Java’s “write once, run anywhere” promise is kept by the JVM. Its performance is kept by the JIT compiler. The gap between “Java is slow” (the 1998 opinion) and “Java is competitive with C++ for many workloads” (the 2013 reality, and more so now) is almost entirely the C2 compiler. Understanding what C2 does — and when it stops doing it — matters if you’re writing performance-sensitive Java. ...
The incident happened at 08:31 on a Tuesday — Frankfurt open, high volatility session. Our tick-to-quote latency spiked to 340ms for about 2 seconds. The SLA was 1ms at p99. Trading desk noticed before our monitoring did. The culprit: a full GC triggered by a promotion failure. We had 12GB heap, CMS collector, and no one had looked at GC logs since the initial deployment. ...
The service started in 3 seconds in development, but the first live trade after deployment took 800ms instead of the expected sub-10ms. The second trade was fine. We couldn’t reproduce it in load tests. The culprit was classloading. The trade execution path touched 47 classes that had never been loaded before. Loading them, verifying the bytecode, and running static initialisers took 800ms — once, at first use. Understanding exactly how that happens is worth the time. ...