105 Process states and Management

 Here’s a complete guide on Process Management – Process States, focused on FAANG-level interview preparation, with clear explanations, real-world examples, and 20 high-quality Q&A to test and reinforce your understanding.

🔧 1. Key Topics to Cover (Process Management & Process States)

🚦 2. Process States in Linux (FAANG-focused)

📚 3. FAANG-Ready Interview Questions + Answers

✅ Basic to Intermediate

Q1. What are the different states a process can be in? Explain.

Answer:

• Running (R): Actively on CPU or ready to run.

• Sleeping (S): Waiting for I/O; can be interrupted by signals.

• Uninterruptible Sleep (D): Waiting on resources like disk; cannot be interrupted.

• Stopped (T): Halted by a signal (e.g., SIGSTOP).

• Zombie (Z): Terminated but not reaped by parent.

• Dead (X): Process terminated and removed from system.

Q2. How can you identify zombie processes on Linux?

Answer:

ps aux | grep 'Z'

Or:

ps -eo pid,ppid,state,cmd | grep Z

Zombie processes show as state Z and can only be cleared if the parent process is terminated or calls wait().

Q3. What is the difference between zombie and orphan process?

Q4. How does a process move from running to waiting (sleeping)?

Answer:
A process transitions from running to sleeping when it requests I/O or is waiting for a resource (disk, network, user input). The scheduler then switches to another process until I/O is ready.

Q5. Explain the lifecycle of a Linux process.

Answer:

1. Created – via fork() or exec()

2. Ready – added to scheduler queue

3. Running – executes instructions

4. Waiting – for I/O or other resource

5. Terminated – exits using exit()

6. Zombie – if not reaped by parent

✅ Advanced FAANG-Level

Q6. How do you handle a zombie process in a production server?

Answer:

1. Identify with ps aux | grep Z

2. Check PPID

3. Kill parent process to allow init (PID 1) to adopt and reap the zombie.

Q7. How to find which syscall a process is waiting on?

Answer:
Use strace:

strace -p <pid>

This shows the system calls being waited on.

Q8. What does the D state (Uninterruptible sleep) mean and why is it dangerous?

Answer:
It indicates the process is waiting for I/O and cannot be killed or interrupted (even with kill -9). This can be a sign of hardware issues, NFS hang, or I/O lock.

Q9. How does the Linux kernel schedule processes?

Answer:

• Linux uses the CFS (Completely Fair Scheduler).

• It maintains a red-black tree to assign CPU time.

• Each process gets a fair share of time based on niceness and priority.

Q10. What happens when you run kill -9 <pid> on a zombie?

Answer:
Nothing. Zombie processes are already terminated. They can’t be killed. You need to terminate the parent process so the kernel can clean up the zombie.

✅ Real-World + Debugging

Q11. How do you debug high CPU usage from a process?

Answer:

top or htop
pidstat -p <pid> 1
strace -p <pid>
lsof -p <pid>

These tools help track what the process is doing, system calls, and open files.

Q12. How do you change the priority of a running process?

Answer:

renice -n 10 -p <pid>

Niceness range: -20 (highest priority) to 19 (lowest priority).

Q13. What’s the use of the pstree command?

Answer:
Shows process hierarchy, useful for visualizing parent-child relationships:

pstree -p

Q14. What signal is sent by default using kill command?

Answer:
SIGTERM (15) – allows process to gracefully shut down.

Q15. When would you use nohup?

Answer:
To run a process after logout/session close:

nohup ./my_script.sh &

✅ Behavioral + Situational

Q16. You found a process in D state for hours. What do you do?

Answer:

• Use strace or dmesg to check blocked I/O.

• Check if NFS, disk, or DB is hung.

• Consider restarting the service or rebooting if stuck.

Q17. A user reports background jobs disappearing after logout. Why?

Answer:

• Use nohup or disown to detach processes from terminal session.

nohup script.sh &
disown

Q18. How to monitor all states of processes?

Answer:

ps -eo pid,user,state,cmd | sort

Also use top, htop, watch.

Q19. What causes a process to become a zombie?

Answer:
Child process exits, but parent hasn’t called wait() to read its exit status. Kernel keeps it to return info when parent requests.

Q20. How can you force the cleanup of multiple zombie processes?

Answer:
Find the parent with:

ps -eo ppid,pid,state,cmd | grep Z

Then kill or restart the parent to allow init to reap the child processes.

📌 Summary Cheat Sheet