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Trouble Understanding Inactive Memory in MacOSX

I'm just wondering why does the Activity Monitor or Free Memory app says that I have 867 MB free (while writing this question).

Currently I'm running the following apps: Chrome (1 tab), Skype, Activity Monitor.

Here is a shot from activity monitor. Can someone explain to me, with 8GB RAM how is it possible to have these statistics?

activity monitor

Free: 869.1MB
Wired: 1.23GB
Inactive: 3.75GB
Used: 7.15GB
VM Size: 254.82GB
Page ins: 4.34GB
Page outs: 0bytes
Swap used: 0bytes

I have a mid-2012 MacBook Air with a 1.8 GHz i5, 8 GB of RAM, upgraded to Mountain Lion.

P.S. Why is the VM size 243GB? I don't even have that much space on my drive.


Mac OS X allocates and uses RAM differently than is reported by Windows and can be confusing at times. Microsoft originally built Windows to keep as much memory free as possible. Apple's memory model follows a guideline that empty RAM is a wasted resource that could be better utilized to enhance performance.

Here is a quick and somewhat simplistic breakdown of how your RAM is currently being used:

  • Wired: memory reserved by the operating system
  • active: memory used by currently running programs, e.g., Chrome, Skype, Activity Monitor
  • Inactive: memory used by programs you previously ran since your last reboot, but have since quit. Most people use a few programs all the time and may switch between them by quitting one program and then launching another. Rather than releasing the memory to be "free", the inactive programs are cached and ready to run again. Some or all of this memory will be combined with Free memory as needed by the system or programs.
  • Free: memory as of yet unused since the last boot.
  • Used: total of wired, active and inactive.

The VM Size is the "total memory footprint available to the computer". When you use VM (virtual memory), the processor swaps unused portions of chip memory with hard disk memory, giving the usability of a larger memory footprint when it might not be available.

Your total Swap used is 0 bytes used, so you're managing to do everything in real or chip RAM without having to use the slower SSD. The zero page outs shows you made a good choice going with the 8GB or RAM.

The VM statistics are reset every time you reboot.

Hope this helps demystify your computer.

additional info added in response to Ando's comment

Let me give you a different perspective on your memory use:

Rather than ask "1.2GB, really?", ask how many services, caches and other system files the Mac loads on boot rather than individually as needed. As long as the amount used remains constant, you should be fine. If the wired amounts tends to dramatically climb as you use it, then you have something to worry about.

Additional wired memory can be used by system preferences, menu bar programs, and other background processes.

Comparing your system to a 2GB Linux machine is similar to comparing your computer to my Mac Pro with 24GB of RAM, multi-terabyte internal arrays, numerous graphic cards, etc. Both were designed and configured for different purposes and footprints.

You can get a good idea of the individual app footprints in the Activity Monitor window above the RAM statistics.

Hope this helps!

third edit in response...

I don't have a lot of experience with Chrome, but don't forget that it has a full instance of Flash living within it. Most of my circle works on iOS or OS X so I run FaceTime instead of Skype.

Here are some procedures to follow:

  • Check and see if you have the latest versions, especially for Chrome. Known memory leaks tend to get corrected over time.
  • When diagnosing memory issues, it's best to begin with a fresh boot.
  • As you run Skype and Chrome, track their memory uses over time.
  • As you open another tab, how much additional memory does it grab?
  • When you close that tab is all the additional memory released?

Rather than spend a lot of time worrying about minimum memory footprints and such, track system performance with your clock... Does the computer still feel fast and peppy to you?

| improve this answer | |
  • Tom, thank you for your broad explanation of how Macs use memory. Do you think its normal for Mountain Lion to be using 1.2GB of ram just to run (for what exactly?!) and one more thing - 1tab chrome, skype and activity monitor - 1GB seriously?! I don't get it. How is it possible to have 2gb used, for what is my question. There are computers with 2GB which are running all sorts of stuff. I wish i had my linux web server with 2GB of ram - it would be able to launch a satellite in space. I don't get it. – Ando Dec 3 '12 at 0:34
  • Glad to be here. I remember when all this was a mystery to me. – TomUnderhill Dec 3 '12 at 0:43
  • So what do you think about the 2 gigs being used by the system and two small apps? – Ando Dec 3 '12 at 1:36
  • Reply to the third edit. I've noticed that Chrome consumes a lot of memory over time. Currently i have three tabs open and that costs me 480MB (three websites, no heavy if any flash). After a fresh reboot I have about ~6GB free. I don't have startup apps, nor i have anything in the dashboard. The reason I'm concerned is that I'm that type of user that opens a ton of apps when working. 10+tabs/~10apps like photoshop/illustrator/word/outlook/chrome/skype/netbeans/ftp/terminals hence the 8GB. I just wanna know where my ram goes. I also think that mem leaks affect battery life when mem not in use. – Ando Dec 3 '12 at 4:01

The VM size is a theoretical amount of memory allocated by the system to all the different applications that claim it. However, only the "used" portion of this memory actually exists.

Your inactive memory is VERY high. There is probably an application that has a memory leak and hasn't free'd is memory. Try running the purge command in terminal to clear out most of the inactive memory.

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  • after purge i got inactive: 680MB Active: 1GB Wired:1.23GB and free:5.05GB I'm still confused if it is normal to have Chrome(3tabs currently), skype, terminal and activity monitor which eat up 3Gigs. I used to be a windows user, and in the worst versions of windows, the situation was way better. – Ando Dec 3 '12 at 0:21
  • Chrome usually leaves A LOT of inactive memory. I don't know why but it's quite annoying. The important part is that you don't start swapping memory, cuz then system performs drops significantly. – Alexander - Reinstate Monica Dec 3 '12 at 4:21
  • OSX is known to have a very efficient memory management. The use of 'memory cleaners' (of which purge is one) should be discouraged. Purge should be used as a diagnostic tool, not as a solution to free up inactive memory. When more memory is needed, just add more memory to your machine (Or buy a new machine... Air) – CousinCocaine Sep 12 '13 at 12:50
  • Real efficient: apple.stackexchange.com/questions/52013/… Because 2.21GB of inactive memory is not worth sacrificing to free up any more than 70 MB of RAM, right? – Alexander - Reinstate Monica Sep 12 '13 at 18:25

Browsers take up huge amounts of memory. I run Safari, Firefox, and Chrome and each of them end up using 1GB+ within a few hours of heavy use.

As long as you are not seeing a lot of page outs, you have sufficient memory. That said, my personal experience is that beach balls increase when free memory gets to zero so I restart my browser if free memory is zero and I start to see beach balls.

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