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A common phenomena in my day to day usage (and several other's according to various posts throughout the internet) of OS X, the system seems to become slow whenever there is no more 'Free' memory available. Supposedly, this is due to swapping, since heavy disk activity is apparent and that vm_stat reports many pageouts. (Correct me from wrong)

However, the amount of 'Inactive' ram is typically around 12.5%-25% of all available memory (^1.) when swapping starts/occurs/ends.

According to http://support.apple.com/kb/ht1342 :

Inactive memory

This information in memory is not actively being used, but was recently used.

For example, if you've been using Mail and then quit it, the RAM that Mail was using is marked as Inactive memory. This Inactive memory is available for use by another application, just like Free memory. However, if you open Mail before its Inactive memory is used by a different application, Mail will open quicker because its Inactive memory is converted to Active memory, instead of loading Mail from the slower hard disk.

And according to http://developer.apple.com/library/mac/#documentation/Performance/Conceptual/ManagingMemory/Articles/AboutMemory.html :

The inactive list contains pages that are currently resident in physical memory but have not been accessed recently. These pages contain valid data but may be released from memory at any time.

So, basically: When a program has quit, it's memory becomes marked as Inactive and should be claimable at any time. Still, OS X will prefer to start swapping out memory to the Swap file instead of just claiming this memory, whenever the 'Free' memory gets to low.

Why? What is the advantage of this behavior over, say, instantly releasing Inactive memory and not even touch the swap file? Some sources (^2.) indicate that OS X would page out the 'Inactive' memory to swap before releasing it, but that doesn't make sense now does it if the memory may be released from memory at any time? Swapping is expensive, releasing is cheap, right?

Can this behavior be changed using some preference or known hack? (Preferably one that doesn't include disabling swap/dynamic_pager altogether and restarting...)

I do appreciate the purge command, as well as the concept of Repairing disk permissions to force some Free memory, but those are ways to painfully force more Free memory than to actually fixing the swap/release decision logic...

Btw a similar question was asked here: http://forums.macnn.com/90/mac-os-x/434650/why-does-os-x-swap-when/ and here: http://hintsforums.macworld.com/showthread.php?t=87688 but even though the OPs re-asked the core question, none of the replies addresses an answer to it...

^1. UPDATE 17-mar-2012 Since I first posted this question, I have gone from 4gb to 8gb of installed ram, and the problem remains. The amount of 'Inactive' ram was 0.5gb-1.0gb before and is now typically around 1.0-2.0GB when swapping starts/occurs/ends, ie it seems that around 12.5%-25% of the ram is preserved as Inactive by osx kernel logic.

^2. For instance https://apple.stackexchange.com/questions/4288/what-does-it-mean-if-i-have-lots-of-inactive-memory-at-the-end-of-a-work-day :

Once all your memory is used (free memory is 0), the OS will write out inactive memory to the swapfile to make more room in active memory.

UPDATE 17-mar-2012

Here is a round-up of the methods that have been suggested to help so far:

The purge command

'Used to approximate initial boot conditions with a cold disk buffer cache for performance analysis. It does not affect anonymous memory that has been allocated through malloc, vm_allocate, etc'.

This is useful to prevent osx to swap-out the disk cache (which is ridiculous that osx actually does so in the first place), but with the downside that the disk cache is released, meaning that if the disk cache was not about to be swapped out, one would simply end up with a cold disk buffer cache, probably affecting performance negatively.

The FreeMemory app and/or Repairing disk permissions to force some Free memory

Doesn't help releasing any memory, only moving some gigabytes of memory contents from ram to the hd. In the end, this causes lots of swap-ins when I attempt to use the applications that were open while freeing memory, as a lot of its vm is now on swap.

Speeding up swap-allocation using dynamicpagerwrapper

Seems a good thing to do in order to speed up swap-usage, but does not address the problem of osx swapping in the first place while there is still inactive memory.

Disabling swap by disabling dynamicpager and restarting

This will force osx not to use swap to the price of the system hanging when all memory is used. Not a viable alternative...

Disabling swap using a hacked dynamicpager

Similar to disabling dynamicpager above, some excerpts from the comments to the blog post indicate that this is not a viable solution: 'The Inactive Memory is high as usual'. 'when your system is running out of memory, the whole os hangs...', 'if you consume the whole amount of memory of the mac, the machine will likely hang'

To sum up, I am still unaware of a way of disabling Mac OS X from using swap when there still is 'Inactive' memory. If it isn't possible, maybe at least there is an explanation somewhere of why osx prefers to swap out memory that may be released from memory at any time?

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MotinMotin

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5 Answers

By definition, inactive memory is memory that is ready to be paged out, and paging it out might involve writing it to swap. This is not any kind of problem or issue that should be optimized; it is in fact OS X working as designed.

Unfortunately, tech support writers are not kernel developers, and the Apple Knowledge Base support article quote is just wrong when it claims that Inactive memory is memory unused by programs. When you quit a program, all of its resident memory becomes Free; it doesn't stop over in Inactive. However, the second link to the developer site describing how memory management works is a good resource, if read fully.

There are many misconceptions about what 'inactive memory' means in OS X. Contrary to the misconceptions, not all inactive memory is empty, unused, cache, or purgeable. In fact,

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