Meaning of the buffers/cache line in the output of free

Question

Why does my server show

            total      used      free   shared buffers    cached
    Mem: 12286456  11715372    571084        0   81912   6545228
-/+ buffers/cache:  5088232   7198224
   Swap: 24571408     54528  24516880

I have no idea on calculating the memory in linux. I think it says that 5088232 is used where as 7198224 is free, meaning it is actually consuming 5GB of RAM?

Answer 1

The display columns mean:

• total: Your total (physical) RAM (excluding a small bit that the kernel permanently reserves for itself at startup); that's why it shows ca. 11.7 GiB , and not 12 GiB, which you probably have
• use: memory in use by the OS
• free: memory not in use

total=use+free

• shared/buffers/cached: This shows memory usage for specific purposes; the numbers are included in the value for "use". This is why the second line gives the numbers for used/free minus the sum cached+buffers.

The first line is about your physical memory (RAM), the last line "swap" gives the same information about swap space usage (i.e. memory contents that have been temporarily moved to disk). The second line gives the numbers for used/free minus the sum cached+buffers (because that value is often more meaningful).

To actually understand what the numbers mean, you need a bit of background about the virtual memory (VM) subsystem in Linux. Just a short version: Linux (like most modern OS) will always try to use free RAM for caching stuff, so "free" will almost always be very low. Therefore the line "/+ buffers/cache:" is shown, because it shows how much memory is free when ignoring caches; caches will be freed automatically if memory gets scarce, so they do not really matter.

A Linux system is really low on memory if the free value in "/+ buffers/cache:" gets low.

For more details about the meaning of the numbers, see e.g. the questions:

http://serverfault.com/questions/23433/in-linux-what-is-the-difference-between-buffers-and-cache-reported-by-the-fr

http://serverfault.com/questions/9442/why-does-red-hat-linux-report-less-free-memory-on-the-system-than-is-actually-ava

Answer 2

The buffers/cache line shows you how much memory is used by programs (first of those columns), and is free for programs to use (second column). It's a fairly esoteric number, and not something you'd really look at much in practice.

Answer 3

For anyone needing an even more simple, step-by-step explanation, check http://thoughtsbyclayg.blogspot.com/2008/09/display-free-memory-on-linux-ubuntu.html, his explanation helped me a lot.