Java / GraalVM Interview questions

Why should you use Profile-Guided Optimization (PGO) with Native Image?

Because a native image is compiled entirely ahead of time with no live runtime profiling feedback loop like a JIT gets, the compiler has to make static guesses about which branches are hot and which methods are worth inlining aggressively - guesses that are often wrong for real-world traffic patterns.

PGO closes that gap in two passes: first you build an "instrumented" image that records real execution profiles (branch frequencies, call-site targets) while running representative workloads, then you rebuild the final image using --pgo=default.iprof, feeding those recorded profiles back into the AOT compiler so it makes the same kind of informed inlining and branch-layout decisions a warmed-up JIT would.

native-image --pgo-instrument -jar app.jar
./app  # run representative workload, produces default.iprof
native-image --pgo=default.iprof -jar app.jar

The result is peak throughput noticeably closer to a fully warmed-up JVM, without giving up Native Image's fast-startup, low-memory profile - though it does require a realistic workload during the profiling run, since profiles from an unrepresentative run can mislead the final compile.

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Why does a native image need PGO instead of relying on live JIT profiling? Because it's compiled entirely ahead of time, with no live runtime feedback loop to guide optimization decisions

✓ Correct! Well done.

Because PGO replaces the need for a garbage collector

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Because native images cannot execute conditional branches otherwise

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Because PGO is required to enable reflection support

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What is required for PGO's recorded profile to be useful? Running a representative real-world workload during the instrumented profiling run

✓ Correct! Well done.

Running the smallest possible unit test only

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Skipping the instrumented build entirely

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Disabling the Graal compiler during profiling

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Java / GraalVM Interview questions

Why should you use Profile-Guided Optimization (PGO) with Native Image?

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