6.5 Process Priorities: nice & renice (103.6)

6.5 Process Priorities: nice & renice

Quick Summary: In a multi-tasking operating system, the Linux Completely Fair Scheduler (CFS) dynamically allocates CPU execution time among competing processes. Scheduling decisions are guided by niceness (NI), an integer value spanning from -20 (highest priority / least nice to others) to 19 (lowest priority / most nice to others). Administrators launch new programs with custom priority offsets using nice and alter the absolute priority of active processes using renice. Unprivileged users may only decrease their own priority, while root retains unrestricted priority control.

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1. Linux CPU Scheduling, Dynamic Priority & Niceness

The standard Linux scheduler is the Completely Fair Scheduler (CFS). Rather than using fixed time slices, CFS models an "ideal multi-tasking CPU" on hardware by tracking the virtual runtime (vruntime) of each task. Tasks with smaller virtual runtimes are scheduled next on available CPU cores.

Dynamic Priority vs. Static Niceness

When viewing process tables in top or ps, two related priority columns appear:

• NI (Niceness): The user-configurable static priority modifier spanning from -20 to 19 (default: 0).
• PR / PRI (Kernel Priority): The internal dynamic priority value used by the kernel scheduler.

For standard user-space processes scheduled under SCHED_OTHER / SCHED_NORMAL, the kernel computes dynamic priority using the direct mapping formula:

$\text{PR} = 20 + \text{NI}$

Niceness-to-Kernel Priority Mapping Spectrum:
Niceness (NI):  -20 ─── -15 ─── -10 ─── -5 ──── 0 ──── +5 ─── +10 ─── +15 ─── +19
                 │                               │                               │
                 ▼                               ▼                               ▼
Priority (PR):    0 ───── 5 ──── 10 ──── 15 ──── 20 ──── 25 ──── 30 ──── 35 ──── 39
              [Highest CPU Time]            [Default]            [Lowest CPU Time]
              [Root Privilege Only]                             [Available to All Users]

The Niceness Spectrum & Privilege Rules

⚠️ LPIC-1 Trap — Niceness Logic is Inverted:

• A LOWER numerical value means HIGHER CPU priority (a value of -20 is aggressive, not nice to other processes).
• A HIGHER numerical value means LOWER CPU priority (a value of 19 is polite and nice to other processes).
• Unprivileged users can only make their processes more nice (increase NI from 0 up to 19). They cannot lower niceness (even back to 0 once raised!).

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2. Launching New Processes with nice

The nice command launches a new process with a specified niceness offset. It cannot modify an already running process.

Command Syntax

nice [OPTION] [COMMAND [ARG]...]

Specifying Niceness Values

Modern Linux systems support the clean -n (or --adjustment) flag:

# 1. Launch a CPU-intensive backup with lower priority (NI = 10):
$ nice -n 10 tar -czf /backup/archive.tar.gz /data

# 2. Launch a high-priority database process with negative niceness (Root only):
$ sudo nice -n -10 mysqld_safe &

# 3. Default behavior when no flag is provided (Increments niceness by 10):
$ nice tar -czf backup.tar.gz /data
# Runs with niceness NI = 10

Legacy Syntax Traps on the LPIC-1 Exam

On the LPIC-1 exam, you will encounter both modern -n syntax and legacy dash syntax:

• nice -10 command → Runs command with a positive niceness of 10 (low priority).
• nice --10 command → The first dash is the flag specifier; the second dash is the negative sign! This runs the command with a negative niceness of -10 (high priority, root only).

# Legacy syntax comparison:
$ nice -15 gzip bigfile.iso      # Nice = +15 (Low priority)
$ sudo nice --15 gzip bigfile.iso # Nice = -15 (High priority)

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3. Modifying Running Processes with renice

The renice command alters the scheduling priority of one or more already active processes.

Key Architectural Difference Between nice and renice

Exam Rule — Relative vs. Absolute Values:

• nice applies a relative adjustment / offset when launching a new program.
• renice sets the ABSOLUTE target niceness value on running processes!

renice Command Syntax & Target Flags

renice [-n] priority [[-p] pids] [[-g] pgrps] [[-u] users]

Practical renice Demonstrations

# Lower the priority of running PID 4812 to an absolute niceness of 15:
$ renice -n 15 -p 4812
4812 (process ID) old priority 0, new priority 15

# Alternative shorthand syntax without -n or -p:
$ renice 15 4812

# Elevated priority (Root only): Set niceness of PID 1024 to -5:
$ sudo renice -n -5 -p 1024
1024 (process ID) old priority 0, new priority -5

# Lower the priority of all processes belonging to user 'john':
$ sudo renice 10 -u john

Privilege Error Demonstration

If an unprivileged user attempts to assign a negative niceness or lower an existing niceness value, the kernel denies the syscall:

$ renice -n -5 -p 4812
renice: failed to set priority for 4812 (process ID): Permission denied

💡 LPIC-1 Exam Fill-in-the-Blank Alert: What command is used to change the priority of an already running Linux process? Answer: renice