[oivindi]

I´ve been monitoring my system activity lately with the Activity Monitor, and I noticed something that I´m not at all familiar with with regards to memory usage in Panther / OS X.

Please note that I haven´t been observing this at all before, so I´m guessing it´s normal behaviour - I just want it explained...

Memory usage is, as you know, split into four: Wired, Active, Inactive and Available. There´s also a category called "used" (I think - my system language is Norwegian).

What happened yesterday when I was running a DVD-to-divx program, was that while the software was doing its processing the amount of available RAM slowly counted down from about 800MB - I have 1GB total - to about 15-20MB (!).

Why does this happen? And why isn´t the RAM released after for example quitting the software?

It seems the amount going from "available" ends up in "inactive".

What´s the difference between for example "available" and "inactive"?

Any explanation of this phenomenon would be greatly appreciated.

Thanks!

/Øivind/

[alien]

Hei fellow nordmann.

This question has been asked many times before here at the forums. Try searching a little.

[msuper69]

If your system is working ok for you, don't worry about it.

Let the system software worry about memory utilisation; that's the whole point of using an OS like OS X.

[malvolio]

For most purposes, that "inactive" memory is available. Once you quit the DVD-to-divx program, OS X is perfectly happy to start using the memory that was freed up for other programs. But it will designate that memory as "belonging" to the DVD-to-divx program until such time as another app or process needs it, just in case you decide to relaunch the program and run the whole thing over again.

[JKT]

Wired = memory used for core system processes - therefore not available to other apps.

Active = memory that is currently in use by active processes (e.g. the app you are working in, any apps running processes in the background)

Inactive = memory that has been used by an application, but the processes are currently dormant or the app has been closed. Left in RAM so that if you use the app again, it is quicker to launch and run. This memory will be the first to get paged out to the disk if RAM is filled.

Free (unused) memory = RAM that is available for use

When you first boot up, especially with 1GB RAM, you will see a lot of free RAM. With prolonged uptime you will start to see that nearly all the RAM will be occupied by wired, active and inactive memory. This is completely normal. If a process requires RAM that isn't physically available, then inactive memory will get paged to the hard drive (as part of a swapfile). If that isn't enough, then you will start to get disk thrashing as the app has to constantly page out and page in memory from the hard disk.

If an app's inactive memory was paged out to the hard disk, when it is needed again (e.g. on reopening of the app), it will be paged back into memory from the hard drive.

If you want an always visible cue as to what is happening with your memory, I recommend downloading the freeware Memory Monitor which sits in your Dock and shows you a running profile of your systems memory usage: