Monday, October 15, 2012

Linux ate my RAM!!!

Have your ever thought that a Linux Server is eating your RAM? Well, don't panic, your RAM it's just fine.

Here are some answers to some sort of questions that maybe you're asking yourself about:

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 immediate. 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 agree 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 isYou'd call itLinux calls it
taken by applicationsUsedUsed
available for applications, and used for somethingFreeUsed
not used for anythingFreeFree

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 free -m and look at the row that says "-/+ buffers/cache" in the column that says "free". That is your answer in megabytes:

root@anneke:~ # free -m
             total       used       free     shared    buffers     cached
Mem:          1504       1491         13          0         91        764
-/+ buffers/cache:        635        869
Swap:         2047          6       2041
root@anneke:~ #

If you don't know how to read the numbers, you'll think the ram is 99% full when it's really just 42%.

Hope this helps to clear some confused minds, like was mine.

Thanks a lot to the knowledge of Gerardo A. and