Monday, July 13, 2009

When is "last"?

One of the extreme programming rules is Optimize Last. The rule is stated as

Do not optimize until the end. Never try to guess what the system's bottle neck will be. Measure it!
Make it work, make it right, then make it fast.

When is "last"? Is it one week before the application goes into production, at the end of an iteration, at the end of the story, or at the end of a pairing session?

Another question: what and how much should be optimized?

Though the answers to these questions may very a little from project to project, in my experience "last" should mean both at the end of the stories and end of iteration and just the bottlenecks should be optimized with the aid of a profiling tool.

Optimization strategy:
  1. Obtain non-functional performance requirements for the system very early in the project. If there is know target, are we not wasting our time optimizing? By the way, "as fast as possible" is not a sufficient requirement.
  2. Set up automated performance tests. You may even want to go as far a having it run as part of your continuous integration build and having the build fail if certain performance thresholds are not met.
  3. Near the end of story completion, profile the new code. I use ANTS Profiler for C# and JProfiler for Java. Obvious bottlenecks with minimal code changes for optimization should be implemented without hesitation. Continue to fix bottlenecks until the application is meeting its performance requirements.
  4. Near the end of the iteration, profile the whole application. Consider having team members rotate through application-wide optimization responsibilities every iteration.


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