Memory vs Real Memory in macOS Activity Monitor

Puzzling over the memory numbers in Mac Activity Monitor? Trying to understand which apps take up most of the memory on your Mac? Find out here how to interpret memory figures and statistics (e.g., “memory” vs “real memory” vs “real private memory”) on macOS.

First, I See Particles of Light

The term “particle” sure is suggestive… These light particles may be tiny, minuscule and teensy, but they are somewhere—are they not?—flying around carrying… light?

As bosons, photons (the light particles) have no mass, however, and no extension, of course. They occupy no space, and like waves on the ocean’s surface or people throwing their hands up in a stadium, are what happens to space or time, in a manner, rather than anything in them.

Now, before we puzzle over interpreting the quantum-physical nature of electromagnetism, how about tacking the interpretation of another matter that seems to be nowhere at all at any time (computer memory)?

How to Interpret “Memory” vs “Real Memory” in macOS Activity Monitor

Activity Monitor lists the following statistics for system and application memory usage.

Data on the Activity Monitor “Memory” Tab

Physical Memory

This is the amount of RAM (random access memory) installed in the computer as physical chips that can hold volatile data. It is measured in GB, and typical numbers are 4 to 32 GB.

More physical memory is better, as it allows the Mac to access more data faster.

Memory Used

The used memory is the amount of physical memory that holds data. It is made up of 3 parts: wired memory, app memory and compressed memory.

App Memory

Application memory is the amount of physical memory actively in use by applications. That includes the app in the foreground, for instance, music playing in the background or the browser window you’ve just minimized.

When physical memory becomes tight, app memory not recently used can be compressed.

Wired Memory

Wired memory is the amount of physical memory used by macOS itself (not individual applications running on it). This comprises the system kernel that managers memory itself, for instance, or drivers for hard disks and displays, for instance.

This memory cannot be compressed, and apps cannot use it.

Compressed

Compressed memory is app memory that has been comprised to reduce its size in physical memory. The data is still kept in physical memory, but made smaller (like a ZIP file). Both compressing the memory and uncompressing it (when you return to an app you had in the background, for example) takes time, but less time than swapping the data to the hard disk would take.

Only limited amount of app memory can be compressed, because a fraction of physical memory must always be free and unused, some memory will be used by the app currently in use, and wired memory is reserved for the operating system.

Cached Files

macOS keeps recently used files in or preloads files from the hard disk to physical memory for faster access. For example, a web site’s content may be kept in memory after you close its tab or navigate away from it so returning to it will be near-instantaneous (instead of reloading the page from the internet or the disk).

This memory is free to use for apps or the operating system, i.e., app memory will not be compressed or swapped out instead of cached files.

Swap Used

App (and to some degree operating system) data that

• does not fit into physical memory and ideally
• has not been used in a while

will be moved to the hard disk instead. This frees up physical memory for use by the operating system and the applications currently in use, but it is also a slow process: swapping out memory to the hard disk takes time and resources, and—more importantly—loading memory from the swap area can take very long.

This is why switching to an app you have had minimized or in the background for a while can take as long or longer than launching the application in the first place.

If you see a large amount of swap used while you are using the computer as you normally would and experience the slow returning to apps with applications you have used recently, the Mac may have a shortage of physical memory.

Activity Monitor Memory Columns

Real Memory (RSIZE)

Real memory is the amount of physical memory currently in use by the app. This includes both memory shared with other processes and applications as well as memory used only by the individual app. Consequently, the number is hard to interpret and can serve only as a rough guide to how much space an app takes up in RAM.

This corresponds to resident_size in task_info().

Real Private Memory (RPRVT)

Private real memory is the space used in RAM for one application or process exclusively. This does not include memory in use by the app together with another process. You can use this is a rough guide to the relative sizes of apps in physical memory.

Real Shared Memory (RSHRD)

Shared memory counts the date in physical memory used together with another process. For instance, one browser process will use private memory for web site data or the text you type on the page as well as shared memory, for instance for interface elements common to all browser processes.

Note that $$real memory ≠ real shared memory + real private memory, but $$real memory ≥ real private memory.

It is not possible to add up shared and private memory to arrive at the physical memory used by a process. The physical memory will always be at least as bit as the private physical memory, however.

Purgeable Memory

macOS lets programs mark memory as purgeable. Purgeable memory is never written to a swap file when memory gets tight. Instead, it is simply discarded to free memory fast.

Types of data in purgeable memory are caches and other data that is kept in memory just in case, will likely not be used, and can be easily re-created when needed.

Compressed Memory

How much memory has been compressed in physical memory for an application or process.

Memory

The Memory column counts all memory to which the app or process has written. This includes data currently in physical memory (RAM), compressed in RAM or written to the swap file on disk.

This number is the best condensed gauge for how much memory an app uses, and how much it contributes to your Mac running out of physical memory or having to resort to compression and swapping. Use this to identify web site process that hog memory as well, for example.

The memory usage reported by Activity Monitor corresponds to the command line command footprint. Using footprint, you can get more detailed insight into what makes up that memory usage (as well as an alternative method to compute the number).

Virtual Memory (Shown When You Inspect a Process)

Virtual memory for an individual process counts all memory requested by a process, whether it is currently in use or has long been freed for new use by this or another process. This can also include large amounts of memory reserved “just in case” by a process but not currently in use.

This number is consequently largely meaningless when you try to find out how much memory an application is currently using.

How to Interpret “Memory” vs “Real Memory” in macOS Activity Monitor: FAQ

What do the memory pressure colors mean?

The memory pressure diagram and its colors are a quick guide to the state of available physical memory:

• Green: There is enough physical memory to use the currently running applications swiftly.
• Yellow: Physical memory is running low, and macOS is compressing memory to free space with some performance impact.
• Red: Data is written from physical memory to a swap file, which can slow down the compeer considerably both during swapping out and reading back the data.
  

What should I look for to identify apps and processes that hog memory on my Mac?

To find the apps, processes and web pages that use lots of memory, look for (and sort by) the columns Memory (for overall memory demands) and Real Memory or Real Private Memory (for current RAM usage) columns.

(How to interpret memory vs real memory in Activity Monitor tested with macOS Sonoma 14.0–14.3 and Ventura 13.3; updated April 2024)