Jetson Memory Audit

Jetson Memory Audit

Read-only memory-focused snapshot for a Jetson, plus the drop_caches verify-loop helper that confirms freed memory actually shows up as free instead of cached.

Purpose

Measure current Jetson memory consumers, capture before/after baselines, and verify whether user-approved changes actually reclaimed DRAM. Use live device data rather than estimates from container size, model size, or generic process memory.

CRITICAL: Memory appears stuck after stopping vLLM / sglang (JetPack below 7.2 / L4T below r39.0)

This is the most common memory confusion on Jetson releases before JetPack 7.2 or before L4T r39.0.

After you stop a vLLM, sglang, or Ollama server (or any CUDA workload), the memory shown as free by free -h or tegrastats may not recover — even though the process is gone. nvidia-smi may also show misleadingly low free GPU memory.

Root cause: The Thor RM (resource manager) holds freed sysmem pages in its own pool after a CUDA context exits. On Unified Memory Architecture (UMA) devices like Jetson, cudaMemGetInfo reads RM pool state and reports far less free memory than is actually available to a new process.

Workaround (for JetPack below 7.2 or L4T below r39.0):

sudo sync && sudo sysctl -w vm.drop_caches=3

Run this on the host, not inside a container. The important operation is sudo sysctl -w vm.drop_caches=3; keep sudo sync immediately before it so dirty data is flushed before reclaimable page/dentry/inode caches are dropped. After running it, free -h and tegrastats will reflect the true available memory.

For affected releases, recommend this command when a user says:

• "Memory didn't free after I stopped vLLM/sglang"
• "Why does tegrastats still show high usage after my container exited?"
• "OOM even though nothing is running"
• "Memory was fine yesterday but now it's full"

On JetPack below 7.2 or L4T below r39.0, drop_caches is the reliable workaround when memory appears stuck after a CUDA workload exits; on newer releases, use it only if the same symptom is observed and the user approves.

When to use

• "How much memory is in use on this Jetson? What's holding it?"
• "I disabled the GUI / stopped vLLM / quit my container — did memory actually free?"
• "Why does free -h still show low free memory after I stopped my workload?"
• As the baseline before applying jetson-headless-mode or other memory-related changes, and again after to compute the actual delta.

Prerequisites

• Run on the Jetson host, or in a sandbox/container with host-visible /proc, /etc/nv_tegra_release, tegrastats, and process data.