# Gc Log Analysis > Configuring and reading JVM unified GC logs: the -Xlog:gc* baseline with decorators and rotation, the cause field, the before->after->capacity triple and headroom, pause distribution, Eden-refill and old-region-growth estimates derived from region lines, adaptive tenuring pressure via gc+age, and correlation with -Xlog:safepoint. Use when a log needs to be interpreted, when GC logging is not enabled and the right GC tag-sets have to be chosen, when full collections or Metadata GC Threshold appear, when the heap floor rises after equivalent reclamation points, when an allocation or promotion proxy has to come from the log rather than a profiler, when a log starts above zero uptime, or when an analysis script reports zero pauses. Does not cover collector mechanics (gc-fundamentals), collector choice and heap sizing (jvm-gc-tuning), or allocation profiling (jfr-and-async-profiler). Syntax is unified-logging; cross-layer pause attribution is pause-attribution. - Skill: `robsonkades/gc-log-analysis` (Agent Skill, multi-file: 5 files) - Install (CLI): `npx skillmds@latest add robsonkades/gc-log-analysis` - Raw SKILL.md: https://api.skillmd.com/api/skills/robsonkades/gc-log-analysis/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Marketing & Growth - Author: robsonkades (https://skillmd.com/u/robsonkades) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/robsonkades/gc-log-analysis --- # GC Log Analysis ## Purpose Get testable hypotheses out of low-overhead JVM evidence. GC logging needs no agent and can persist across the incident, but its overhead and volume depend on tag level, sink and I/O; measure the production configuration. Enabling it after an incident cannot reconstruct the missing interval, so a bounded baseline belongs in the service template. ## Workflow 1. **Identify the JDK build, collector, process identity, decorators and captured window.** Analyze available evidence with its limits; if logging is missing, arrange an authorized bounded capture without claiming it reconstructs the past. Do not upgrade to match a recipe. 2. **Read the cause, collector, event type and adjacent lines before forming a hypothesis.** The cause routes the investigation but is not a root-cause verdict; one trigger can lead to several mechanisms and collectors use different vocabularies. 3. **Separate duration from frequency.** Many short pauses violate no per-pause threshold and still consume throughput: one hundred 20 ms pauses per minute is 2 seconds per minute stopped, over 3%, and no duration-based alert fires. 4. **Read the `after` number in collector context, and more than it, its trend.** In `before->after(capacity)`, compare equivalent reclamation points under equivalent load. A rising floor is a retention hypothesis, not proof: delayed concurrent reclamation, adaptive sizing, humongous occupancy and phase selection can change the number. 5. **Check reported committed-capacity headroom** — capacity minus `after`, not necessarily distance to `-Xmx`, usable evacuation space or contiguous allocation capacity. 6. **Reconcile with the observed pause.** If they disagree, go to the safepoint log; the reported pause excludes Time-To-SafePoint. 7. **If you need to know how much is allocated and how much survives**, the log can provide bounded proxies: Eden refill and old-region growth come from the `Eden regions` and `Old regions` lines and the region size in `gc,init` — see `references/rates-from-the-log.md`. **If you need to know who allocated**, it cannot — take JFR with `jdk.ObjectAllocationSample`. ## Rules - Candidate baseline: `-Xlog:gc*:file=gc.log:time,uptime,level,tags:filecount=5,filesize=20m`. Both `time` **and** `uptime` decorators — one correlates with incidents, the other with process lifetime. Size rotation and retention from event rate, disk budget and incident look-back; protect against both overwriting the relevant window and filling the volume. - Consider `-Xlog:safepoint:file=safepoint.log:time,uptime` alongside it when the latency SLO needs TTSP attribution. Validate its volume and sink as part of the same budget. - Unified logging is **synchronous by default**. The thread emitting the line — including a GC thread inside the safepoint — waits for the write, so on slow or throttled I/O the instrument can add latency to the pause it is measuring. `-Xlog:async` (JDK 17+) moves writes to a logging thread; it does not remove formatting/queue overhead. Check the target release's drop/stall mode and dropped-message notices before treating a capture as complete. - Never read only the mean pause. Report count, total stop-the-world pause fraction, an explicitly defined percentile estimator and max, split by event type and operating regime. Small samples make p99 effectively one of the largest observations; attach the sample size and window. - Three-argument `match()` is not POSIX awk; unsupported implementations may reject its syntax. OS name does not identify the awk implementation. Use `RSTART`/`RLENGTH`, inspect stderr and every pipeline stage's status, and test against known events plus empty/unsupported input. - `new threshold` below `max threshold` in a `gc+age` line means adaptive tenuring chose an earlier promotion age; that can be healthy. Call it premature only when promotion, old-generation pressure or downstream collection cost is harmful and the age table shows survivor pressure. A larger young/survivor budget is then one candidate; validate it because lowering the pause target can shrink young and worsen the same mechanism. - Raising the heap is not the first response to frequent short pauses. Allocation rate and ergonomic young sizing are common causes; a larger heap can change frequency, young size, concurrent-cycle timing and failure headroom, with collector-specific pause effects. Confirm the mechanism: estimate allocation rate from the log and compare it with the Eden target `(N)` and ergonomic logs; a small target alone does not establish why it was chosen. See `references/rates-from-the-log.md`. - A first uptime above zero can reflect rotation, late activation, truncation or an excerpt. Inspect file continuity and process identity; rotated suffixes are a ring, not a chronological sort. Use measured window boundaries, not process lifetime, and avoid duplicate overlap. - Treat log-derived rates as estimates with explicit blind spots: region rounding, humongous allocation, collector phase and rotation boundaries. Cross-check surprising values with JFR or another independent counter before changing capacity. - A production GC log reveals load pattern, installed capacity and deploy cadence. Uploading it to a third-party analysis service is a security decision, not a convenience one. Prefer local tools by default. - Pre-JDK-9 GC logging flags are a mixed trap: some survive as deprecated aliases, others were removed — and a removed flag stops the JVM from starting at all. ## References - [Log analysis recipes](references/log-analysis-recipes.md) — POSIX awk scripts for pause distribution, cause counts and headroom, plus the JFR commands for allocation sources. Read when you have a log in hand and need numbers out of it. - [Rates from the log](references/rates-from-the-log.md) — the anatomy of a G1 line, the arithmetic for Eden refill, old growth, survivor occupancy and STW pause share with the caveats that change the number (humongous, mixed collections, rotation), a validated POSIX awk recipe, and a symptom-to-cause table. Read when an allocation or promotion proxy has to come out of the log, when a young pause grows without changing frequency, or when a log starts at an uptime well above zero. - [Cause field reference](references/cause-field.md) — what each cause means and what to investigate next, including the ZGC log format change. Read when the cause in parentheses is unfamiliar or the log format does not match the examples you know.