If the server becomes slow or unresponsive, execute the top command.
The first thing to look is the load average.
The load average typically looks like this: load average: 0.49, 0.41, 0.35 . The first number represents the server load average currently, the 2nd number for load average 5 minutes ago and 3rd number for 15 minutes ago. if you have 1 CPU and the load average is 1, it means that the server has been busy constantly. Four 4 core machine if the average is 4, then all cores are busy.
If MySQL is consuming high CPU, we can go to the MySQL command prompt an:d execute the following commands to see what processes are executing:
show processlist;
show full processlist;
If a particular process has gone out of control, the time column in the output would show how long the process(query) had been executing.
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From Lullabot Videos