http://www.linuxnix.com/2013/05/find-ram-size-in-linuxunix.html


free is a command which can give us valuable information on available RAM in Linux machine. But many new Linux users and admins misinterpret its output. In this post we will walk through its output format and show you actual free RAM.

Free command examples

Example 1: Display RAM details in Linux machine
Output:
Let us see what this table for you.
Line 1: Indicates Memory details like total available RAM, used RAM, Shared RAM, RAM used for buffers, RAM used of caching content.
Line 2: Indicates total buffers/Cache used and free.
Line 3: Indicates total swap memory available, used swap and free swap memory size available.
Let us dig more in to these lines to better understand it as Linux user prospective.
Line 1:Mem: 8027952 4377300 3650652 0 103648 1630364
8027952 : Indicates memory/physical RAM available for your machine. These numbers are in KB's
4377300 : Indicates memory/RAM used by system. This include even buffers and cached data size as well.
3650652 : Indicates Total RAM free and available for new process to run.
0 :  Indicates shared memory. This column is obsolete and may be removed in future releases of free.
103648 : Indicates total RAM buffered by different applications in Linux
1630364 : Indicates total RAM used for Caching of data for future purpose.
Puzzled with buffers and cache?

What is the difference between buffers and Cache?

A buffer is a temporary location to store data for a particular application and this data is not used by any other application. This is similar to bandwidth concept. When you try to send burst of data through network, if your network card is capable of sending less data, it will keep these huge amounts of data in buffer so that it can send data constantly in lesser speeds. In other hand Cache is a memory location to store frequently used data for faster access. Other difference between a buffer and a cache is that cache can be used multiple times where as buffer is used single time. And both are temporary store for your data processing.
Line 2: -/+ buffers/cache: 2643288 5384664
Then why my Linux machine is showing 4377300 as used RAM. This is because Linux counts cached RAM, Buffered RAM to this used RAM. But in future if any application want to use these buffers/cache, Linux will free it for you. To know more about this, visit this site.
5384664 : Indicates actual total RAM available, we get to this number by subtracting actual RAM used from total RAM available in the system.
So from today on words don't complaint that Linux ate your RAM, its our understanding of free command output which is the culprit and the teacher who thought us Linux. If any one asks what is the free RAM available, we have to give this number(5384664) instead of first line number(4377300) for free RAM available in your machine.
Line 3: Swap: 15624188 608948 15015240
This line indicates swap details like total SWAP size, used as well as free SWAP.
Swap is a virtual memory created on HDD to increase RAM size virtually. To know more about swap click here for creating swap partition and here for swap file creation.
Example2: Display RAM in human readable formats like in KB's, MB's, GB's, TB's
Output:
Note: If you observe with option –tera, the RAM size shows as 0, this is because my RAM is just 8GB and the output of free command will not support floating point numbers.
Example3: My actual RAM is 8GB but it is showing 7GB when using -g option, whats wrong with my RAM. This is because free command by default counts 1024 as power instead of 1000. To see difference use –si
With out –si
Output:
with –si
Output:

Example 4: Want to see combined size of both swap as well as RAM use -t option
Output:
Example 5: I want to see continuous varying of RAM usage for every second, use -s option to mention number of seconds
Output:
This is not a good way to display continuous output, we can use watch command to do iterations
In out next post we will see other RAM details.

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