How to Read and Understand the Linux free Command Output

 The free command is a valuable utility in the Linux arsenal, providing insights into your system's memory usage. It presents a snapshot of the memory utilization and helps you gauge the health of your system's RAM. In this article, we'll explore how to read and interpret the output of the free command to make informed decisions regarding your system's memory management.

The Basics of the free Command

Before diving into interpreting the free command output, let's understand how to use it. Open a terminal and simply type:

 

free

 

By default, this command displays memory statistics in kilobytes. To make the output more human-readable, you can use the -h flag to display the values in megabytes and gigabytes:

 

free -h

 

To display Memory in Gigabytes

 

free -g 

 

Sample output:

 

[root@node1 ~]# free -h

      total        used        free      shared  buff/cache   available

Mem:  15G          751M        1.2G        272M         13G         14G

Swap: 0B           0B          0B

 

Interpreting the free Command Output

The free command output is divided into several columns. Here's what each column represents:

1. total: This column represents the total amount of physical RAM on your system. It's the sum of used and free memory.
2. used: This column shows the total amount of RAM currently in use by the system.
3. free: This column displays the amount of RAM that is currently unused or available for new processes.
4. shared: Shared memory is memory that can be used by multiple processes. This column indicates how much memory is shared among processes.
5. buffers: Buffers are memory blocks used to speed up data transfer between the RAM and hard drive. This column shows the amount of memory used for buffering.
6. cached: Cached memory is memory used to store data from recently accessed files for faster access in the future. This column represents the amount of memory used for caching.
7. available: This column is available starting from free version 3.3. It indicates the estimated amount of memory that is available for starting new applications, without swapping.
   

 

Key Points for Understanding Memory Usage

Now, let's delve into some key points to help you interpret the free command output effectively:

1. Total Memory vs. Used Memory: Comparing the total and used columns gives you an immediate sense of how much memory your system is actively using. If used is close to total, your system might be experiencing high memory utilization.
2. Free Memory: The free column shows the amount of memory that's currently not being used. This value fluctuates as applications and processes run and release memory. A low free memory value doesn't necessarily indicate a problem unless it's consistently low.
3. Buffers and Cached Memory: The buffers and cached columns represent memory used for optimization purposes. This memory is typically released when needed by applications. It's normal for these values to be non-zero.
4. Shared Memory: Shared memory is used by processes that need to communicate with each other quickly. It's typically a small fraction of the total memory.
5. Available Memory: The available column, if available in your free output, provides a useful estimate of how much memory is readily available for new applications. It's particularly valuable to check if you're concerned about system responsiveness.
6. Swap Space: Keep in mind that the free command output doesn't provide information about swap space. If you see consistently high memory usage and a significant amount of swapping (use swapon -s to check swap usage), you may need to consider adding more physical RAM to your system.

 

When to Take Action

Understanding the free command output helps you identify potential memory issues. Here are some scenarios when you might need to take action:

1. Low Free Memory: If the free column consistently shows very little free memory, it may indicate that your system is running out of physical RAM. Consider adding more RAM or optimizing your software to reduce memory usage.
2. High Swap Usage: Frequent and significant swapping can lead to a noticeable slowdown. Monitor your system's swap usage (swapon -s) and consider addressing the root cause, such as reducing memory-hungry processes or adding more RAM.
3. Excessive Cached Memory: If the cached value is extremely high, it might suggest that your system is using a substantial portion of RAM for file caching. While this is generally beneficial, you might want to evaluate if it's impacting your application's performance.
4. Available Memory Drops Significantly: If the available column suddenly drops to a very low value while running applications, it could indicate a memory bottleneck. Investigate which processes are consuming memory and optimize them if necessary.
5. OOM kicks in: There are OutOfMemory-killer messages in the logs. You can check it with:

# grep -i kill /var/log/messages*

kernel: Out of memory: Kill process 8910 (mysqld) score 511 or sacrifice child

kernel: Killed process 8910, UID 27, (mysqld) total-vm:2457368kB, anon-rss:816780kB, file-rss:4kB

or

dmesg | grep oom-killer