MacMost Now 390: Understanding Memory Use with Activity Monitor

You can use Activity Monitor to see how your Mac is using memory. But what do free, active, inactive and wired mean? How about page ins and page outs? Plus, learn to use them to figure out if you need more memory.
Video Transcript / Captions
Closed captioning for this video is available on YouTube: MacMost Now 390: Understanding Memory Use with Activity Monitor.

Hi this is Gary with MacMost Now. On today's episode, let's look at your Mac's memory usage in Activity Monitor. So Activity Monitor can be found in your applications utilities folder and useful for a lot of different things like seeing what processes and applications are running in your Mac. One thing you can use it for is to see how much of your memory is being used. So when you run Activity Monitor, it looks like this and shows you processes running at the top of the screen. The bottom of the screen, you have several tab choices including system memory. Click on that and you get to see exactly how your Mac is using memory. So first let's look at the left side. You can see it says free, wired, active, inactive and used. So basically divide that into two pieces, free and used, the top and bottom parts. And that adds up to your total amount of memory. In this case I have 10 Gigs in my MacPro. So the difference between active and inactive is active memory is memory that's now being used by the applications for something, its inactive memory is memory that was used by the applications recently and maybe used again but your Mac is allowed to clear that out if something else needs it and the applications will reclaim it later if it needs it then. So basically wired is important stuff that your Mac cannot remove from memory, active is stuff that's not as important but it's being used at the moment and inactive stuff is not being used at the moment so your Mac can reclaim it if necessary. So before we go further, it's important to understand the concept of virtual memory. So you have physical memory, say you have 4 Gigs of RAM in your Mac and every time you run an application, it would take up another portion of RAM. Eventually you would run out of space and you try to run another application, and it would say 'sorry can't run application, out of memory'. We used to get this all the time, maybe 20 years ago before the introduction of virtual memory. So the way virtual memory works is it uses all of the free space you have on a hard drive as virtual RAM. So in other words you might have a hundred Gigs of free space, so your Mac pretends it has a hundred Gigs of RAM. And it uses your hard drive for that. Now it takes longer for it to access the information on your hard drive than it does on your RAM so what it does it it says I'm gonna use a hundred Gigs of RAM on the hard drive. And the 4 Gigs that access the most often, I'm gonna put that in memory so I can access it quickly only when I need to go outside of that 4 Gigs of the most commonly accessed memory will actually call to the hard drive and ask a bit of memory there. Think of it as how you may take notes in a class. You may write down everything the instructor says, but the most important piece of information, the one's you know you're gonna need, you keep up here as well. And if there's something that you can't remember, you can refer back to your notes and its all there. In this case the note-book is your virtual memory in your hard drive and your RAM is your memory up in your head. So now back to Activity Monitor. On the right side we can see VM size which is basically the amount of free space I have in my hard drive that's available to the user's virtual memory. At the bottom there is swap used. That's how much of that that I'm using for virtual memory right now. None of it I've got 10 Gigs of RAM and not using at all at the moment so I'm not using any virtual memory. Now page ins and page outs give you an idea of how your Mac is using your virtual memory. So pages are blocks of memory. Page ins are pages used in RAM. So here I'm using 3 Gigs and of course I am using memory cause I'm using applications in the system so there's gonna be a number here always. And this is fine no matter how big it gets, it's fine because were using a RAM. Now page outs are when a page is used outside of memory. In other words its read in from the memory of the hard drive. So here I'm at zero which is great meaning I'm not using virtual memory at all. Now if there's a small number here, that's fine because applications are going to use virtual memory even if you have free space available. Some applications will definitely use virtual memory because they know some information is just isn't enough for it to take up hard drive space. Now if your page outs gets to be large, like say they get to be about the size of page ins or larger, that means you're using virtual memory a lot. And that's when you may want to consider adding more RAM to your MAC. So here's a look at monitoring your MAC's memory usage using Activity Monitor and a little bit about virtual memory as well. Hope you found it useful and until next time this is Gary Rosenzweig with MacMost Now.

Comments: 12 Responses to “MacMost Now 390: Understanding Memory Use with Activity Monitor”

    9 years ago

    Nice explanation. Thanks Gary.

    9 years ago

    Excellent and very informative.

    Rich O
    9 years ago

    Awesome explanation! much appreciated! thanks.

    Dale A
    9 years ago

    You answer some of the questions I have always wanted to ask!

    Phyllis S.
    9 years ago

    I love MacMost Now. I follow on Twitter and Facebook. Gary keeps me up to date with easy to follow explanations. I’m waiting for his iPad manual!

    8 years ago

    Very good, very complete and simple to understand. Particularly liked the look
    at page in/page out which often gets overlooked by explanations of the pie chart.

    7 years ago

    I’d like to monitor the performance of the video memory in my Mac Pro. Does anyone know of a solution for me?

    7 years ago

    you’re so awesome! thanks alot

    7 years ago


    What about Swap Used:? What does that measure?

      7 years ago

      That’s how much memory has been copied to the hard drive — virtual memory, in other words.

        7 years ago

        Thank you for the quick reply. I just noticed you mention swap used. I must have been up too late.
        This is a great overview for anyone wondering about their RAM and it’s capacity sufficiency. :)

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