I noticed that if I use Mac OS X for a while, I get really low on memory and things start to get slow. I have been suspecting that something was not quite right, because in “Activity Monitor” I would look at the memory usage and it didn’t seem like I was using that much memory.

I added up all the memory from all processes shown in “Activity Monitor.” It adds up to only 1769.2 MB, even though I’m supposed to have 4GB memory. I only have 100MB of memory free. Why is that? It’s showing 625MB as wired, 1.63GB as active, and 1.65 GB as inactive. It says that 3.90 GB are being used.

  • The next time that happens try purge (flushes disk cache or inactive memory) or sudo periodic weekly (removes memory-mapped scratch files) instead of iFreeMem, and report back if either of them helped. – Lri Apr 14 '11 at 17:14

You don't have a problem here.

100 + 625 + 1024 * (1.63 + 1.65) = 4083.72, about 4GB. “Used” are all those categories but "free" combined.

Inactive memory was used before, but is no longer in use. The system can quickly reallocate it if there's a need. See Apple’s support article on this.

That’s why other memory monitors—such as iStat menus if I recall correctly—display both “free” and “inactive” as unused/free.

As for the difference between “Wired + Used” and the sum of all processes,

  • Make sure you’re displaying all processes in Activity Monitor.
  • Don’t add the numbers up by hand. You can select all processes, copy and paste into Numbers to sum real memory up (filter through a text editor first to remove " MB" first to allow calculations, if necessary).

Your machine getting slow has likely another reason.

  • No, it's caused by the memory. I ran an application called iFreeMem and I got over 2G freed. Then my system started to act normal. – tony_sid Apr 14 '11 at 6:11
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    @OSXJedi Read the Apple article. Those programs do nothing else but allocate as much memory as possible (in your case, 1.65GB+100MB), and then release it. Also, see here. – Daniel Beck Apr 14 '11 at 6:13
  • It solves my problems though. No more momentary freezes caused by the system trying to manage memory. If you read about iFreeMem it says that some programs do not exit cleanly, and the memory they were using becomes unusable unless something like iFreeMem is used. – tony_sid Apr 14 '11 at 6:19
  • @OSXJedi I'd write that too if I were to charge 10 GBP for such a program. Have you considered the actual issue being your hard disk? – Daniel Beck Apr 14 '11 at 6:25
  • @OSXJedi Try the following: Stop all disk activity (downloading, running games, active web browsing, etc.). Run the iFreeMem tool and watch disk activity in Activity Monitor at the same time. Is there any disk activity while the tool runs? – Daniel Beck Apr 14 '11 at 17:25

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