Saturday, February 13, 2010

Is High CPU Utilisation A Bad Sign ?

Is high CPU utilisation a bad sign ? Can a single CPU utilisation graph tell you the whole story ? In order to tell a real story, one need to piece other performance data together.

Here is a typical monthly graph of the total CPU utilisation and memory utilisation put together in the same scale (percentage). You can see that this server has sufficient memory. As for the CPU, it hit 90-100% at the early hour everyday. As we all know, CPU utilisation can only max at 100% and we have no idea whether it exceeded the 100% mark.

It is the load averge that can tell you the amount of work that your server performs. With RRDtool CDEF, you can flag out if the cpu load exceeds the no. of CPUs. The CDEF definition is in reverse polish notation, which may take a while to get used to it. Here is the same graph with additional information (indicated as red) to show if load is higher than the number of CPUs.

It is definitely possible to have various colour codes to indicate different level of load, eg. 1.0 < load/#CPUs < 1.5, 1.5 <= load/#CPUs < 2.0, ... They can all be done in CDEF.

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