I’ve recently had some ask the question, ‘How do I know how much memory my Macintosh has?’ It’s very easy to find out how much memory your mac has – just go to the apple menu (top left of your screen) and select About this Mac.

You’ll then be given a screen that displays your processor speed and how much RAM is installed.

1. Go to your utilities folder.
   (Click on your desktop anywhere so that the finder is active.
   Hold down apple-shift-U, this will open your ‘utilities folder.)
2. Open ‘Activity Monitor’
3. Press apple-1 so that the main window of Utility Monitor is open.
4. Click on the ‘System Memory’ tab at the bottom of the window. This will display a little pie graph of how much memory you are currently using.

 
There are two important items to take note of ‘Free:’ and ‘Page outs:’

Free tells you how much free memory you currently have, the higher the better, and
Page outs tells you how many times your computer has had to slow down because it has run out of memory and needed to move some of it’s memory back to the hard disk temporarily to free up some space. (This would be the equivalent of your brain being too full so you have to write your thoughts down on paper to free up your headspace!) This really slows down your computer.

If the page out count is increasing as you watch it, this is not good, it means your computer has run out of free memory, and so temporarily is using the hard disk as memory. The faster the page out count is increasing, the worse the problem – your computer is struggling along and not performing as well as it could. You need more memory.

If you have had lots of free memory since the last time you restarted your machine, the Page outs count will be 0. This means your computer had enough memory and has not run out. You dont’ need more memory.

It may be that your Pagein/outs count is high, something like 50431/10850, but steady and this and not increasing. What this means is that some time in the past when you opened a lot of programs, you ran out of memory, but now you have enough. Reboot, check the Pagein/outs count again. It should be 0/0. If it is not 0/0, have a look at it while you open a program or two and see how fast it increases, the faster, the worse your system performance will be.

Here is a screen shot of my computer. My page in/out count is quite high – 66825/697, but I still have 189 MB free, and the page in/out count is not increasing at the moment. This means that sometime since the last reboot, my computer ran out of memory, but currently it is OK. If I reboot the page in/out count will return to 0/0, and stay there unless I go crazy opening lots of programs. I don’t’ need more memory.

To quote from apple:
Moving data from physical memory to disk is called paging out (or swapping out); moving data from disk to physical memory is called paging in (or swapping in)… Extended periods of paging activity reduce performance significantly; such activity is sometimes called disk thrashing.

One more note: Under Leopard rather than giving a ‘count’ of page in/outs, it gives a size in MB of the amount of RAM that has been pages in or out – the numbers are smaller but the same principals apply.