Ever wondered what all the different memory types are that are shown in the pie chart at the bottom of the system memory tab in your Mac OS X Activity Monitor? If you’re used to the memory usage information that Windows shows, then the OS X descriptions might be a bit confusing. The reason for this is because there are actually more types of memory shown.
OS X Memory Types
In Windows all you see are what is free and what’s in use.
So what are these two extra types of memory that OS X shows?
This is exactly what it sounds like. Free memory is memory that hasn’t been written to yet. This will generally be at it’s highest straight after you turn on your computer.
This is the first memory option that you probably won’t be used to seeing. Wired memory is what is required if an application cannot be run, or has portions that cannot be run from the hard disk in the form of a swap drive. This means that this portion of memory will always be used as long as the application is running, it cannot be pushed to the hard drive if it isn’t being used. For the most part this shouldn’t cause you any problems as OS X is very good at handling your RAM efficiently.
Active memory, like free memory is exactly what it sounds like, memory that is currently being used by applications. This memory can be pushed to the swap drive if need be, such as if an application is minimised for a long time and the memory is needed for other programs that are being used more frequently.
This is the second memory option you may not be used to seeing. Logically speaking you could probably expect inactive memory to be the same as free memory, if you didn’t already have a free memory option. Have a look at the total used memory in the screen shot above. If you add it all up, inactive memory is included in the used memory. This can be quite confusing if you don’t know what it actually is.
Since it’s clear that inactive memory isn’t free memory, it appears a lot like you have this portion of memory that isn’t available to be used. In the screen shot above, you will see at the time I took it, I had 527.6mb of RAM listed as inactive, so out of my 4gb of RAM, that’s 1/8th of my total that appears to just be missing. At the time of writing this 810mb is inactive. So what’s going on?
Inactive memory is memory that has been previously written to but hasn’t yet been wiped to make it free memory. So if you have Safari open, then you close it. An hour later you may want to open it again, if the inactive memory containing Safari hasn’t been written over by that time, then Safari will load faster than if it had to be completely loaded into the RAM.
Basically the concept is that until the RAM is needed for something else, the application stays in it, even if you’ve already closed it. This is why inactive memory can sometimes get so high, yet free memory be so low, especially if you haven’t rebooted in a while.
Inactive memory is also used if an application does not get accessed in a certain period of time, despite being open. If this is the case and the application does not require wired memory, then it gets moved from active memory into inactive memory. After some time if it has not been pulled back to active memory and the inactive memory is needed for something else, it is then pushed to the swap drive.
What do the other options mean?
You may have noticed some other options beside the pie chart in the Activity Monitor, such as:
- VM size
- Page ins
- Page outs
- Swap used
So what are they? You might recognise the term “swap” from Windows, I also mentioned it above, but the others may not be as familiar, though Windows does use what is effectively the same functionality to help improve performance, particularly on low end computers without much RAM.
The VM size stands for virtual memory size and is the maximum amount of space that OS X is allowed to use on the hard drive for memory allocation should there not be enough RAM. It doesn’t really matter if it’s a big number or not, you’ll notice on mine that it is 183.77gb. That seems huge compared to Windows which recommends using approximately double your physical RAM capacity. OS X automatically sets this size and it’s largely irrelevant. For example, I only have 6gb fee on my hard drive at the moment, so there is no way that 183.77gb could be used, even if I could manage to figure out some way to actually use that much memory! As long as it’s big enough for your needs it doesn’t really matter how big it is.
Page ins occur when something needs to be read from the swap drive into the memory. For example when you need to edit a document you have had minimised for a couple of hours, it may be stored in the swap drive. Once you pull it back up, it will be moved from the swap drive into active memory so that you can edit it quickly, much quicker than if it stayed in the swap drive.
Page outs are the opposite of page ins. Page outs are when something is moved from memory into the swap drive, usually from inactive memory. The inactive memory then becomes free for some other application to use it.
This is the total amount of space on your hard drive that is actually being used as virtual memory to help speed up your computer. In my case at the time I took the screen shot, I was using 3.34gb of space on the swap drive.
I hope this helps you make a bit more sense of your Activity Monitor!
For more information, see Apple’s Knowledgebase article on reading system memory usage.