Why is Arrays.fill 265 times slower on G1GC?

There's a strange beauty in performance anomalies like this. We tend to think of code as a deterministic sequence of instructions—write a loop, fill an array, done. But in modern managed runtimes, your program is just the visible tip of a vast, churning ecosystem. The garbage collector, the JIT compiler, the memory hierarchy—they're all conspiring (sometimes against you) to make your code run. When `Arrays.fill` suddenly becomes 265 times slower on G1GC, it's not the loop that changed; it's the invisible tides of memory management shifting beneath it. G1GC is a marvel of engineering, a garbage collector that treats your heap like a fragmented universe of regions, tracking references with card tables and remembered sets. When you fill an array, you're not just writing bytes—you're potentially dirtying cards, triggering write barriers, and forcing the collector to notice the change. Under the wrong conditions, a simple fill can cause a cascade of bookkeeping that dwarfs the actual work. It's like discovering that writing a single word on a piece of paper requires filing paperwork in triplicate with a government agency that never sleeps. This is the "weird and wild" part of computing: performance is an emergent property, not a simple sum of instruction costs. The same operation can take microseconds or milliseconds depending on the state of the runtime's hidden state machine. It's a reminder that our abstractions are leaky in ways that defy intuition. We build layers upon layers—source code, bytecode, machine code, GC barriers, CPU caches—and sometimes the layers interact in ways that produce astonishing, almost surreal results. What's particularly fascinating is that this kind of anomaly is not a failure of engineering, but a feature of complexity. The universe of software is full of such emergent surprises, much like the physical universe is full of phenomena that only appear when you look closely enough. A performance cliff isn't a bug to be fixed; it's a window into a deeper reality. So the next time your code runs slowly, don't just blame the algorithm—ask what invisible forces are at play. The answer might be weirder than you expect. Source: [Why is Arrays.fill 265 times slower on G1GC?](https://www.reddit.com/r/hackernews/comments/1w74aq2/why_is_arraysfill_265_times_slower_on_g1gc/)