My Mac is slow, does it need more memory?

Memory chipsSome people have found that after upgrading to the latest version of OS X on their Apple Mac, it runs more slowly than it used to.

One of the reasons for this could be that the Mac is running short of memory for apps and a quick look at Activity Monitor shows that all the memory in use. Is it really out of memory? It might not be and it's not obvious how to read Activity Monitor's memory display.

Take a look at the screen shot below, which is Activity Monitor in Mavericks running on a MacBook with 4GB of memory. The table at the bottom is the important part and it shows the memoru usage. On the left side it clearly says that there is 4GB of physical memory and that 3.99GB is used. So it is using all buy 0.01GB of memory.

OS X Activity Monitor memory usage

Clearly the Mac is out of memory and is struggling to run the operating system and apps. No it isn’t and the left hand side of this table is very misleading. Ignore everything that it says!

On the right-hand side is the real memory usage and it shows that there is plenty of free memory. This isn’t clear from the figures unless you know how to interpret them.

App Memory is the amount of memory that is being used by applications that are running.

Wired Memory is memory that is currently in use and cannot be used by apps or swapped to disk (swapping blocks of memory to disk is a way of freeing up memory for running more apps).

The amount of memory currently being used is the sum of App Memory and Wired Memory:

1.62GB + 596.2MB = 2.22GB

So what is File Cache? You can think of this as the free memory. When you run an app, it is loaded into memory from the disk drive. If you quit the app, it stops running, but it stays in memory.

If you run the app again it doesn’t need to be loaded from the disk drive and it can simply be activated in memory again. This process is much quicker, particularly with old style mechanical disk drives, but less so with solid state disks.

The File Cache can contain more than one inactive app and with a gigabyte or more to play around with, OS X can hold several apps in memory just in case you want to use them again. How many times a day do you check your email? Some apps are run frequently and so it makes sense to store them in memory.

Suppose you run another app on disk. It obviously needs memory to run, so it grabs whatever it needs from the File Cache. This might mean that one or more apps currently in the File Cache are overwritten, but they are merely copies of files on disk and they can be reloaded if they are needed again.

Basically, File Cache is free memory, or at least memory that can potentially be used by any apps you want to run.

There is a chart in the middle of the table at the bottom and in the screen shot it is green. This is good and green means that there is plenty of memory for running apps. If you see amber sections in the chart, it means that OS X is beginning to run out of memory. If you see a lot of red then it means that OS X is seriously short of memory.

If Mavericks is slow on your Mac and you suspect that it is short of memory, a quick look at the chart on the Memory tab of Activity Monitor will tell you. If it is frequently red you need to upgrade the RAM in your Mac.

Intro image: public domain from



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