How do you measure the amount of busy waiting?

I don't think there is a good general tool for this. In this specific case, I went spelunking for all the points where we had thread contention over resources, and discovered that for several resources quite a lot of CPU cycles were being expended to no use. The goal is really to eliminate the underlying resource contention - we added per-thread caches I various places, swapped out the logging system, and were able to ~double the system throughput during times when top showed the system to be "fully loaded"