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I have these stats, but is the system really using almost 4GB?

40 MB free, 1 GB Wired, 2 GB Active, 988.9 MB Inactive, Used: 3.96 GB, VM size: 193.83 GB, Page ins: 9.71 GB, Page outs: 2.59 GB, Swap used: 3.19 GB

This Inactive memory... Can't I use it? How should I interpret these stats?

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This looks like a duplicate of:… – Nix May 12 '12 at 21:58
See… about the swap usage when there's inactive memory. – Nickolay Dec 30 '12 at 22:57

I suspect you probably are using Mac OS 10.7 which is very memory hungry. As noted by kccqzy, the purge command may reclaim some inactive memory if you have XCode installed.

Something you should try is closing and reopening applications which are memory hungry or have a tendency to leak memory (Safari, Firefox). Alternatively, restarting your computer would also accomplish the same though take considerably longer.

To answer the question, are you really using 4GB of memory. Yes. Technically more if you take note of the "Swap used:" stat.

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I'm using Mac OS Snow Leopard 10.6 I think. So I conclude I'm really using all the memory and needing to buy more. – Ed Pichler May 16 '12 at 19:02

Yes, you can. Fire up and run purge. It will force disk cache to be purged. It is likely that the inactive memory will become much less and more free memory will become available. A temporary side-effect is that your system will be a bit slower just after running purge.

EDIT: If you cannot find the command, you may have to install Xcode, which is available for free from Mac App Store or Apple Dev Website.

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command not found – Ed Pichler May 15 '12 at 2:28
Forgot to mention Xcode. Edited. Sorry. – kccqzy May 15 '12 at 11:59
And some references, if you need to be convinced: ( and (…) – kccqzy May 15 '12 at 12:04

You're only using three GB, not four. Both Inactive and Free memory are available for applications to use. The difference is that Inactive memory is memory that used to be in use. It will retain its contents until it is needed for something else. That way, if the same program that used it before runs again, OS X doesn't need to reload anything that is still in memory. Free memory, on the other hand, is memory that currently has nothing of value in it.

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You missed the 3+ GB of swap used. Judging from that and the high page-ins/page-outs the system needs more memory. – Nickolay Dec 30 '12 at 22:56

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