Types of memory: Why do private and shared not add up to real memory?

Question

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?

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MacOS Catalina 10.16.6

Answer 1

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.