Allocation Profiling

Allocation Profiling

Find which code produces heap bytes, decide whether those bytes are avoidable, and test whether reducing them improves the reported problem. Allocation is neither retained heap nor GC cost. A large allocation site is a candidate, not automatically the cause of latency.

The reference baseline is OpenJDK 25 GA and async-profiler 4.1, not a minimum target version. Before version-sensitive advice, inspect Maven/Gradle release and toolchain settings, resolved logging/profiler dependencies, CI/runtime images and actual JVM flags. Keep the project's Java and dependency versions; using this skill does not authorize upgrades or enabling preview/experimental features. HotSpot-specific events and flags are not portable Java guarantees.

Workflow

1. Define the comparison. Record the JDK build, collector, heap and relevant flags, profiler version, workload/payload mix, concurrency, warm-up state, affected time window, and target metric. Request missing facts that change the tool or decision. With source alone, provide a hypothesis and collection plan; do not invent a rate or confirmed fix.
2. Establish the rate and denominator. Prefer existing counters or a recording from the affected window. Report bytes/s and bytes/completed operation when that denominator is meaningful; account for background work and failed requests. Compare like workloads. GC-log Eden consumption is only an estimate of young allocation and can miss direct-old allocation. Read allocation tools before collecting or interpreting events, counters, empty profiles, or virtual-thread data.
3. Attribute bytes under representative load. Use weighted allocation stacks to rank sites, preserving the sample population, interval/throttle, filters and window. Verify events and stacks are present and the target path ran. Reconcile totals with independent counters where available. A missing site may be undersampled or filtered; it does not prove that the JIT eliminated it. First-instrument and general capture planning belong to jfr-and-async-profiler.
4. Test the connection to the symptom. Separate allocation churn from survival and retention. Neither a live-allocation profile nor an old-object sample measures promotion rate. Correlate latency windows with GC/stall timelines; coincidence supports a hypothesis, not exclusive causality. Read collector symptoms for humongous allocation, stalls, TLAB attribution, or conflicting measurements. Route retention/root questions to heap-dump-analysis.
5. Choose one falsifiable change. State the site, estimated contribution, mechanism and expected effect. Read reducing allocation before proposing code, reuse or flags. Preserve behavior; a semantic shortcut is not an allocation fix. Use jit-inlining-and-escape-analysis when the decision needs C2 inlining or scalar-replacement analysis.
6. Validate both bytes and outcome. Repeat a matched, warmed workload with comparable instrumentation; compare site bytes, total bytes/op, throughput, CPU, retained heap and the original latency/GC metric. Repeat enough to distinguish the effect from variation. A smaller percentage alone is insufficient, and moving bytes to another site is not a reduction. JMH -prof gc can test an isolated mechanism; it does not establish a service latency improvement. Report inconclusive results and the next discriminating measurement.

Minimum result

For each actionable finding, give the evidence artifact/window and measurement semantics, the observation, the hypothesis and why it remains uncertain, the proposed adjustment, and the measurement that would confirm or refute it. A short paragraph or table row is enough. State separately what was executed and what remains a plan; do not label a source-based prediction as a measured result.

Capture constraints

Start with a bounded capture on a representative canary and an explicit CPU/latency/disk budget; shorten or stop it if that budget is exceeded. Recording defaults are not proof that an event is enabled in an existing capture. Do not bypass attach/container controls when collection fails; report the error and use an available recording or a controlled reproduction. Profiles may expose class, method and thread names and other JFR context; store and retain them according to the service's telemetry policy.

When reviewing this skill or practicing its ambiguous decisions, use validation cases. These are behavioral evaluation inputs, not evidence that the skill has already improved an agent's performance.