Challenge, Easy,  on  Linux
ProblemSolutionDiscussion

Freezing a process lets you safely pause it without stopping or killing it. This can be useful if you want to temporarily suspend a heavy job during peak load, hold a service still while you reconfigure its environment, or "stop the world" so you can inspect the system's state during a security audit or a bug investigation.

In this challenge, you'll launch a resource-hungry program in its own cgroup, freeze it, observe its footprint, and then thaw it back to life.

The binary you'll run is called omnihog - a simple program that greedily consumes CPU and memory when started. Launch it in a dedicated cgroup, freeze (pause) it, take a look around the system (including the process's CPU and memory usage while frozen), and then thaw it back (resume).

Good luck!

Hint: Placing a process in a dedicated cgroup 💡

If you need a refresher on creating cgroups and placing a process into them, revisit this challenge: Limit CPU and Memory Usage of a Linux Process.

Linux control group v2 visualized.
Hint: Freeze (pause) and thaw (resume) a process 💡

In cgroup v2, each cgroup (i.e., a /sys/fs/cgroup subfolder) has a cgroup.freeze file. Freezing all processes in a cgroup is as simple as writing 1 to this file. And to thaw the process(es), you just need to write 0 to the same cgroup.freeze file.

Note that while freezing a cgroup freezes not only all processes in it, but also all processes in all child cgroups, in this challenge, you're expected to freeze the omnihog's own cgroup directly.

Hint: Observe the resource usage while frozen 💡

While frozen, threads do not make forward progress, but they still consume memory. You can inspect the omnihog process's memory and CPU usage while frozen using the ps or a similar command.

Challenge on Linux
Discussion  Discord
cgroups

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How to Author Challenges on iximiuz Labs

Instead of providing a subpar online editing experience, iximiuz Labs offers a helper CLI tool called labctl, allowing you to use your favorite text editor (or a full-featured IDE) to write content from the comfort of your local machine.

Install labctl CLI

curl -sf https://labs.iximiuz.com/cli/install.sh | sh

This will download and install the latest version of the labctl CLI. You only need to do this once per workstation.

Authorize labctl

labctl auth login

This will open a browser window asking you to authorize labctl to access your account. You need to do it after a fresh install of labctl and repeat it whenever the auth session expires.

Pull challenge content

labctl content pull challenge linux-freeze-and-thaw-processes

This will create a local copy of the challenge content in a directory named linux-freeze-and-thaw-processes. You only need to do this once per challenge.

Stream your changes

labctl content push -fw challenge linux-freeze-and-thaw-processes

Run this command in a separate terminal to continuously upload your changes to the server while editing the challenge in your favorite text editor or IDE.

You can also use labctl to create, list, and delete your content. Learn more about the available commands: labctl content --help