Understanding buffers/cache in linux `free -m`

Question

Just looking for some quick help regarding free -m output on Linux. Here's an example on one of my servers:

  $free -m
               total       used       free     shared    buffers     cached
  Mem:         14881      14813         68          0        262        883
  -/+ buffers/cache:      13667       1213
  Swap:         4095        240       3855

I understand the kernel can and will use available memory for disk caching to speed up the system.

Normally, I look at the second value in the "free" column, in the above case it's 1213. It's normally a lot higher than this.

I guess I am looking to understand what the two outputs next to '-/+ buffers/cache' mean. From what I can see, the system truly is low on memory (only 1213MB available for applications to use) and the OS actually needs 13GB for caching?

Answer 1

This has been answered in serverfault. Here is the relevant excerpt:

total = used + free

• shared / buffers / cached: This shows memory usage for specific purposes, these values are included in the value for used.

The second line gives first line values adjusted. It gives the original value for used minus the sum buffers+cached and the original value for free plus the sum buffers+cached, hence its title. These new values are often more meaningful than those of first line.

[End of excerpt]

Focusing on your situation,

• 14813 (used memory) - 262 (buffered) - 883 (cached) = 13668 (used by applications).

In the event an application needs more memory, it can be taken either from free memory or from cached/buffered, so:

• 262 (buffered) + 883 (cached) + 68 (not used at all) = 1213 (available to applications).

The system does not really need 262+883=1145 (not 13GB) for cache and buffers, but since it's there, it takes it (but gives it back if applications need more). Take a look at the Linux ate my RAM! page.

You can also use top (I personally prefer htop) to see the memory status and which applications are taking most RAM.

As an example, here is a screenshot of htop running in my RaspberryPi and the free -h execution:

raspberry ~ # free -h
             total       used       free     shared    buffers     cached
Mem:          183M       178M       5,3M         0B        63M        42M
-/+ buffers/cache:        72M       111M
Swap:           0B         0B         0B