A system administrator must maintain the health of their server. One of the critical components is the memory. When server memory is full utilized, it can slow down the server performance. To monitor memory utilization, Linux has another command that called 'free'.
Free command is a tool that can display the amount of free and used memory in the system. The output of free command is similar with top command. Most of Linux distribution already have free command.
How to run it
To run it, just type free on your console. Running it without an option will show you a default view with kilobyte units.
From the above screenshot we can read :
Memory (in kilobytes units)
- Total : 1026740
- Used : 843396
- Free : 183344
- Shared : 0
- Buffers : 52704
- Cached : 376384
Swap (in kilobytes units)
- Total : 1045500
- Used : 3376
- Free : 1042124
When you see the buffers/cache free memory is low or you see that the swap free is low, then a memory upgrade is needed. This means that memory utilization is high. Please notice that the shared memory column should be ignored because it is obsolete.
Display memory information in another units
As we mentioned before, by default free will show information in kilobytes unit. Free also provide us -b (bytes), -k (kilobytes), -m (megabytes), -g (gigabytes) and --tera (terabytes). To show the information in the unit we want, just pick one of them and put the option behind free command. Here’s a sample output in megabytes unit.
$ free -m
This trick is also applied to -b, -k, -g and --tera option.
Display memory information in human readable
Free also provide us with -h option which means human readable. So what is the difference with previous option, such as -m (megabytes) option? The most visible difference is that -h option will add human readable unit after the numbers. Let’s take a look a sample of it.
$ free -h
As we can see together, there is G (gigabyte) letter behind 1,0 number. When the number is not reached gigabtye, free is smart enough to know it and put the appropriate unit behind each numbers. M letter behind - let say - 929 number tell us its 969 Megabytes.
Display free with delay
As one of the statistic tool, the best way to capture memory utilization is using a delay. To do this, we can use -s option followed by N seconds that we want. We can always combine more than 1 options to make the output is fit with our needs. Let say we want to capture memory utilization every 3 seconds and human readable. So the command will be like this :
$ free -hs 3
Display low and high memory utilization
If we want to show low and high memory statistics, we can use -l option. Here’s a sample.
$ free -l
Display Linux total memory
When we need the information of total for every column, we can add -t option behind free command. This will add 1 more row at the bottom which display it.
$ free -t
Besides vmstat, Free command is another simple statistic tool for capturing memory utilization. With this you can grab a quick information about what happened in your Linux memory. Free is using /proc/meminfo as a base for showing memory utilization information. As usual, you can always type man free on your console to explore more detail about free.