How to disable Mac OS X from using swap when there still is "Inactive" memory?

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, Active memory can be cached or purgeable as well, if it has been recently accessed. Much inactive memory also contains data that cannot be simply discarded. If it were discarded, programs would crash, because the discarded pages would have contained valid data (as the quote from the OS X developer's side says,) and programs expect data they have stored in (virtual) memory to not just disappear.

Inactive memory contains the same types of data as active memory. The only difference is that OS X has noticed that some chunks of memory have not been read from or written to in a while.

The reason that OS X classifies some memory as inactive and other regions as "active" has to do with paging out. When memory runs low, you are going to have to page out some data. The question is, which data? If you page out data that a program turns out to immediately need again, it wastes time and accomplishes nothing. So you want to page out memory that a program won't immediately need to use again.

Anticipating which pages are likely to be unneeded in the future is difficult because a program can use its virtual memory however it likes and not tell the OS anything about what its plans are. But as a heuristic, most programs are "sticky" in their memory usage; if they haven't used some piece of memory in a while they are likely to continue not using that memory, and likely to continue using memory that they have recently used.

So when the OS decides to page out some data, it takes the strategy of swapping pages that haven't been used recently. This is why OS X sorts the memory that is being occupied by programs into two piles of "active" and "inactive." The above posted link to the Developer site, if read fully, tells how that process happens:

• When memory starts getting low, the OS starts going through the active memory pages, and sets a flag on each.
• If a program reads or writes to a page, the flag is cleared.
• If, after some delay, the flag is not cleared, that page gets sorted into the "inactive" pile.
• If an "inactive" page is accessed by its program, it is put back into the "active" pile.
• When memory runs out, the "inactive" pages are paged out.

Note that this sorting process to decide which memory to swap out is similar across all modern operating systems. Linux has the same two lists of active and inactive pages, as described in Understanding the Linux Virtual Memory Manager. Windows might use something a bit different with more than two classes of recency; I can't find a recent, reliable technical description at the moment. More implementations are discussed at the Wikipedia page entitled "Page replacement algorithm". The only difference with OS X was how the statistics were shown: someone decided it would be a good idea to show separate numbers for active and inactive in top or Activity monitor. In retrospect this was probably not such a good idea (and this has changed in OS X 10.9.)

This process of setting and clearing flags and maintaining active/inactive heaps does take a little bit of processor power. For that reason, OS X doesn't do it when there is a lot of free memory. So the first programs you start up will show up as all "active" memory until free memory starts running low.

So, as you start from a blank slate, and open more and more programs, you can expect to see the following progression in Activity Monitor:

• First, there is a lot of "free" memory and very little inactive. This is because the memory flagger hasn't started running.
• As the amount of free memory drops, OS X will start running its memory flagger, and you will start to see the amount of "inactive" rising. Each bit of "inactive" was previously "active."
• When you run out of free memory, pages from the "inactive" pile will be paged out. The memory-flagger will also be running full tilt sorting out memory into active and inactive. Typically, you will see a lot of "inactive" while swap is being written to, indicating that the memory-flagger is doing what it is supposed to.

Pages must be classified as inactive before they are swapped out. That is what the quote from the Apple Developer site means when it says "These pages contain valid data but may be released from memory at any time." This is in opposition to Active pages, which will not be released until after they have been demoted to Inactive. There are various ways of releasing pages; if the page was mapped from a file and has not been modified, it can be deleted immediately and re-read on demand. Similarly if it is memory that had been previously swapped out and not modified since it was swapped in. Programs can also explicitly allocate cache and purgeable memory, to store data that can be forgotten and recreated on demand (but the reason a program would allocate cache is if it takes significant time to recreate that data.) But much of inactive memory is memory that programs have written valid data to, and paging out this data requires writing to swap.

Therefore looking at the amount of "inactive" memory in Activity Monitor, and seeing that there is a lot of inactive at the same time as the computer is writing to swap, only tells you that the system is working as designed.

There is also a confusion between inactive memory and file cache. I'm not sure why there is that confusion, because Activity Monitor already lists them under separate headings. Cache is memory used to store recent data that have been read to or written from the file system, in case they need to be accessed again. When memory is low, OS X does tend to get rid of the the cache first. If you have swap thrashing, and Activity monitor shows a big pile of cache (NOT inactive) then that would be a problem. But inactive memory is a different thing.

If in doubt, ignore the distinction between "inactive" and "active." Regard them as being one lump of "memory used by programs" and add the two numbers together. This is what every other operating system does when telling you about memory usage.

NOTE for OS X 10.9: Mavericks introduced "memory compression" which is, more or less, another layer of swap. Active pages now get classified inactive, then compressed (which might show up as Kernel memory depending on what tools you are using,) then written to swap as memory usage increases. Mavericks has also has stopped showing separate numbers for active and inactive in Activity Monitor, since it turns out not to be a useful thing to look at, especially given the misconceptions surrounding it.

There is currently no easy way of adjusting the swappiness (or so it is called) behavior of macos X. There are a few hacks available though (requires developer account & SDK):

http://cestdelamerde.com/archives/22-Killing-Mac-OS-X-Swapping-How-To-Disable-dynamic_pager.html

http://dropsafe.crypticide.com/article/3848

Good luck!

Postscript. I guess you may want to read this answer (also by me) to have a more general review of what active, inactive and other memories in MacOSX: Wired Memory vs. Active Memory in OS X

Not a permanent solution, but at least it can help reclaim some inactive memory, potentially avoiding the dreaded swapping: http://itunes.apple.com/nz/app/freememory/id460931672?mt=12

The tool is free and pretty easy to use. Once started, select its "Free memory" option in the system toolbar/menu.

Unlike the ActivityMonitor memory display, it only shows free memory, which seems to be a better indicator of swapping going on or not.

Starting with OS X 10.5 there are evident memory management problems in MAC OS X. The web was already then cluttered with complaints about system slowing down dramatically after some time. Back then i had slower machine, Mac Mini with 1GB RAM, so i (wrongly) concluded that it was due to inferior hardware.

Now i have 2010 MBP, core i7, 8 GB RAM, dual GPU. Mac os X Snow Leopard was pain, but after migrating to OS X Lion, working some serious stuff on MAC started to be a nightmare.

I finally managed to reproduce the problematic scenario, so i run the test and recorded the screen, into video.

MAC OS X Lion performance problem - broken memory management

I run the tar+bzip command, which is basic unix stuff, on the large amount of picture files, in my Pictures/ folder. Just before start, i run the "purge" command, to delete inactive/cached program data.

You can see on the video that free memory starts to drop very fast, and inactive is constantly rising. If you take a look at "bsdtar" command, it takes only a fragment of RAM, so the problem is not in this process. You cannot say that it is a program memory leak, because then the problem would not be in inactive ram, rather in active/wired.

When the free memory dropped below 100mb, i started some apps, like Safari, iPhoto and MS Word, and you can see in the video, that it takes even minutes to start an app, when normally (when there is free RAM), it would take some 3-5 secs to load.

I run the same scenario and the same commands on my Linux Centos 6 box, no problem there ! Memory usage is some 10-20mb, no problems with cache/buffer.

The memory management must be very broken in Mac OS X !