Interview QA

Interviewer: tell me all ways to cleanup memory in Linux?

Cleaning up memory in Linux is a critical skill for DevOps and system engineers, especially when systems are under memory pressure or to optimize performance. Here’s a detailed breakdown of all major ways to clean up or free memory in Linux:

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🔧 1. Clear PageCache, dentries, and inodes (drop_caches)

# Sync before clearing caches (recommended)
sync

# Clear PageCache only
echo 1 > /proc/sys/vm/drop_caches

# Clear dentries and inodes
echo 2 > /proc/sys/vm/drop_caches

# Clear PageCache, dentries, and inodes
echo 3 > /proc/sys/vm/drop_caches

✅ Best used for reclaiming cache memory (won’t kill processes).

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🔥 2. Kill High-Memory Processes

Find and kill memory-hungry processes:

# Find top 10 memory-consuming processes
ps aux --sort=-%mem | head

# Kill a specific process
kill -9 <PID>

✅ Useful when a specific process is hogging memory.

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🧠 3. Free Up Swap Memory

To clear swap and move data back to RAM:

# Disable swap
swapoff -a

# Re-enable swap
swapon -a

✅ Helpful when swap usage is high and you want to reset it.

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🚰 4. Flush Disk Buffers and Cache (sync)

sync

✅ Ensures all data is written to disk before clearing caches.

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🗂 5. Use systemctl restart or service to Restart Services

If a service is leaking memory:

systemctl restart apache2   # Example: restart Apache

✅ Restarts the service and reclaims memory.

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📉 6. Use oom_kill or earlyoom

Configure OOM Killer or use earlyoom for automated memory cleanup:

• earlyoom: kills processes when memory is critically low.

• /proc/sys/vm/oom_kill_allocating_task: enable OOM killer tuning.

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📦 7. Clear Cache via free and sync combo

Check usage:

free -h

Force cleanup:

sync; echo 3 > /proc/sys/vm/drop_caches

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🧽 8. Memory Leak Debugging and Cleanup

Use tools like:

• valgrind

• top, htop

• smem

• vmstat

• meminfo in /proc

✅ Identify memory leaks for long-term cleanup.

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🧹 9. Reboot the System

reboot

✅ Last resort but guarantees memory reset.

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🛠 BONUS: Automate with Cron

You can set periodic cache cleanup:

# Add to crontab (runs daily at 2 AM)
0 2 * * * root sync; echo 3 > /proc/sys/vm/drop_caches

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Summary Table

Method	Purpose	Risk
drop_caches	Free file system cache	Safe
Kill processes	Reclaim memory from apps	Medium
swapoff && swapon	Clear swap	Medium
sync	Flush disk buffers	Safe
Restart services	Reclaim app memory	Low
oom_kill / earlyoom	Auto kill on low memory	Low-Medium
Reboot	Complete cleanup	High

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Let me know if you want a bash script or systemd timer to automate these.

interviewer: tell me all ways to cleanup CPU in Linux?

Cleaning up CPU usage in Linux is essential when your system becomes sluggish due to high processor load. If an interviewer asks "Tell me all ways to clean up CPU in Linux?", here’s a comprehensive, categorized answer you can give:

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✅ 1. Identify High CPU Consumers

Start by finding the root cause:

top         # Real-time CPU usage
htop        # Better UI (requires install)
ps aux --sort=-%cpu | head

Check system load:

uptime
vmstat 1 5

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🔥 2. Kill or Nice High CPU Processes

Kill a high-CPU process:

kill -9 <PID>

Lower a process’s CPU priority (increase niceness):

renice +10 <PID>    # Values from -20 (highest) to 19 (lowest priority)

✅ Prevents process from hogging CPU without killing it.

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📉 3. Limit CPU Usage with cpulimit

Install and run:

sudo apt install cpulimit  # or use yum/dnf on RHEL-based distros

cpulimit -p <PID> -l 30    # Limit to 30% CPU

✅ Helps manage resource-hungry apps like ffmpeg or compression tools.

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📊 4. Use taskset to Pin Process to Specific CPU Core

taskset -c 0,1 <command>   # Bind to core 0 and 1

✅ Useful for isolating CPU-intensive apps.

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⚙️ 5. Check and Fix Stuck Services or Daemons

Use systemctl or service:

systemctl status <service>
systemctl restart <service>

✅ Some background services can go rogue and consume CPU.

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🐞 6. Identify and Debug CPU Spikes

• Use perf top, pidstat, or strace to trace syscalls.

• Investigate with:

pidstat -u 1
strace -p <PID>

✅ Helps when CPU is high but process is unknown or stuck.

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🧠 7. Reduce Unnecessary Background Services

List all enabled services:

systemctl list-unit-files | grep enabled

Disable unneeded ones:

systemctl disable <service>

✅ Frees both CPU and RAM.

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📦 8. Uninstall Heavy or Unused Applications

For example:

sudo apt remove apache2

✅ Avoid resource-hogging software that isn’t needed.

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🪫 9. Use Cgroups to Restrict CPU for Apps

Create a CPU-limited group (example with systemd):

# Create systemd slice
mkdir /sys/fs/cgroup/cpu/myapp

# Limit CPU usage to 20%
echo 20000 > /sys/fs/cgroup/cpu/myapp/cpu.cfs_quota_us
echo 100000 > /sys/fs/cgroup/cpu/myapp/cpu.cfs_period_us

# Move process to cgroup
echo <PID> > /sys/fs/cgroup/cpu/myapp/tasks

✅ Fine-grained control over CPU.

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🔄 10. Reboot (Last Resort)

reboot

✅ Clears up stuck processes and resets everything.

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🔐 BONUS: System Optimization Tips

• Enable performance governor only when needed (cpupower frequency-set)

• Tune kernel parameters: /etc/sysctl.conf

• Use lightweight alternatives (e.g., nginx instead of apache)

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🧾 Summary Table

Action	Description	Risk
Kill process	Stops high CPU app	Medium
Renice/CPULimit	Lowers app priority/CPU	Safe
Restart service	Fixes stuck services	Low
Cgroups	Isolate and limit apps	Low-Medium
Disable background services	Frees system resources	Low
Use taskset	CPU core binding	Safe
Debug with strace, perf	Advanced troubleshooting	Advanced
Reboot	Resets CPU load	High

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Would you like a shell script to monitor and auto-restrict high-CPU processes?

interviewer: tell me all ways to cleanup Disk in Linux?

If an interviewer asks "Tell me all ways to clean up disk in Linux?", they’re testing your practical knowledge of storage management, cleanup commands, and troubleshooting. Here's a complete and structured answer:

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🧹 All Ways to Clean Up Disk in Linux

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🔍 1. Find Out What’s Using Disk Space

Check disk usage:

df -h

Find largest directories/files:

du -h --max-depth=1 /         # Top-level usage
du -ah / | sort -rh | head -n 20  # Top 20 largest files
ncdu /                        # Interactive disk usage viewer

✅ First step before cleanup: know where the space is going.

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📦 2. Clean Package Manager Cache

For Debian/Ubuntu:

sudo apt clean        # Remove all cached .deb files
sudo apt autoremove   # Remove unneeded packages

For RHEL/CentOS:

sudo yum clean all
sudo dnf autoremove

✅ Recovers hundreds of MBs or even GBs from package cache.

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🧹 3. Clear System Logs

sudo journalctl --vacuum-time=7d         # Keep only last 7 days
sudo journalctl --vacuum-size=500M       # Or limit size
sudo rm -rf /var/log/*.gz /var/log/*.[0-9] /var/log/*.log.old

✅ System logs can grow indefinitely if not rotated.

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📁 4. Delete Temporary Files

sudo rm -rf /tmp/*
sudo rm -rf /var/tmp/*

✅ Regular temp files can accumulate and waste space.

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🧹 5. Clear Trash/Recycle Bin

rm -rf ~/.local/share/Trash/*

✅ Especially relevant for desktop environments (GNOME/KDE).

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🔧 6. Uninstall Unused Software

List installed packages:

dpkg --get-selections | grep -v deinstall

Remove unused ones:

sudo apt remove <package>

✅ Frees both binaries and dependencies.

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🧱 7. Identify and Clean Orphaned Packages

Debian/Ubuntu:

sudo deborphan

RedHat-based:

sudo package-cleanup --leaves

✅ Removes packages no longer needed.

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🐳 8. Clean Docker Resources

docker system prune -a        # Remove all unused containers/images/networks
docker volume prune           # Remove dangling volumes

✅ Docker can eat up gigabytes of space if left unchecked.

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🧼 9. Remove Old Kernels

On Ubuntu:

sudo apt --purge autoremove

✅ Old kernels not in use can be safely removed.

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📦 10. Clean Snap Packages (Ubuntu)

sudo snap list --all
sudo snap remove <old-version>

Use a script:

snap list --all | awk '/disabled/{print $1, $2}' |
  while read snapname version; do sudo snap remove "$snapname" --revision="$version"; done

✅ Snap stores multiple versions by default.

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🧠 11. Find and Delete Duplicate Files

Install fdupes:

sudo apt install fdupes
fdupes -r /home/user
fdupes -rdN /home/user     # Delete duplicates

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📦 12. Remove Unused Flatpak Apps

flatpak uninstall --unused

✅ Like Snap, Flatpak can take up hidden space.

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📁 13. Mount External Volumes Correctly

Sometimes disk full errors are due to writing to root (/) instead of a mounted path. Always check:

mount | grep <mountpoint>
df -h

✅ Prevents accidental writes to the wrong disk.

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🔄 14. Reboot to Release Deleted Files Still in Use

If a large file was deleted but still held open by a process:

lsof | grep deleted

Kill the process or reboot:

kill -9 <PID>

✅ Disk space is not released until the file handle is closed.

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🧾 Summary Table

Cleanup Method	Command/Tool Example	Safe
Find large files	du, ncdu, find, df	✅
Clean package cache	apt clean, yum clean all	✅
Clear logs	journalctl --vacuum-*	✅
Delete temp files	rm -rf /tmp/*	✅
Clear trash	rm -rf ~/.local/share/Trash/*	✅
Uninstall unused apps	apt remove	✅
Remove old kernels	apt autoremove	✅
Clean Docker data	docker system prune	⚠️
Remove orphaned packages	deborphan	✅
Delete duplicate files	fdupes -rdN	⚠️
Remove old Snap/Flatpak revisions	snap remove, flatpak uninstall	✅
Reboot or close deleted files	`lsof	grep deleted`

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Would you like a bash script to automate all safe disk cleanups?