Any meter that gives you an averaged value has to average the results over a period of time. If you don’t precisely understand that averaging, then you can get into a lot of trouble.
The two graphs below show exactly the same data with the only difference being the sample length of the meter. In the chart below the data was averaged every minute. Notice the very impressive spike in utilization in the middle of the graph. During this spike this resource had little left to give.
In the chart below the same data was averaged every 10-minutes. Notice that the spike almost disappears as the samples were taken at such times that part of the spike was averaged into different samples. Adjusting the sample length can dramatically change the story.
Some meters just report a count, and you’ve got to know when that count gets reset to zero or rolls over because the value is too big for the variable to hold. Some values start incrementing at system boot, some at process birth.
Some meters calculate the average periodically on their own schedule, and you just sample the current results when you ask for the data. For example, a key utilization meter is calculated once every 60 seconds and, no matter what is going on, the system reports exactly the same utilization figure for the entire 60 seconds. This may sound like a picky detail to you now, but when you need to understand what’s happening in the first 30 seconds of market open, these little details matter.
Below you will see a big difference in the data you collect depending on how you collect and average it. In the one-second average (red line) you are buried in data. In the one-minute average (sampled in the yellow area) you missed a significant and sustained peak because of when you sampled. The 10-minute average (sampled in the green area) will also look reassuringly low because it averages the peaks and the valleys.
Take the time, when you have the time, to understand exactly when the meters are collected and what period they are averaged over. The best way to do that is to meter a mostly idle system and then use a little program to bring a load onto the system for a very precise amount of time and see what the meters report. The better you understand your tools, the more precisely and powerfully you can use them.
This hint and many others are in: The Every Computer Performance Book which is available at Amazon, Powell’s Books, and on iTunes.
