Memory vs Real Memory!?

Discussion in 'OS X Mavericks (10.9)' started by Traverse, Jun 28, 2014.

  1. Traverse macrumors 604


    Mar 11, 2013
    Activity Monitor by default shows a "memory" column. I always thought that it's numbers seem a bit low.

    I just discovered on Macrumors (thanks W0lf) that there is another column called "real memory" that uses like 2x as much in some instances (see screenshot).

    Which one is actual usage? What's the difference?

    Also what is "private memory" which is another option.

    Thanks, I'm just trying to learn more about computers.

    Attached Files:

  2. w0lf macrumors 65816


    Feb 16, 2013
    I believe the difference is that "Memory" represents active unpurgable memory whereas "Real Memory" is the total of active memory and virtual memory for a process.

    I could be wrong though as Apple doesn't really have documentation that directly talks about the activity monitor column.

    Here's probably the best sumuary of activity monitors memory stuff:
  3. pmau, Jun 28, 2014
    Last edited: Jun 28, 2014

    pmau macrumors 68000

    Nov 9, 2010
    Real Memory is what is commonly referred to as RSS or Resident Set Size in Unix.
    This is the number of "pages" (regions of physical memory) that are currently occupied by the process.

    Processes also have "virtual" memory that counts as part of of its size but is paged in on demand. For example a memory-mapped file (i.e. a movie) will be paged in chunk by chunk whenever an address of its virtual mapping is accessed.

    Real memory can be paged to swap, because it is not file-backed (it's allocated on the heap), which reduces process size but is paged in from swap on demand.

    Also, real memory regions can be pinned to be resident all the time.
    Hope this helps.


    Sorry, forgot to mention:

    "Memory" is usually smaller than "Real Memory" because System Frameworks are mapped in each process (indicated as "Real Memory"), but are shared between processes. Therefore the "Memory" column only counts regions that are not shared, i.e. the memory allocated only by this process.
  4. Traverse thread starter macrumors 604


    Mar 11, 2013
    Thank you both for the information.

    I'm surprised Apple doesn't make this information more readily available.
  5. grahamperrin macrumors 601


    Jun 8, 2007
    Activity Monitor and understanding memory (content that's normally seen in Help Centre) is not entirely correct. At least: the explanation of virtual memory is wrong.

    Better: How to use Activity Monitor – Apple Support (referred from

    It's terribly difficult to find memory-related documentation that's both user-friendly (plain english) and not open to misinterpretation.

    It's not Apple's style, but a comic strip approach could be good for people who have minimal technical knowledge and wish to learn a little more without being overwhelmed.

    Food for thought, from 2008: The Google Chrome Comic
  6. m4v3r1ck macrumors 68020


    Nov 2, 2011
    The Netherlands
    Great post, love the Comic Style. Thanks for sharing! :cool:
  7. Traverse thread starter macrumors 604


    Mar 11, 2013
    Thank you for the Apple support link, they make those really hard to find sometimes.

    I remember reading that comic years ago, good times...:)

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