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.
Best Answer
This is a design decision that is made to handle situations where operators of the device choose to ignore low storage space warnings or are not present to see them and react appropriately.
From your comments, you described that the system works well enough to let you inspect storage, choose to delete apps and data and continue working - that’s the case in my experience. The system sacrifices the ability to launch new apps, install updates and do other non-critical functions when storage space is close to empty.
In most cases, the system is able to send a sequence of disk space “warning” and disk space “critical” signals to the OS, the first party apps and third party apps requesting they purge any cached storage, but in the end, you can overload the filesystem and this is the last ditch attempt to get the device in the hands of someone that will realize that files or apps need to be deleted to resume normal operations.
For the nerds that program, there are entire API and processes around this to let an app know how much space is available for opportunistic storage and critical storage:
One of the better articles on this is:
In the end, it’s up to the programmer of an app if they want to check and let you know they may stop working or if they want to be simple in design and let you figure out if your device is full in other manners. Neither decisions is completely right or completely wrong. The OS will nag you about low storage, so most apps let that handle education these days and they just postpone large downloads when things get tight.