Like habit 2, Start at the Top, this habit looks easy to understand and to keep. But, due to the constant desire for productivity, I and most others I know in the performance community have broken it many times. Some times I even get away with it. But trying to keep this habit is important, because when I don't get away with it, breaking this rule results in even more work than I was trying to save.
The concept behind this habit is simple - when you are optimizing your platform or your code, make only one change at a time. This allows you to measure the effect of each change, and only accumulate the positive changes (however small) into your workload. I have seen instances, for example, where 2 small changes applied at the same time to a workload cancelled each other out: one caused a small in performance and the other a small increase. If these changes weren't tested individually, we would have missed out on that performance gain.
Another thing that can happen in a complex workload is that two changes that seem independent can interact with each other. Like many developers know from fixing bugs, changing one thing may affect something else. Keeping all your changes separate can help you identify these interactions more easily.
You may be wondering when it is acceptable to break this habit. I think of performance methodology, and this rule in particular, as similar to the scientific method we learned in school. It's always good to follow it - doing so will help you quantify your successes and failures, stay organized, and defend your conclusions - but, you can still make a big breakthrough without it. In some cases, like when you are making small local changes to source code in completely different modules, or when you are changing two things you are certain won't interact, the habit can be broken. But the advice I give, especially to those involved in long-term optimization projects, is to follow it.
What has your experience been? Please share your "changing multiple things at one time" stories.
Keep watching The Server Room for information on the other 4 habits in the coming weeks.