Demo 02 — GC Monitoring with Prometheus

Two Quarkus apps (G1GC on :8080, ZGC on :8081) + Grafana LGTM + standalone Prometheus scraping /q/metrics. Live GC pause histograms and traces simultaneously.

Required configuration

# application.properties — without this, Grafana GC panels show no data
quarkus.micrometer.distribution.percentiles-histogram.jvm.gc.pause=true
quarkus.micrometer.distribution.percentiles.jvm.gc.pause=0.5,0.95,0.99

Key PromQL queries

# GC pause P99 (ms)
histogram_quantile(0.99, rate(jvm_gc_pause_seconds_bucket[1m])) * 1000

# Heap utilisation %
jvm_memory_used_bytes{area="heap"} / jvm_memory_max_bytes{area="heap"} * 100

Fedora / RHEL notes

All bind mounts require :Z for SELinux. Prometheus uses tmpfs + user: root to avoid rootless Podman named volume permission issues. Do not mount custom config into otel-lgtm — it breaks the internal OTel pipeline and produces blank dashboards.

Generate load

# Allocate heap and watch the GC panels react
curl "http://localhost:8080/allocate?mb=100&iterations=10"

# Virtual threads — 500 concurrent tasks
curl "http://localhost:8080/virtual-threads?tasks=500&workMs=5"

Reference

• Demo source
• Grafana JVM dashboard 4701