Why does my mac use virtual memory when I still have free physical memory?

Question

I have a 2011 i7 quad core mac, I was hoping it would scream. Most of the time it does. However when trying to edit within FCPX I get a very disappointing experience with many pauses and pin wheels if I don't close every single other program.
I have 8GB of physical memory and when i'm experiencing these problems I see that i still have 1-2 gb of physical memory free or inactive. At the same time FCPX is only using 2gb of memory. I just happened to keep an eye on the VM page in/outs and noticed them going up.
Right now i'm doing some browsing and emailing, that's about it.. its sat with over 4gb of memory free or inactive and yet still the page in/outs is still going up occasionally. It's currently at over 2 million page ins, and over 1 million page outs.
So with so much physical memory free why is this happening!? At the moment the mac feels nice and responsive, but if i start trying to use FCPX i'll start to experience these slowdowns, stalls... whenever i see these i see my main hdd is being accessed whilst the pinwheel is displayed.. I mean i get it, its VM, the hdd is too full, a bit fragmented perhaps, its stalling... but i've got gigs of memory sitting free or inactive... why wont the OS use it!!!
Would my experience improve if i took the plunge and got 16gb of memory instead of 8gb!?
Thanks for your help!

Answer 1

Because without virtual memory, managing computer RAM is a royal pain in the ...
Virtual memory cost you nothing, and gains you huge benefits, even if you do not notice it
What cost you is when you need more real RAM than is available, and things are thown out of RAM, either back to the original file it came from (Read Only information), or pushed out to the swapfiles (/var/vm/*).  Then the system has to wait for slower disk access.  But even this is better than not being able to run the apps until you quit something else.
(speaking as someone that starting his professional life working with 1" punch paper tape, 80 columns cards, 7-track and 9-track mag tapes, 1MB disks (you heard me right 1 Megabyte), etc..., and trust me when I tell you that virtual memory is a god send to software development).
There are a lot of problems running a modern operating system with out virtual memory.  For example all the shared libraries and frameworks that provide services to an application would all need to be compiled into the application, which means every application gets bigger and instead of having a single copy of the shared library or framework, you would have dozens of copies wasting your RAM.
Without virtual memory, you would be required to find a contiguous chunk of RAM to run your application.  Think of this like going out to dinner by yourself, you can find any available table, but if you go to dinner with your extended family, you need a table for 10 to 15, and if you are going to dinner with your high school graduation class, you will need hundreds of seats all next to each other and a very large table.  In the later situations you have to wait until the resturante has enough contiguous space, which means you have to wait until other diners finish.  There may be lots of empty tables, but they are not together, and your group wants/needs to sit together.  Virtual memory allows gathering any 4K chunk of RAM, building a virtual memory map for all those random 4K chunks, and make it look like one big contiguous chunk of RAM, so you can run your application right away, no waiting.
Going back to shared libraries and frameworks.  This code will need to have addresses resolved so they branch to the correct locations during execution, and it will need to have addresses resolved on where its program variables are located in RAM.  Using virtual memory, you can local a shared object into RAM, then place it in everyone's virtual memory map at the exact same RAM address.  This means everyone can use the exact same code, and since everyone is using it at the same RAM address, it makes life so much easier for the operating system (translation, less work, less wasted CPU time, faster execution).
When a program wants to grow, for example a web browser loading a web page (and its images) into RAM, it needs to allocate additional RAM.  In the contiguous RAM model, you need to get control of the RAM that imediately following your program, but if that RAM is being used by someone else, you have to wait until that program goes away.
Virtual memory provides protection from another program looking at and modifying your program's RAM.  Malware would just love for virtual memory to go away.
You want virtual memory.  What you do not want is excessive paging activity.
If you are concerned, then you can launch Applicaitons -> Utilities -> Terminal.  Once you have a terminal command prompt, enter the following command:
sar -g 60 100
which will tell you the number of 4k pages written to /var/vm/pagefile ever minute for 100 minutes (modify the numbers to suit your tastes).  You can then go about your normal usage, and come back later to see how much you have been using the pagefiles.  If you have mostly zeros, and an occasional small burst, this is noise, and not worth worrying about.  If you have sustained pageout activity, with higher numbers, then you should either consider running less things all at the same time, or looking for an application that is being greedy with its memory use (or has a memory leak), OR get more RAM for your Mac if you need to do all those things at once.
But do not complain about virtual memory.  Life would be much worse without it.  Then again if you have a better idea, write a research paper, and get operating system vendors (as well as hardware vendors) to implement your ideas.  I am serious, as I've seen many accepted computing ideas be overturned by good new ideas.