MacOS – Disable inactive memory swapping

macosmemoryvirtual-memory

Background:

OSX keeps some information in RAM that is no longer used, but was recently used. This is called inactive memory, and is intended to improve user experience by speeding up some actions:

For example, if you've been using Mail and then quit it, the RAM that
Mail was using is marked as Inactive memory. 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 it from the
slower drive.

Sounds questionable (just how much can you possibly speed up Mail on a quad-core machine?), but at least that couldn't possibly hurt since such memory can be treated like free memory! Actually, it can:

In OS X, when the number of pages in the free list dips below a
computed threshold, the kernel reclaims physical pages for the free
list by swapping inactive pages out of memory.

This makes me question whether OSX' memory handling is in fact superior; after all the same page mentions that

Paging of any kind… affects performance negatively because it
forces the system to spend a lot of time reading and writing to disk.
Reading a page in from the backing store takes a significant amount of
time and is much slower than reading directly from RAM. If the system
has to write a page to disk before it can read another page from disk,
the performance impact is even worse.

Issue:

This (paging out inactive memory and the subsequent severe performance degradation) is likely to occur even during trivial tasks such as multi-tab browsing, even if the number of simultaneously open tabs is kept within reasonable (both Safari and Chrome appear to be guilty of adding to inactive memory whenever a tab is closed).

The machine becomes hardly usable when this happens – even opening up Terminal and purging memory can take a few minutes.

Question:

Can swapping inactive memory be turned off? Or, with inactive memory not being essential, can it be disabled completely?

Alternatively, is there a daemon or a preferably Apple app available that monitors inactive memory usage (an OSX oxymoron) and when it goes over an arbitrary amount (half a gig?) a purge is performed?

Best Answer

I've been researching this very question for some time, and I've come to the conclusion that while there are several so-called "solutions" offered in various places, none of them really fixes the problem or makes the symptoms go away. The best I've found is techniques that change when inactive memory is swapped, such as the use of the purge command you referred to. I believe that MacLemon is correct that swapping can be disabled only in entirety and not selectively, and so disabling the swapping of inactive memory would in practice be equivalent to disabling virtual memory altogether—which could be a workable solution on a machine with really large amounts of RAM, but is impractical on machines with a low maximum RAM capacity, such as MacBooks or Minis.

The reason that none of the ad-hoc purging solutions make the situation significantly better is that there are really only two ways to force OS X to clear inactive memory: the purge command, or forcing allocation of all free memory (and since I'm not certain what method purge uses, these two may actually be more or less the same). purge, as you mentioned, takes a non-trivial amount of time to complete. Allocating all free memory simply accelerates the process by which the contents of inactive memory would be swapped out naturally, and so still takes up the same large amount of system resources. The only advantage of either of these solutions is that they allow you to choose when the swapping occurs, so it can be done before you would be normally trying to allocate that memory to something else. Whether that is actually useful to you depends on several factors, so the simplest way to find out is to just try it.

I've been testing a few of the memory-clearing utilities out there, and I've found that for me, manually forcing inactive swap requires more active monitoring of memory levels than is practical while I'm actually working, and using a utility that automatically forces the swap when free memory drops below a certain threshold is no better than letting the OS do it on its own, as I still have no control over when the swapping will occur and my apps will SPOD. So while there is an app that will do exactly what your alternative question asks for, that doesn't actually make the situation any less painful.

Until Apple's OS development team decides that the memory management system isn't working the way it's supposed to, and they figure out a way to make it work properly, the only real solution is to identify which apps are generating the most inactive memory and stop using them. In my situation, this has meant changing web browsers. I've been testing a variety of them, and Chrome is so far the one that seems to generate the least inactive memory, probably in part because every tab and every extension runs as a separate process, allowing the OS's native memory management to treat each one separately in terms of prioritizing swapping. Safari is the worst I've tried; starting with Safari 5.1, I could open a few tabs, do absolutely nothing, and watch the inactive memory rise rapidly in Activity Monitor—it would easily go from <1gb inactive to >3gb inactive within about five minutes, run the swap cycle, and then do the exact same thing. (Granted, memory that is doing nothing is exactly what makes it inactive in the first place, but it shouldn't create more inactive memory than was allocated active in the first place.) Safari 6 under Mountain Lion is a bit better, but not better enough to be worth switching back. Firefox, not being based on WebKit, ought to be better about this than either Safari or Chrome, but it has its own legacy of memory management problems, including a history of memory leaks, that make it no better in practice.

What would really fix the issue is if there was an option, likely a hidden option in the OS, telling the OS to simply dump the contents of inactive memory when needed instead of swapping its contents to disk. But I don't expect Apple to ever make such an option available.

Related Solutions

MacOS – force the Mac to make better use of the inactive memory

You can think of inactive memory like free memory that's been reserved for a recently closed application, but ready to be consumed by other uses when the 'truly' free memory has been exhausted. If you have 1 GB of memory (for the sake of making this discussion easier) on a freshly booted system, imagine the OS uses 300 MB and you open an app that uses 200 MB. You now have a total of 500 MB used, and 500 free. If you close that app that was using 200 MB, the memory is marked inactive and you'll now have 300 used (OS), 200 inactive, and 500 free. If you open an app that requires 600 MB, it'll use the free memory then pull from the inactive pool.

You can free inactive memory by opening Terminal.app and typing (without the $):

 $ purge

You can, also configure MySQL to use more memory but if you're not having performance problems I'd leave everything alone:

For more information on inactive memory in OS X:

MacOS – Trouble Understanding Inactive Memory in MacOSX

I have found, on my machine, that Mac OS X is a little slow to page the right things out.

Whenever doing anything particularly memory hungry, I will often quit a few applications to help it out. Safari is a huge memory hog if it has been running for a while and is top of the list of applications to quit. In my case this was when I tried to run VMWare Fusion without quite enough actual RAM.

You probably know this but here are a couple of links to Apple documentation which describe memory usage and virtual memory on OS X:

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

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

Here is the explanation that is given: The inactive list contains pages that are currently resident in physical memory but have not been accessed recently. It's hard to get an exact definition of what this means but is seems to include a file system cache as well as actual inactive memory.

Inactive memory is probably a difficult thing to handle on an interactive GUI driven OS where a user may choose to switch to any running application at any time and providing responsiveness in this sort of situation is important.... but the flipside of that is that in a situation like yours the OS doesn't quite know what the right thing to do is.

It is possible to purge (some of) the inactive memory by using the purge command. This may need to be installed by installing the CHUD tools but you may already have that installed. I haven't tried using the purge command so YMMV...

I've rambled on a bit and may or may not have answered your question. After writing all this out, I found another question on this site which much of the same info that might also help.