107 MB of Invisible JVM RAM

Tue, 02 Jun 2026 00:00:00 +0000

Prometheus says 137 MB. Docker says 244 MiB. /proc/PID/smaps is the only witness that doesn’t lie.

The Discrepancy

JVM at 256 MB, Serial GC, after serving 1.66 million requests. Measured at idle:

Docker stats:         244.2 MiB / 256 MiB  (95.38%)
Prometheus reports:
  Heap used:           35 MB  (Eden 4.4 + Survivor 0.1 + Tenured 30.5)
  Non-heap:           101 MB  (Metaspace 71.9 + CompClass 12.7 + CodeCache 16.3)
  Direct buffers:     0.75 MB
  ────────────────────────────
  Total visible:      ~137 MB

Gap:                  107 MB

107 MB that no JMX MBean exposes. No micrometer gauge records it. No Grafana dashboard shows it. To know it exists, you exec into the container and parse kernel memory maps.

Kubernetes on tsvinc

107 MB of Invisible JVM RAM

The Discrepancy