🕒 Linux/Unix: Process Scheduling – nice, renice, chrt Explained with Examples

🧲 Introduction – Why Learn Scheduling Commands in Linux?

Linux is a multi-tasking operating system, where multiple processes run concurrently. Sometimes, you need to prioritize or deprioritize processes for optimal performance. That’s where nice, renice, and chrt come in—they control the CPU scheduling priority of processes.

🎯 In this guide, you’ll learn:

• How Linux handles process priorities
• Difference between nice values and real-time priorities
• How to schedule tasks using nice, modify them using renice, and apply real-time policies using chrt

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🧠 Understanding Scheduling in Linux

• Nice value: Determines priority for non-real-time processes (range: -20 [highest] to 19 [lowest], default is 0)
• Real-time priority: Managed by chrt, works with FIFO and RR (Round Robin) policies

Tool	Affects	Range / Modes	Privilege Required
nice	New processes	-20 to 19 (default 0)	Yes (for -ve)
renice	Running processes	-20 to 19	Yes (for others)
chrt	Real-time tasks	FIFO, RR, Other	Yes (root needed)

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🟢 nice – Launch Process with Custom Priority

✅ Syntax:

nice -n [value] command

📌 Description:

Runs a command with a custom nice value. Higher value = lower priority.

🧪 Example 1: Run with lower priority

nice -n 10 long_running_task.sh

📤 Output: (No output unless the script prints something)

🧪 Example 2: Run with higher priority (requires root)

sudo nice -n -5 myscript.sh

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🟡 renice – Change Priority of a Running Process

✅ Syntax:

renice [nice_value] -p [PID]

📌 Description:

Changes the nice value of an already running process.

🧪 Example:

renice 5 -p 1234

📤 Output:

1234 (process ID) old priority 0, new priority 5

🧠 Change priority for all user’s processes:

renice 10 -u john

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🔴 chrt – Set or View Real-Time Scheduling

✅ Syntax:

chrt [options] priority command

📌 Description:

Assigns a real-time scheduling policy (FIFO, RR, OTHER) and priority to a process.

🧪 Example 1: Run with FIFO policy

sudo chrt -f 50 ./critical_task

📤 Output: (Runs in real-time FIFO mode at priority 50)

🧪 Example 2: Check current scheduling policy

chrt -p 1234

📤 Output:

pid 1234's current scheduling policy: SCHED_OTHER
pid 1234's current scheduling priority: 0

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⚙️ Scheduling Use Cases

Task	Use This Tool	Example
Lower priority for backup	nice	nice -n 15 rsync ...
Raise priority for compiler	renice	renice -5 -p 3231
Real-time video rendering	chrt	chrt -f 80 ./render
Balance multi-user CPU load	renice	renice 10 -u username

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📌 Summary – Recap & Next Steps

Linux lets you tune process scheduling to manage system performance and responsiveness. With nice, renice, and chrt, you can control which tasks get CPU time and when—ensuring critical processes run smoothly.

🔍 Key Takeaways:

• nice starts a command with a custom priority.
• renice changes priority of already running processes.
• chrt assigns real-time priorities using FIFO/RR policies.
• Lower nice values = higher priority; chrt offers stricter scheduling.

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❓ FAQs

❓ What’s the default nice value in Linux?
✅ It’s 0.

❓ What’s the lowest and highest nice value possible?
✅ Highest priority = -20, Lowest priority = 19.

❓ Can any user lower the nice value (raise priority)?
❌ No, only root can assign negative nice values.

❓ What is the difference between nice and chrt?
✅ nice changes user-space CPU priority, while chrt controls real-time scheduling policies.

❓ How do I check the priority and scheduling policy of a process?
✅ Use:

chrt -p <PID>

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