Answers to previous questions define types of real memory used by a process roughly as follows:

  • Real memory: the amount of physical memory allocated to a process (eg the number of pages)
  • Real shared memory: the amount of physical memory shared between multiple processes
  • Real private memory: the amount of physical memory not shared with other processes

I would have expected that real = real_shared + real_private, but this doesn't seem to be the case. I've noticed that real memory can be more or less than shared + private, depending on the process.

Can someone please clarify the differences between these types of memory and why they don't add up?

MacOS Catalina 10.16.6

  • What version of macOS? – mmmmmm Jul 21 '20 at 15:57
  • There's more than just shared and private memory. There's reserved which is reserved by the OS, wired which is allocated and reserved to/by an App. There's more, but I'm forgetting them at the moment - point is Apple has given you an infographic display of what they think is important to the user, not a complete view of everything that's going on. – Allan Jul 22 '20 at 14:36

The answer to your question is given in Mac OS X Process Memory Statistics by Mike Ash:

RSIZE: The total amount of physical RAM used by this process. (This is not equal to RPRVT + RSHRD because they measure address space, but this measures actual memory.)

Note :

  • RSIZE is Real memory
  • RSHRD is Real shared memory
  • RPRVT is Real private memory

You can find these abreviations by hovering the mouse on the Activity Monitor column headers.

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