I spent quite some years performance tuning large Java systems, pre-warming server JVMs and then many hours staring at visualgc, a small Sun engineers tool for

I spent quite some years performance tuning large Java systems, pre-warming server JVMs and then many hours staring at visualgc, a small Sun engineers tool for watching the various memory pools including generational GC, it was very satisfying work, and also pretty useful for uncovering bugs and race conditions when the systems were under load. The GC advances that came along helped a lot, especially over the stop-the-world early days of Java GC, along with all the JVM tuning parameters that were gradually exposed for tweaking. But visualgc allowed for a real feel for how the generational GC was running just by watching the saw-tooth shaped graphs. Interestingly most of the garbage was string copying, especially with one companies system that had more abstractions than a Spring Oak has leaves, say no more least I have nightmares.