linux – What killed my process and why?

As a Linux user, there may come a time when one of your processes mysteriously terminates. Understanding what killed your process can save you hours of frustration and help you troubleshoot issues effectively. In this blog post, we’ll explore common reasons processes are terminated and how to investigate the cause.

Common Reasons for Process Termination

1. Out of Memory (OOM) Killer

Linux systems use the Out of Memory (OOM) killer to reclaim memory when the system runs out of RAM. If your process consumes too much memory or the system is under heavy load, the OOM killer may terminate it.

How to Check:

• Inspect the system log:

dmesg | grep -i ‘killed process’

Look for entries like:

[12345.678901] Out of memory: Killed process 12345 (my_process) score 1000 or sacrifice child

2. Signals (e.g., SIGKILL, SIGTERM)

Processes may be terminated by signals, either from the system, another process, or a user.

How to Check:

If the process was terminated by kill:

• Check logs or scripts for commands like kill -9.
• Investigate who sent the signal using tools like auditd or reviewing system logs.

3. Segmentation Faults (SIGSEGV)

Segmentation faults occur when a process tries to access invalid memory. This is often due to bugs in the code.

How to Check:

Examine the dmesg output:

dmesg | grep -i segfault

Debug the process using gdb:

gdb ./my_program core

4. Resource Limits (ulimit)

Processes exceeding resource limits set by the ulimit command may be terminated.

How to Check:

• View current limits:

ulimit -a

Adjust limits if necessary:

ulimit -n 1024 # Example: Increase file descriptor limit

5. Cron Jobs and Systemd Timeouts

If your process is managed by cron or systemd, it might be terminated due to timeouts or misconfigurations.

How to Check:

• For cron:

• Check the cron logs:

cat /var/log/cron

• For systemd:
  • Check the status of the service:

systemctl status my_service

Review the configuration for timeouts in

/etc/systemd/system/my_service.service.

Tools for Investigation

1. dmesg

Displays kernel messages, including information about process terminations.

2. journalctl

Provides detailed logs for services and the system.

journalctl -u my_service

3. strace

Trace system calls to understand process behavior.

strace -p <pid>

4. Core Dumps

Enable and analyze core dumps to debug crashes.

ulimit -c unlimited

Core dumps are usually saved in /var/lib/systemd/coredump/ or similar directories.

Preventing Unexpected Termination

• Monitor resource usage with tools like top, htop, or vmstat.
• Implement proper error handling and testing in your code.
• Set appropriate resource limits and monitor them.
• Use systemd configurations to restart critical processes automatically.

Restart=on-failure

Understanding why a process was killed in Linux requires a mix of detective work and familiarity with system tools. By following the steps outlined above, you can uncover the root cause of mysterious process terminations and take steps to prevent them in the future. Armed with these insights, you’ll be better equipped to keep your Linux systems running smoothly.