Load Testing in the Dinosaur Age

[article]
The Story of an Early Load Test
Summary:

So how did they test the capacity of the computer dinosaurs of yore? Randy Hayes was there, and he gives us a taste of what it was like. Even with automation, a simplistic approach can be very tedious. Randy also demonstrates that simple automated load tests don't necessarily test what you think they do, and they don't necessarily give accurate results until you've taken steps to ensure that they exercise the system the way real users would.

Can you imagine a full Broadway production that only runs for a single night? It would have to be a really bad show! But that's exactly what load testing used to be like, and still is if you automate the old-fashioned way. Writing test scripts, setting up computers to generate the load, running the test scripts by hand, and evaluating the results is like a really awful Broadway production. Problem is, the next time you make changes to your software you have to do the whole thing all over again. At least a bad show never comes back-this one does, but not often enough to keep the whole cast and crew hanging around!

That reminds me of a story. Back in 1981 (the dinosaur age in Internet time), I worked as a software test engineer for a company called Basic Four in Irvine, California. Basic Four manufactured a "minicomputer" that was the size of a small refrigerator and about half as fast as that old 386 machine in your closet. As many as 100 dumb terminals could be attached to a single machine and used for data entry tasks (or so the marketing guys claimed). My job was to prove that the computer really could support 100 terminals.

Proving that it couldn't did not seem to be an option. So I dutifully wheeled in several racks of CRT terminals, exactly 100, from the manufacturing floor and painstakingly connected all of the cables and cords. My boss cheerfully suggested that we post an announcement on the cafeteria bulletin board, asking for volunteers from the factory floor to help us with the test. We could even offer free pizza to entice them! "Forget it," I said, "most of the people on the floor don't speak English and have better taste in food. And it seems like a silly thing to do if we have to repeat it every time we change the software-can we really afford that much pizza? If we offered free beer we would draw more people for sure, but it might mess up the test results if they throw up on the keyboard or get tipsy. So I'll just throw together a little program to write to the disk drive and display characters on the CRT-at least it will be similar to what happens in real life."

So that's what I did. One after the other, I logged in at each terminal and started up my test program. The screen began to fill up with dots and then I moved on to the next one. I began to notice that the dots on all the screens moved just a little bit slower with each one I added. By the time I got to about 30, they were barely even moving at all. "Oops!" I thought, "My test program must be too demanding on the CPU-back to the drawing board."

So I rewrote the test program to be a bit more realistic. I started with what a fast typist might do (say, eighty words a minute) and calculated the average delay between keystrokes. Then I wrote a routine that put the program to sleep for that period of time and woke it back up long enough to send a dot to the screen. After eighty such iterations, I told it to send eighty bytes of data to the disk to simulate saving a record. Then, of course, I had to go back to all the terminals, one by one, to kill the previous version of the program that was running and start up the new one.

By then it was

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