What the Mean Really Means

When analyzing response time, or latency, you need much more information than an average provides. The average, commonly the arithmetic mean, shows the index of central tendency. But, as I found in earlier posts, the tendency is often not central, but may be skewed by outliers, or split by multiple modes. How often these factors [...]

Posted on July 12, 2013 at 10:07 am by Brendan Gregg · Permalink · 2 Comments
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Modes and Modality

It is a truth universally acknowledged that the average is the index of central tendency. But what if the tendency isn’t central? I’ve worked many performance issues where the latency or response time was multimodal, and higher-latency modes turned out to be the cause of the problem. Their existence isn’t shown by the average – [...]

Posted on July 8, 2013 at 12:59 pm by Brendan Gregg · Permalink · 2 Comments
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Detecting Outliers

In computer performance, we’re especially concerned about latency outliers: very slow database queries, application requests, disk I/O, etc. The term “outlier” is subjective: there is no rigid mathematical definition. From [Grubbs 69]: An outlying observation, or “outlier,” is one that appears to deviate markedly from other members of the sample in which it occurs. Outliers [...]

Posted on July 1, 2013 at 10:43 am by Brendan Gregg · Permalink · 7 Comments
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