[hobbit] CPU Utilization, Less Averaging

[Henrik Stoerner]

On Fri, Feb 16, 2007 at 12:31:16PM -0600, James Wade wrote:
> Can I set the vmstat collection to average less
> and collect more data points?

You can change the collection to run every 15 seconds, but you will have
to change the RRD file definition also so it knows that you intend to
feed it data that often. Otherwise it will just discard your extra
datapoints.


> The users here don't agree with the CPU Utilization Graph,
> they don't like the way the Average doesn't fluctuate much.
> 
> Basically, they are comparing the Load graph with the Utilization 
> Graph and want to see the graph correlate more with the Load Graph.

Your users are comparing apples and oranges - those two numbers have
very little to do with each other. 

Here's the explanation of the "load average" that you see in the "Load"
graph:

    System load averages is the average number of processes that are
    either in a runnable or uninterruptable state.  A  process  in a 
    runnable state is either using the CPU or waiting to use the CPU. 
    A process in uninterruptable state is waiting for some I/O access, 
    eg waiting for disk.  The averages are taken over the three time 
    intervals.  Load averages are not  normalized  for  the number of 
    CPUs in a system, so a load average of 1 means a single CPU system 
    is loaded all the time while on a 4 CPU system it means it was idle 
    75% of the time.

The CPU utilisation graph IS normalized over the number of CPUs - that
in itself makes a difference (unless you have single CPU systems). But
more importantly, a process will show up on the "CPU utilisation" graph
ONLY when it is using CPU time; and it will show up on the "load
average" graph when it is using CPU time, AND when it is waiting to be
scheduled for some CPU time, AND while it is waiting for I/O to
complete.

Since most server tasks are very I/O bound you will often see little
correlation between the two graphs. In fact, if the correlation becomes
too close it is usually a sign that your system needs more CPU power.


Regards,
Henrik