JIT Warmup

ELI5 - The Vibe Check

A JIT compiler starts out running your code the slow, simple way, an interpreter, while it quietly watches which parts get called the most. Once it's confident a piece of code is hot, it compiles that specific part down to fast machine code on the fly. JIT warmup is that awkward in-between period where your program is slower than it will eventually be, because the compiler hasn't finished figuring out what deserves the expensive optimization treatment yet.

Real Talk

JIT warmup is the period after a program starts during which a just-in-time compiler is still profiling code execution and progressively recompiling frequently executed methods into optimized machine code, meaning performance is measurably worse than the program's steady state. This is why benchmarks on JIT-compiled languages like Java, C#, or JavaScript's V8 engine must discard early iterations, and why short-lived processes like serverless functions suffer disproportionately from JIT overhead they never get to amortize.

When You'll Hear This

"The first thousand requests were slow, that was just JIT warmup, throughput climbed after that." / "Serverless cold starts are painful partly because you eat JIT warmup on every fresh instance."