How to check memory usage on your Mac

September 13, 2011 in Mac OS X

Does your Mac run slowly, sometimes turning your cursor into that darn spinning beach ball? It may be because your running applications have used up all your memory. In the video below I show two tools you can use to check how much of your Mac’s memory is being used, and what numbers to look for.

As a bonus you’ll also see how easy it is to download and install a new widget on your Dashboard.

Do you do any of these?

  • Use your Mac for many days (or weeks?!) without restarting it
  • Run many applications simultaneously
  • Open dozens of browser tabs or windows, or many large documents
  • Use a web-based email program like Gmail

If so, congratulations, you are getting your money’s worth! Heck, I do ALL of those. But the more of those you do, the more likely your Mac will start slowing down due to lack of memory. But keeping apprised of your Mac’s memory situation is easy. Watch this video to see how.

Video too small? Watch full screen by clicking the YouTube Full Screen button button here

Monitoring your Mac’s memory

The Activity Monitor application that comes with Mac OS X (in the Utilities folder) is an adequate way of checking your memory usage but I prefer the Dashboard widget, iStat Pro (click this link to download). I usually check my iStat Pro readings a few times a day. It only takes a couple seconds if you know what to look for.

Whichever tool you use, the two most important measurements to check are your “free” memory and “swap used”. To see the swap number in iStat Pro, you must set its memory section to “Advanced”. Do this by clicking on the widget’s “i” button (in the upper left), clicking on the Sections tab, and changing the Memory setting to Advanced. If you notice your Mac slowing down and you see low “free” memory and high “swap used”, it’s time to start freeing up some memory.

Activity Monitor showing low free memory and high swap used

Activity Monitor showing low free memory and high swap used

iStat Pro showing low free memory and high swap used

iStat Pro showing low free memory and high swap used

Freeing up memory

The most thorough way to free up your Mac’s memory is to perform a restart. But if you don’t want to take the time for a full restart, here are some quicker things you can do to free up some memory and possibly speed up your Mac.

  • Close unneeded browser tabs or windows, especially the ones with long histories
  • Close web-based email tabs (like Gmail) and start a new session in a newly opened tab
  • Quit and restart applications that have been heavily used (lots of tabs, lots of documents, etc.)
  • Log out of your computer user name and log back in

After one or more of those steps, you’ll get closer to the memory conditions you had when you first turned on your computer.

iStat Pro showing high free memory and low swap, after a full restart

iStat Pro showing high free memory and low swap, after a full restart

So take a quick look at your Mac’s memory usage a few times a day and you’ll soon learn which of your activities tends to use up memory and at what memory levels your Mac starts to slow down. Then, the next time your Mac slows down you’ll be able to diagnose if it’s caused by memory issues. If not, then a bad Internet connection or slow website could be slowing your Mac. But if it is, at least you have some control over your memory usage and now you have some actions you can take to help solve the problem.

The Geek Box

Warning: The following is a week of my graduate-level operating systems course condensed into one paragraph!

When too many applications run for too long, they can use up all the available memory in your computer. After that, when one of those applications needs more memory (for instance, maybe you opened yet another tab in Safari) and there is no more unused memory, the system has to temporarily move (or “swap out”) the least recently used (but still running) application out of memory to a special location on your disk. Most of the time, using this “swap space” magically lets you do more on your Mac than if you were restricted to the computer’s physical memory.

But as the amount of swap space used grows, your Mac’s speed can degrade. In extreme cases, your Mac can get into a vicious cycle of swapping out application A to make room for application B, then swapping out B to make room for A, and so on. This is called “thrashing” and is often the cause of the dreaded spinning beach ball cursor freezing your Mac for a long time. That’s when shutting down some applications or (even better) restarting your Mac is the next order of business.

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