When your server is under load, you need to identify the bottleneck. This can be CPU, Memory or Disk IO. Here are some of the useful commands that help you identify cause of high server load.
uptime is the first command you use to check the server load.
boby@hon-pc-01:~$ uptime 22:50:05 up 2:02, 1 user, load average: 0.97, 1.01, 1.00 boby@hon-pc-01:~$
What you need to look is load average values. This is load in last 5, 10 and 15 minutes respectively.
top is 2nd most used command to identify load. top give you much more details, what process running, how much CPU usage, IO usage etc..
There are alternative for top commands that provide more features like htop and atop.
vmstat provide details about processes, memory, paging, block input/output, interrupts, and CPU activity. You can run it with command
sar can be used to record and analyse server load. To install sar, run
yum install sysstat