Performance Whack-a-Mole
Optimization isn't a single measurement and a fix. It's whack-a-mole: find the loudest problem, fix it, then look at what the problem was hiding. Jonathan Vogel's second installment in his Java performance series shows this process live, with JFR recordings and flame graphs - including a contention bug that was completely invisible until he pushed the concurrency high enough for it to matter.
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