Interview Prep · Phase 5
Heap, stack, metaspace and direct memory; every collector from Serial to ZGC and when each is the right answer; reading a GC log; the leak patterns that recur across every codebase; and the tooling that turns a guess into a measurement.
Every region the JVM allocates, which are per-thread and which are shared, why the heap is generational, where Metaspace lives since Java 8, and why total process memory always exceeds Xmx.
Read tutorialHow each collector works, the throughput-versus-latency trade-off that separates them, what G1 regions and pause targets really do, and how ZGC achieves sub-millisecond pauses on huge heaps.
Read tutorialEnabling unified GC logging, reading a G1 log line by line, calculating allocation and promotion rates, identifying every Full GC cause, and the tuning changes that are usually wrong.
Read tutorialThe seven leak patterns that recur in every codebase, why a garbage-collected language leaks at all, how each one is diagnosed from a heap dump, and the code change that fixes each.
Read tutorialEach OutOfMemoryError message, what it actually indicates, the most likely cause, and the first three things to check — plus why OOMKilled by the kernel is a different failure entirely.
Read tutorialCapturing a heap dump safely in production, the difference between shallow and retained size, reading the dominator tree, using path to GC roots, and OQL queries that answer real questions.
Read tutorialHow the JVM reads cgroup limits, why MaxRAMPercentage beats Xmx in a container, how CPU quota affects GC and pool sizing, and why CPU limits cause latency spikes through throttling.
Read tutorialRunning JFR continuously in production, why traditional samplers suffer safepoint bias, reading a flame graph, allocation profiling, and choosing between CPU and wall-clock sampling.
Read tutorialHow many bytes an object really costs, why compressed oops stop working above 32GB, how the JIT proves an object never escapes and removes the allocation, and what deoptimisation is.
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