In this post on the Linux resources series we are going to talk about monitoring our CPU usage. When everything slows down this might be a good first suspect to check, you can use the iostat command with the -c option to see current CPU load, the second parameter specifies the time interval in seconds between each report. If you don’t have iostat you will need to install the sysstat package.
This shows a breakdown of the current CPU activity, for a description of what each value means check the man page for iostat.
Using top
If we want to know which process are using most of the CPU we can use the top command, which will show us information about what’s going on in our system including: running process, cpu, memory usage, uptime…
It will automatically refresh every 3 seconds and it is sorted by the %CPU column by default, you can sort by a different column by pressing the F (capital f) key and then the corresponding key for the column you want to sort by. You can also kill a process directly from top pressing k and then the pid of the process.
One detail that is relevant for CPU usage on top and from the uptime command is the load average (sometimes shortened as loadavg), this gives us an idea on how busy our CPU is over time. There is three numbers for load average, they represent the loadavg of the last 1min, 5min, and 15min, so how does this load average work? Well, this value represents the number of process that are ready to run by the cpu but that are waiting on the queue because the cpu is busy executing some other process, every CPU or core in our system can process 1 unit of work so if we have only a single core CPU and our load average is constantly under 1.0 we are ok, but if the load average is constantly over 1.0 it means that our CPU has more work than it can handle. For a more detailed explanation of this subject head over here.
