my system, the top 3 process alone report over 3GB "Compressed", but the system reports only 1.36GB total.
    – matthias
    Commented Nov 28, 2016 at 20:42

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