What's going on?
Linux is borrowing unused memory for disk caching. This makes it looks like you are low on memory, but you are not! Everything is fine!
Why is it doing this?
Disk caching makes the system much faster! There are no downsides, except for confusing newbies. It does not take memory away from applications in any way, ever!
What if I want to run more applications?
If your applications want more memory, they just take back a chunk that the disk cache borrowed. Disk cache can always be given back to applications immediately! You are not low on ram!
Do I need more swap?
No, disk caching only borrows the ram that applications don't currently want. It will not use swap. If applications want more memory, they just take it back from the disk cache. They will not start swapping.
How do I stop Linux from doing this?
You can't disable disk caching. The only reason anyone ever wants to disable disk caching is because they think it takes memory away from their applications, which it doesn't! Disk cache makes applications load faster and run smoother, but it NEVER EVER takes memory away from them! Therefore, there's absolutely no reason to disable it!
Why does top and free say all my ram is used if it isn't?
This is just a misunderstanding of terms. Both you and Linux agrees that memory taken by applications is "used", while memory that isn't used for anything is "free".
But what do you call memory that is both used for something and available for applications?
You would call that "free", but Linux calls it "used".
|Memory that is||You'd call it||Linux calls it
|taken by applications
|available for applications, and used for something
|not used for anything
This "something" is what top and free calls "buffers" and "cached". Since your and Linux's terminology differs, you think you are low on ram when you're not.
How do I see how much free ram I really have?
Too see how much ram is free to use for your applications, run
and look at the row that says "-/+ buffers/cache" in the column that says "free". That is your answer in megabytes:
$ free -m
total used free shared buffers cached
Mem: 1504 1491 13 0 91 764
-/+ buffers/cache: 635 869
Swap: 2047 6 2041
If you don't know how to read the numbers, you'll think the ram is 99% full when it's really just 42%.
How can I verify these things?
See this page
for more details and how you can experiment with disk cache.