Troubleshooting High RAM Usage on Ubuntu: A Case Study

March 06, 2026 2022 technical

Have you ever looked at your system monitor and realized that nearly all your RAM is gone? Today, I hit 13 GiB of usage out of 15 GiB, despite not having any major applications visibly open.

In this post, I’ll walk through how I diagnosed the issue and recovered 9 GiB of RAM in just a few minutes.

1. The Symptom: Where did my RAM go?

Running a simple free -h revealed a concerning state:

               total        used        free      shared  buff/cache   available
Mem:            15Gi        13Gi       304Mi       397Mi       2.3Gi       1.8Gi

Only 300MB of free RAM! While Linux uses memory for caching, the “available” column showed only 1.8GB, meaning the rest was actively “held” by processes.

2. The Investigation: Finding the Culprit

To find out what was eating the memory, I used ps to sort processes by memory consumption:

ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head -n 10

The result was immediate:

PID 3213: qemu-system-x86_64 was using 52.2% of my RAM (~7.8 GiB).

This process is a Virtual Machine (QEMU). On my system, it was associated with Docker Desktop. Even if no containers are doing heavy work, the VM itself reserves a massive chunk of RAM.

3. The Solution: Reclaiming the Memory

To fix this, I followed a two-step approach:

Step A: Stopping the Resource Hog

Since I wasn’t using Docker at the moment, I stopped the service:

sudo systemctl stop docker.socket docker

Step B: Clearing the Page Cache

Linux keeps a “Page Cache” of files you’ve recently accessed. While this is normally good for performance, clearing it can reveal exactly how much “real” RAM you have left.

sudo sync && echo 3 | sudo tee /proc/sys/vm/drop_caches

4. The Result: 9GB Recovered

After these steps, another check with free -h showed:

               total        used        free      shared  buff/cache   available
Mem:            15Gi       4.1Gi       9.7Gi       361Mi       2.2Gi        11Gi

We went from 13 GiB used to 4.1 GiB used. Mission accomplished!

5. Pro-Active Monitoring

If you want to keep a closer eye on your memory without opening a heavy GUI, I highly recommend installing these CLI tools:

htop: An interactive process viewer that is much more readable than standard top.
- How to use: Simply type htop in your terminal.
- Keys to remember:
  - F6: Sort by memory (%) or CPU (%).
  - F3: Search for a specific process (e.g., “docker”).
  - F9: Kill a process safely.
  - q: Quit.
smem: A tool that reports “PSS” (Proportional Set Size), which is the most accurate way to measure memory.
- How to use:
  - smem -tw: Shows total memory usage across the system.
  - smem -u: Shows memory usage per user.
  - smem -rk: Sorts by memory and shows values in MB/GB (human-readable).

Summary Tip: If your RAM is low, always check for hidden Virtual Machines or Docker instances first. They are often the stealthy consumers!

Happy debugging! 🛠️📉