Memory Usage in OS X Explained

Tweaking OS X to get the best performance out of your Mac can be worth your time and money if it allows you to extend your older Mac’s life another year (or a few months until Apple releases that new model). Blindly following tutorials and tips can help but if you really want to get the best performance, you’ll need to know some theory. Let’s decipher OS X’s memory usage in Activity Monitor and look at how it affects your system.

First off – start up Activity Monitor and follow along. If you’ve never used it, it’s located in your Utilities folder (inside your Applications folder).

Activity Monitor by default shows only your processes. That’s great but it shows a very small portion of what’s going on in your Mac. So, look up top for the tab that says “My Processes”. Click on it and choose “All Processes”.

Activity Monitor

Memory Usage Explained:

We’re going to focus on memory usage, so at the bottom of Activity Monitor, click on the “System Memory” tab.

Activity Monitor

Activity Monitor now shows some statistics on your Mac’s memory usage. What does all this mean?

Free: This is the memory amount that is currently unused by the system. It’s free and available for applications and processes to use.

Wired: Wired memory is memory that’s in use by the system and can not be written to disk – it must stay in memory. Wired memory depends on the applications running and how many of them.

Active: Active memory is the amount of information currently being stored in RAM that’s being used or recently used.

Inactive: Inactive memory has been used by the system but not recently. It’s being held in RAM in case it’s needed again but if another application or process needs the memory, it will take it from the Inactive memory and OS X will swap the unused information to disk. If the memory is called again, it will be moved back into Active status.

Used: Used is the total amount of memory currently in use by OS X.

VM size: VM size is the hard disk space being used as virtual memory. Virtual memory is when your system needs more RAM than it has, it used the hard disk as memory. It will store inactive memory there, freeing up more space for Active processes.

Note: If you find yourself running low on free memory, the system will rely heavily on virtual memory and cause a noticeable slowdown.

Page ins: The process of writing information held in RAM to hard drive (virtual memory) and back is called paging. Page ins are when virtual memory is brought back into actual memory (RAM) from the hard drive.

Page outs: Page outs are the opposite of Page ins. When actual memory (RAM) gets full, OS X will Page out the inactive memory to the hard drive.

Swap used: If you noticed the incredibly large number for VM size, and thought “that’s an awful lot of my hard drive”, don’t worry. It’s the limit of how much space OS X can use if it needs. In reality, Swap used is the actual amount of space that OS X is really using for virtual memory.

How memory usage affects OS X:

Memory is one of the cheapest and easiest ways to get an older system to perform better. As the years go by, applications (and OS X) use larger and larger amounts of memory. It’s almost as if OS X “outgrows” your system. When your Mac uses more and more virtual memory, it really slows down.

Looking at the System Memory portion of Activity Monitor can help you decide if more memory will help or not. Look for the following:

– A low amount of Free Memory (under 50mb).
– A large number of Page outs.
– A large amount of Swap used.

Also, keeping a good amount of space free (at least 2-5gb) on your hard drive will ensure that virtual memory works properly. If not, then things really become slow and unruly.

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