Do you need a memory cleaner and are they helpful?

Go to the Mac App Store, go to Categories, select Utilities and the number two app in the free chart is Memory Clean. It is second only to OS X Yosemite. It is a very popular app, but do you really need a memory cleaner? What does it do?

First, you need to understand how the Mac handles memory. I will be using Yosemite for this, but older versions of OS X are very similar, so everything here applies to Mavericks if you are still using it.

Go to the Applications/Utilities folder and run Activity Monitor. select the memory tab at the top and down at the bottom is a table containing various figures showing how OS X is using the RAM in your Mac.

This looks straightforward and in the left column it says that the physical memory is 4.00GB and the memory used is 3.66GB. Subtract one from the other and the amount of free memory is therefore 0.34GB or 340MB.

The Memory Clean utility from the Mac App Store confirms this:

There is a slight difference in MB, but that’s because the memory free has changed a little when I ran Memory Clean.

The problem is that this figure is wrong. Subtracting the memory used from the physical memory does not tell you how much memory is free.

Look at the right-hand side of the table in Activity Monitor. The App memory is the amount of memory used by apps that are running. The Wired Memory figure is also apps and OS X memory usage, (important memory not to be messed with).

The total memory used is App Memory + Wired Memory, which is 1.59GB + 585.1MB or about 2.18GB. Therefore the amount of free memory is 4.00GB minus 2.18GB, which is 1.82GB. This is significantly different to the amount that Memory Clean claims is free.

The confusion arises because of the way that OS X uses memory. Memory that is not used is simply wasted, so OS X puts it to good use. If you run an app, such as Mail, you will see that the App Memory figure increases and this is because more memory is in use.

If you quit Mail, the App Memory figure goes down as memory usage falls. However, the File Cache figure rises. When OS X has lots of free memory that isn’t being used for anything, it will hang on to apps you have quit just in case you want to run them again. Some people check their email 10 times a day and if the app is still in memory, OS X can restart it in an instant without having to find and load it off the disk drive.

Apps are cached when nothing else wants to use the memory. Rather than leave it empty and unused, OS X stores recently run apps in it.

This means that you can think of File Cache as free memory. If you run one or more big memory-hungry apps, OS X can abandon one or more items in the File Cache and give it to the new apps being run. Watch what happens when I run a bunch of apps.

Memory Used has increased from 3.66GB to 3.98GB and App Memory has increased from 1.59GB to 2.34GB because more apps are now running. The apps need more memory, so the File Cache is reduced and falls from 1.50GB to 1.10GB.

This is a balancing act that OS X constantly performs, using free memory as File Cache and reducing it as apps need memory, then increasing it when apps quit.

The scrolling Memory Pressure chart in Activity Monitor shows whether you are short of memory. Green is OK, orange is not so good and red is really bad. Notice that it is green in the screen shots, yet Memory Clean has changed to red in the menu bar to alert me that there is just 24.51 MB of memory free.

Memory Clean red alert! Only 23.42 MB free memory! But the green bar in Activity Monitor says that memory usage is fine. Activity Monitor is right and Memory Clean is wrong. The File Cache is 1.10GB, which, remember, is like unused memory. If yet another app is run, OS X simply discards something in the File Cache and uses the memory made free.

One figure in Activity Monitor I haven’t mentioned is Compressed. When OS X gets really short of memory, it compresses bits that haven’t been used for a while. It’s a bit like adding files you don’t use very often to a zip archive, only OS X does it automatically and at lightning speed on the fly.

So is Memory Clean and other memory cleaners (there are many more) actually useful? I am not convinced by what I saw when using the app. It appears to work because the free memory figure that it displays increases, but as pointed out, this figure is not calculated correctly and so is misleading. I would leave OS X to figure out the best way to use memory rather than try to force it to work in some other way.

Are there rogue badly written apps that mess up memory so badly that a memory cleaner is needed? I don’t know. All I can say is that I don’t have any. If you are the sort of person that leaves their Mac running for weeks at a time, just reboot it every so often.