Over 3GB of swap files with 16GB of RAM?

Constantly? I wouldn't worry so much. Virtual memory in Mountain Lion is highly optimized - especially for SSD drives. Much of that swap space doesn't really exist. There is sophisticated fakery going on underneath to keep RAM-hungry applications from allocating memory they aren't really using.

Do you have a virtual machine installed (Fusion, Virtual Box etc)?

megagram wrote:

Do you have a source for that information? I would love to read about it.. Thanks.

It is pretty standard practice for modern virtual memory systems. I have no idea where I saw it specifically. It was probably some developer documentation. Apple isn't doing anything particularly special. Pretty much all modern virtual memory systems work the same way. It is just that no other people obsess over the details like Mac users do.

I can tell you that you should ignore any suggestions or adivce to try to change virtual memory in any way. As a developer, I have been running Mountain Lion for some time and using it heavily with Xcode. The virtual memory system in Mountain Lion is the best I have ever seen. There is nothing an end user can do to improve it - only damage it. If you are having problems, the cause is elsewhere.

How about answering my question?

See Below

megagram wrote:

K as far as I understood modern OS's, they write memory to disk when physical RAM is exhausted. My physical RAM has not been physically exhausted and yet I still see high numbers of pagefiles. This is my concern; especially since I have an SSD that can't be replaced I'd rather not accelerate its demise with unecessary writes.

That's the thing - it is way more complicated than that. That is how virtual memory worked 20 years ago. It is best to avoid looking too closely at memory values. When people do that they start seeing problems that aren't there.

I'm not sure how you can say mountain lion's virtual memory is optimized for SSDs and that most of the swap space doesn't exist, even though it exists in the form of pagefiles without backing it up with concrete sources.

I say that because Apple is moving to SSDs for their new machines, so that is where the focus is. Even with 16 GB, any application could exhaust your memory if allowed. MacOS X uses those swap files to keep that from happening.

Virtual Box could still be the cause, try this: reboot, do not start VBox, give it an hour or so (doing whatever you normally do) and observe the swapfile, then add VBox to the mix and check again.

I doubt you will have any luck stressing Mountain Lion's VM system. While running Xcode heavily I have been able to get my free RAM to less than 10 MB, but it recovered right away with no noticeable performance impact.

You really don't need to worry about "swap used". I was unable to find a clear definition of entirely what this value represents. There are a number of possibilities, any or all of which are plausible. It could be memory-mapped files that are on disk to begin with but are now considered to be in memory. They could be initialization code like shared libraries that are in the application memory space but are shared. Therefore, there is no need to duplicate them in RAM. They can be permanently swapped to disk. They could be backing store of the application such that if it does need to be swapped, the OS doesn't have to worry about writing it. I don't know the real answer.

I wouldn't worry about it unless you're seeing a corresponding performance hit. If you are, then if you can reproduce it consistently, that might be worth filing a bug about or something. Otherwise, it is probably just writing cold pages out to disk for performance reasons.

Well I *am* seeing a slowdown. With Snow Leopard there were 3 swapfiles totalling 256 M Bytes. With Snow Leopard there are 6 files totaling 2 G Bytes, and I have seen this go up to 7 files and 3 G Bytes. And I do not have a lot running. Free memory is usually pretty low as reported by top, a few hundred MBytes. I have 4 G of RAM. Unlike Snow Leopard, these swapfiles never go away even after quitting applications. Applications were slow to launch as swapfiles were being written to or read from. I think Mountain Lion has a serious virtual memory problem.

Nathan Goldshlag wrote:

Well I *am* seeing a slowdown.

The cause is not related to virtual memory. The longer you persist in thinking that it is, the longer you will suffer from your slowdown.

I beg your pardon. It takes unnaturally long to launch an app and I look at the 1G swapfile and see that this one and the others have just been updated and you tell me it is unrelated? Sorry. Writing to disk is relatively slow.