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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?

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seeing the tags I assume they still haven't fixed the paging of inactive memory in Mountain Lion? –  Pieter Aug 20 '12 at 12:53
    
@Pieter: apparently there's nothing to fix because current implementation is just as Apple intended :) –  o.v. Aug 20 '12 at 21:56
    
they confirmed this as 'working as intended'? I don't get what the benefits are of swapping inactive memory to disk. –  Pieter Aug 21 '12 at 12:36
    
"… subsequent severe performance degradation …" – despite what's in the Mac Developer Library, I'm not convinced that all degradations are severely negative. Suspect that in some cases there's a more negative effect from the OS attempting to restrain its approach to external swap files: Mac virtual memory: how to make OS X create swap (page) files as freely with a normal boot as it does with a safe boot (swapfile1 etc) –  Graham Perrin Apr 27 '13 at 9:37
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@Graham Perrin: when I say severe, I talk from experience (albeit, not necessarily a shared one) - if you work with massive amounts of data and perpetually run out of HDD space before having it archived, the swap file gets more and more fragmented, increasing access time etc. (wouldn't be a performance issue with an SSD I imagine, but sounds like something that would shorten its lifespan). I'm not advocating this usage pattern but this is just something I keep running into. –  o.v. Apr 28 '13 at 0:00

2 Answers 2

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.

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How much RAM does your Mac have? Also, browsers (especially Webkit ones) are known to use huge amounts of memory these days. Inactive memory is put to good use opposed to free RAM which just sits there consuming power. In the case of a browser this enables you to undo closing a tab and still have it's history preserved, etc.

You can completely disable memory swapping by running sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.dynamic_pager.plist and a restart. You cannot selectively disable swapping.

You can undo this by running sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.dynamic_pager.plist and restarting again.

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"… Inactive memory is put to good use …" – +1. –  Graham Perrin Apr 27 '13 at 9:24

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