When running a load test, there is usually a ton of data that gets generated. You get charts for response times and error rates. You usually also end up with large amounts of web server logs, which may have interesting data embedded in them. If you’re profiling your servers for their CPU, memory, and disk utilization, you’ll likely have additional charts to look at. All of these are necessary to piece together a complete picture of the results of your load test.

The Throughput Chart

But if I could only pick one metric that I could get out of a load test, it’d almost certainly be the data throughput charting that BrowserMob and most other load testing tools provide. This chart shows you how many bytes per minute were transfered during the load test. The following example shows almost 4GB/minute, which is almost 550 mbps in throughput (most sites never peak above 50 mbps):

The reason I like the throughput chart more than any other is because I can usually tell multiple things about the test from this one chart. For example, in this test, I can make the following conclusions without looking at any other data:

• Load was likely ramped up for the first half of the test.
• If load was applied at a constant rate during the second half of the test, response times likely stayed flat during the whole test.
• If load continued to ramp up during the whole test, response times likely increased in the second half of the test, causing throughput to flatten out.
• Near the middle of the test, something drastic happened to decrease throughput. It could be that load levels were reduced temporarily, but it could also be because a page or object in the transaction path suddenly took longer to load, reducing total throughput temporarily.

That’s just some of the reasons why I love the throughput chart in BrowserMob. What’s your favorite metric or chart when it comes to load testing?

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